Supercharged '91 GT Refresh
01-13-2014, 04:48 PM
#166
Nordschleife Master
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I don't know what I am talking about but here are some thoughts.
I think the valve seats need to be recut because the seat cutter indexes of the valve guide. If the new guide bore is not aligned exactly like the old one, then the valve doesn't seal without recutting the seat.
Even when recutting the seats, logic dictates that one wants to get the new guides as closely aligned with the old guides as possible. I think this minimizes the chance that valves end up too deep and the seats have to be changed.
I have heard that some people first thread a bolt to the seal side of the guide, as shallow as will fit. Then, they drill out most of the valve guide from the port side until the drill tip touches the bolt. This will remove most of the interference fit and the guides can be pushed out with least amount of force, creating the least amount of distortion to the aluminum bore. The same people cool the new valve guide with dry ice (why wouldn't my wife's freezer do?) and barbecue the head to 250F so the new guides can be set with the minimum force. Allegedly, this way the new valve guides will be about as closely aligned with the old guides as possible. I am not vouching for this procedure, just noting that this is one story.
I've got a digital mike that has a ball anvil in one end. It's supposedly very accurate. In practice, since it's a hand held mike and not a laser stand, a human operating the mike can never get to the stated precision. When measuring bearing shells, I am getting 0.002mm (0.00008") measurement error standard deviations for bearing shells and 0.003mm (0.00012") measurement error standard deviations for Honda-size rod journal (with a similar two-flat anvil mike.) These are measurement errors for measurements taken in temperature controlled environment within a 20 minute period, so measuring say on different days in slightly different temperatures would probably give much bigger measurement error standard deviations.
Any thoughts about whether what I said about the guides makes sense?
I think the valve seats need to be recut because the seat cutter indexes of the valve guide. If the new guide bore is not aligned exactly like the old one, then the valve doesn't seal without recutting the seat.
Even when recutting the seats, logic dictates that one wants to get the new guides as closely aligned with the old guides as possible. I think this minimizes the chance that valves end up too deep and the seats have to be changed.
I have heard that some people first thread a bolt to the seal side of the guide, as shallow as will fit. Then, they drill out most of the valve guide from the port side until the drill tip touches the bolt. This will remove most of the interference fit and the guides can be pushed out with least amount of force, creating the least amount of distortion to the aluminum bore. The same people cool the new valve guide with dry ice (why wouldn't my wife's freezer do?) and barbecue the head to 250F so the new guides can be set with the minimum force. Allegedly, this way the new valve guides will be about as closely aligned with the old guides as possible. I am not vouching for this procedure, just noting that this is one story.
I've got a digital mike that has a ball anvil in one end. It's supposedly very accurate. In practice, since it's a hand held mike and not a laser stand, a human operating the mike can never get to the stated precision. When measuring bearing shells, I am getting 0.002mm (0.00008") measurement error standard deviations for bearing shells and 0.003mm (0.00012") measurement error standard deviations for Honda-size rod journal (with a similar two-flat anvil mike.) These are measurement errors for measurements taken in temperature controlled environment within a 20 minute period, so measuring say on different days in slightly different temperatures would probably give much bigger measurement error standard deviations.
Any thoughts about whether what I said about the guides makes sense?
As for your measurements of the bearing shells, that is a very tricky measurement with people who know what they are doing. The proper way to do it is to stick the shell into the bore and measure the bore after. That in itself can be tricky. The best way is non-contact with an air gauge. Most people don't want to spend $3000.00 to measure just one size with a range of around .100mm. For every different size add another $1000.00.
What you meed to do is what is called a GR&R with your mic. It will tell you the reproducibility of your measurements. Generally you need 10% or less on your GR&R to have an effective measurement. On a bore of +/- 0.012mm that means you need to be able to take 90 measurements and cannot vary more than .0006mm (which is the limit of the best technology available today). Those numbers are not exact as the calculation in incredibly complex, but that is a good starting point.
So in short, your mic measurement is a best guess as well. At least to about +/- .0025mm
As Greg Brown says that is wear limit and not rebuild limit. If I was blueprinting the engine I would replace the guides. If I just wanted a couple more years out of it I might be inclined to leave them. In reality, to me it would depend on how much they charge to do the job.
01-13-2014, 04:56 PM
#168
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For those interested a link here
http://www.carcraft.com/techfaq/116_...lastigage.html
Seems they are in the Dr Brown camp -surprise surprise.
If one is going to take on something like this then not having the correct tools makes zero sense to me- especially on a high end/spec motor. I presume Porsche maintain very high tolerence controls when building their motors so why compromise that?
There again I suppose one could use some emery cloth on the high spots!
http://www.carcraft.com/techfaq/116_...lastigage.html
Seems they are in the Dr Brown camp -surprise surprise.
If one is going to take on something like this then not having the correct tools makes zero sense to me- especially on a high end/spec motor. I presume Porsche maintain very high tolerence controls when building their motors so why compromise that?
There again I suppose one could use some emery cloth on the high spots!
Porsche in their FACTORY service manuals, say to use plastigauge......
01-13-2014, 05:01 PM
#169
The removal and installation of the guides does not *seem* to be as important as the valve seat geometry and whether THAT was done right.
BUT - When SLC motorsports screwed up my heads (they will chime in if they see this and say they had someone else DO the work, but I was *thier* customer), and I had to have them fixed by a relatively well thought of guy in Orange county with a weird accent, he found lots of monkey mistakes.
One of which was a ballooned valve guide.
So its important that you not have monkeys hammering room tempature guides into room temperature heads. Because then the seals have to be hammered on like a monkey. Like what happened to my heads. I learned alot.
Also the new guides are not going to just magically fit your new valves. You have to make sure they will be reamed on the ID properly to match what you want to happen, with reference to clearance on the exhaust and intake sides. And these two clearances are different from what I understand.
What was also found was tool chatter on the valve seats. As if the tool sort of jumps around on the valve seat while attempting to cut it. Neil said that it was from a tool that was worn and not sharp or just bad toolmanship by the operator.
These are the things I think GB and Todd and Simard, etc. talk about when they speak of "experience" - where yes - mistakes are made, but one learns while making it right, either for yourself or your customer.
AO - I think you are looking at new exhaust guides. And that means recut or new seats for that end.
Make you you don't give it to monkeys.
BUT - When SLC motorsports screwed up my heads (they will chime in if they see this and say they had someone else DO the work, but I was *thier* customer), and I had to have them fixed by a relatively well thought of guy in Orange county with a weird accent, he found lots of monkey mistakes.
One of which was a ballooned valve guide.
So its important that you not have monkeys hammering room tempature guides into room temperature heads. Because then the seals have to be hammered on like a monkey. Like what happened to my heads. I learned alot.
Also the new guides are not going to just magically fit your new valves. You have to make sure they will be reamed on the ID properly to match what you want to happen, with reference to clearance on the exhaust and intake sides. And these two clearances are different from what I understand.
What was also found was tool chatter on the valve seats. As if the tool sort of jumps around on the valve seat while attempting to cut it. Neil said that it was from a tool that was worn and not sharp or just bad toolmanship by the operator.
These are the things I think GB and Todd and Simard, etc. talk about when they speak of "experience" - where yes - mistakes are made, but one learns while making it right, either for yourself or your customer.
AO - I think you are looking at new exhaust guides. And that means recut or new seats for that end.
Make you you don't give it to monkeys.
01-13-2014, 06:00 PM
#170
Former Sponsor
I don't know what I am talking about but here are some thoughts.
I think the valve seats need to be recut because the seat cutter indexes of the valve guide. If the new guide bore is not aligned exactly like the old one, then the valve doesn't seal without recutting the seat.
Even when recutting the seats, logic dictates that one wants to get the new guides as closely aligned with the old guides as possible. I think this minimizes the chance that valves end up too deep and the seats have to be changed.
I have heard that some people first thread a bolt to the seal side of the guide, as shallow as will fit. Then, they drill out most of the valve guide from the port side until the drill tip touches the bolt. This will remove most of the interference fit and the guides can be pushed out with least amount of force, creating the least amount of distortion to the aluminum bore. The same people cool the new valve guide with dry ice (why wouldn't my wife's freezer do?) and barbecue the head to 250F so the new guides can be set with the minimum force. Allegedly, this way the new valve guides will be about as closely aligned with the old guides as possible. I am not vouching for this procedure, just noting that this is one story.
I've got a digital mike that has a ball anvil in one end. It's supposedly very accurate. In practice, since it's a hand held mike and not a laser stand, a human operating the mike can never get to the stated precision. When measuring bearing shells, I am getting 0.002mm (0.00008") measurement error standard deviations for bearing shells and 0.003mm (0.00012") measurement error standard deviations for Honda-size rod journal (with a similar two-flat anvil mike.) These are measurement errors for measurements taken in temperature controlled environment within a 20 minute period, so measuring say on different days in slightly different temperatures would probably give much bigger measurement error standard deviations.
Any thoughts about whether what I said about the guides makes sense?
I think the valve seats need to be recut because the seat cutter indexes of the valve guide. If the new guide bore is not aligned exactly like the old one, then the valve doesn't seal without recutting the seat.
Even when recutting the seats, logic dictates that one wants to get the new guides as closely aligned with the old guides as possible. I think this minimizes the chance that valves end up too deep and the seats have to be changed.
I have heard that some people first thread a bolt to the seal side of the guide, as shallow as will fit. Then, they drill out most of the valve guide from the port side until the drill tip touches the bolt. This will remove most of the interference fit and the guides can be pushed out with least amount of force, creating the least amount of distortion to the aluminum bore. The same people cool the new valve guide with dry ice (why wouldn't my wife's freezer do?) and barbecue the head to 250F so the new guides can be set with the minimum force. Allegedly, this way the new valve guides will be about as closely aligned with the old guides as possible. I am not vouching for this procedure, just noting that this is one story.
I've got a digital mike that has a ball anvil in one end. It's supposedly very accurate. In practice, since it's a hand held mike and not a laser stand, a human operating the mike can never get to the stated precision. When measuring bearing shells, I am getting 0.002mm (0.00008") measurement error standard deviations for bearing shells and 0.003mm (0.00012") measurement error standard deviations for Honda-size rod journal (with a similar two-flat anvil mike.) These are measurement errors for measurements taken in temperature controlled environment within a 20 minute period, so measuring say on different days in slightly different temperatures would probably give much bigger measurement error standard deviations.
Any thoughts about whether what I said about the guides makes sense?
There's a bit more to the story than that. Porsche offers the first replacement guide in .010" oversize. The machinist is supposed to measure the hole from the guide that was removed and then machine the new guide to the proper tolerance and then installed.
Not very commonly done. Plus, the factory guides are very expensive.
More likely, the machinist will remove the stock guide and re-install the "standard" size guide....regardless if it fits or not. And the replacement guides will come from an aftermarket source....because of the price difference (PEP is a huge supplier of aftermarket valve guides.) This works great, if the valve guide holes are still exactly the correct size.
The other common thing to do is for the machinist to just order a replacement aftermarket valve guide that is .001" oversize and just stuff those into all the holes. Again, this works great, if all the valve guides holes happen to be .001" oversize.
The reality?
Each guide need to be properly removed by coring that guide and removing it properly, so that the valve guide hole is not made oversize by pounding out the guide....or the hole isn't made hourglass shaped by removing the old guide. Then each hole has to be measured and the correct dimension guide needs to be installed....in a chilled head.
Not very many machinists take the time to do things this way....actually, not very machinists know how to do things this way.
I did all my valve work, in house, for many years.....because I could not find a machinist that would take the time to do the job correctly. Took hours and hours to do a set of heads, correctly. The resulting job was expensive....and because I was cutting each valve seat as an individual, using different angle cutters...the heights varied.
And then I found my "dream machinist". Does everything as I described, plus has the equipment to make each and every valve seat exactly the same, at the exact same height, plus a whole litany of things beyond this. Understands that the oil plugs have to be removed for proper cleaning and replaces as the last step. Understands that aluminum heads need to be "soda" blasted, not bead blasted, so that the beads are not pounded into the aluminum surface and come loose later, ruining the rings and the bores.
And does the entire job for 1/2 of what I charged 10 years ago....
I no longer do my own heads, in house.
01-13-2014, 06:08 PM
#171
Rennlist Member
!00% correct, as far as I'm concerned.
There's a bit more to the story than that. Porsche offers the first replacement guide in .010" oversize. The machinist is supposed to measure the hole from the guide that was removed and then machine the new guide to the proper tolerance and then installed.
Not very commonly done. Plus, the factory guides are very expensive.
More likely, the machinist will remove the stock guide and re-install the "standard" size guide....regardless if it fits or not. And the replacement guides will come from an aftermarket source....because of the price difference (PEP is a huge supplier of aftermarket valve guides.) This works great, if the valve guide holes are still exactly the correct size.
The other common thing to do is for the machinist to just order a replacement aftermarket valve guide that is .001" oversize and just stuff those into all the holes. Again, this works great, if all the valve guides holes happen to be .001" oversize.
The reality?
Each guide need to be properly removed by coring that guide and removing it properly, so that the valve guide hole is not made oversize by pounding out the guide....or the hole isn't made hourglass shaped by removing the old guide. Then each hole has to be measured and the correct dimension guide needs to be installed....in a chilled head.
Not very many machinists take the time to do things this way....actually, not very machinists know how to do things this way.
I did all my valve work, in house, for many years.....because I could not find a machinist that would take the time to do the job correctly. Took hours and hours to do a set of heads, correctly. The resulting job was expensive....and because I was cutting each valve seat as an individual, using different angle cutters...the heights varied.
And then I found my "dream machinist". Does everything as I described, plus has the equipment to make each and every valve seat exactly the same, at the exact same height, plus a whole litany of things beyond this. Understands that the oil plugs have to be removed for proper cleaning and replaces as the last step. Understands that aluminum heads need to be "soda" blasted, not bead blasted, so that the beads are not pounded into the aluminum surface and come loose later, ruining the rings and the bores.
And does the entire job for 1/2 of what I charged 10 years ago....
I no longer do my own heads, in house.
There's a bit more to the story than that. Porsche offers the first replacement guide in .010" oversize. The machinist is supposed to measure the hole from the guide that was removed and then machine the new guide to the proper tolerance and then installed.
Not very commonly done. Plus, the factory guides are very expensive.
More likely, the machinist will remove the stock guide and re-install the "standard" size guide....regardless if it fits or not. And the replacement guides will come from an aftermarket source....because of the price difference (PEP is a huge supplier of aftermarket valve guides.) This works great, if the valve guide holes are still exactly the correct size.
The other common thing to do is for the machinist to just order a replacement aftermarket valve guide that is .001" oversize and just stuff those into all the holes. Again, this works great, if all the valve guides holes happen to be .001" oversize.
The reality?
Each guide need to be properly removed by coring that guide and removing it properly, so that the valve guide hole is not made oversize by pounding out the guide....or the hole isn't made hourglass shaped by removing the old guide. Then each hole has to be measured and the correct dimension guide needs to be installed....in a chilled head.
Not very many machinists take the time to do things this way....actually, not very machinists know how to do things this way.
I did all my valve work, in house, for many years.....because I could not find a machinist that would take the time to do the job correctly. Took hours and hours to do a set of heads, correctly. The resulting job was expensive....and because I was cutting each valve seat as an individual, using different angle cutters...the heights varied.
And then I found my "dream machinist". Does everything as I described, plus has the equipment to make each and every valve seat exactly the same, at the exact same height, plus a whole litany of things beyond this. Understands that the oil plugs have to be removed for proper cleaning and replaces as the last step. Understands that aluminum heads need to be "soda" blasted, not bead blasted, so that the beads are not pounded into the aluminum surface and come loose later, ruining the rings and the bores.
And does the entire job for 1/2 of what I charged 10 years ago....
I no longer do my own heads, in house.
01-13-2014, 06:14 PM
#172
\
Each guide need to be properly removed by coring that guide and removing it properly, so that the valve guide hole is not made oversize by pounding out the guide....or the hole isn't made hourglass shaped by removing the old guide. Then each hole has to be measured and the correct dimension guide needs to be installed....in a chilled head.
Each guide need to be properly removed by coring that guide and removing it properly, so that the valve guide hole is not made oversize by pounding out the guide....or the hole isn't made hourglass shaped by removing the old guide. Then each hole has to be measured and the correct dimension guide needs to be installed....in a chilled head.
01-13-2014, 07:08 PM
#173
Former Sponsor
Not sure that that extra work is really needed. The factory manual just states push out the old and push in the new. Now I have never had to replace a valve guide in a 928 engine before, so I don't know if there are any caveats with that.
As for your measurements of the bearing shells, that is a very tricky measurement with people who know what they are doing. The proper way to do it is to stick the shell into the bore and measure the bore after. That in itself can be tricky. The best way is non-contact with an air gauge. Most people don't want to spend $3000.00 to measure just one size with a range of around .100mm. For every different size add another $1000.00.
What you meed to do is what is called a GR&R with your mic. It will tell you the reproducibility of your measurements. Generally you need 10% or less on your GR&R to have an effective measurement. On a bore of +/- 0.012mm that means you need to be able to take 90 measurements and cannot vary more than .0006mm (which is the limit of the best technology available today). Those numbers are not exact as the calculation in incredibly complex, but that is a good starting point.
So in short, your mic measurement is a best guess as well. At least to about +/- .0025mm
I just did my GT engine and the guides were at .5mm as well. I just put it back in. If I had rebuilt the engine (bored the block, etc. I would have replaced the guides.
As Greg Brown says that is wear limit and not rebuild limit. If I was blueprinting the engine I would replace the guides. If I just wanted a couple more years out of it I might be inclined to leave them. In reality, to me it would depend on how much they charge to do the job.
As for your measurements of the bearing shells, that is a very tricky measurement with people who know what they are doing. The proper way to do it is to stick the shell into the bore and measure the bore after. That in itself can be tricky. The best way is non-contact with an air gauge. Most people don't want to spend $3000.00 to measure just one size with a range of around .100mm. For every different size add another $1000.00.
What you meed to do is what is called a GR&R with your mic. It will tell you the reproducibility of your measurements. Generally you need 10% or less on your GR&R to have an effective measurement. On a bore of +/- 0.012mm that means you need to be able to take 90 measurements and cannot vary more than .0006mm (which is the limit of the best technology available today). Those numbers are not exact as the calculation in incredibly complex, but that is a good starting point.
So in short, your mic measurement is a best guess as well. At least to about +/- .0025mm
I just did my GT engine and the guides were at .5mm as well. I just put it back in. If I had rebuilt the engine (bored the block, etc. I would have replaced the guides.
As Greg Brown says that is wear limit and not rebuild limit. If I was blueprinting the engine I would replace the guides. If I just wanted a couple more years out of it I might be inclined to leave them. In reality, to me it would depend on how much they charge to do the job.
I guess I should preface everything I write with "This is what I've found out about dealing with this particular issue. This is what I also do, when I'm faced with dealing with this particular problem. What I have found out and what I do may have nothing to do with what anyone else has found out or does."
Regarding the use of Plastigauge. Very old school product and has been used to build zillions of engines, in sure. I'm also certain that a zillion houses have been built with a yardstick as the only measuring tool....
I just prefer to measure and calculate clearances. Every machinist I've ever used also prefers this method and would never depend on Plastigauge to measure any of their bearing clearances.
I am somewhat relieved that after you re-used your guides with all of that clearance that you did not use the "rebuilt" nomenclature to describe your engine. Many people take engines apart, clean them up, replace all the seals and gaskets, re-use the bearings, the rings, and all of the other "wear items" and call an engine "rebuilt".
01-13-2014, 08:19 PM
#174
Former Sponsor
01-13-2014, 08:32 PM
#175
Supercharged
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So this is the guy to whom I should ship my extra set of 2R heads I have laying around so they will be perfect when the time comes that I need them. Just swap valves, cams and go? Or I just do it in my basement and then send the next set to him to be done right the second time after my motor goes boom?
Damn Greg! YOu sure know how to inspire confidence?
01-13-2014, 09:11 PM
#176
Former Sponsor
When Jay passed away, I had to find another machinist and was fortunate enough to find one that did the valve work better (and cheaper) than I could do it, in house.
Is it always perfectly done?
No, I'm still the final quality control guy and I've had to have a few things redone....but he is certainly the best I've found.
I'm just trying to arm you with the questions that you need to ask....
Are you going to post pictures of your head gaskets? Especially interested in seeing the fire ring that surrounds the combustion chamber. There's a whole bunch of stuff to be learned, there.
01-13-2014, 09:15 PM
#177
Supercharged
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Okay. Thanks. I appreciate that.
WHat about the guides themselves? Is there a preferred brand to use? Porsche, SI, someone else? And would I normally supply said guides with the head, or do we source them once the old ones have been removed? And do I bring the valves along with the heads? Sorry for the stupid questions, but I'm sure others out there have the same or will when they decide to go down this path.
WHat about the guides themselves? Is there a preferred brand to use? Porsche, SI, someone else? And would I normally supply said guides with the head, or do we source them once the old ones have been removed? And do I bring the valves along with the heads? Sorry for the stupid questions, but I'm sure others out there have the same or will when they decide to go down this path.
01-13-2014, 09:35 PM
#178
01-13-2014, 10:02 PM
#179
Former Sponsor
Okay. Thanks. I appreciate that.
WHat about the guides themselves? Is there a preferred brand to use? Porsche, SI, someone else? And would I normally supply said guides with the head, or do we source them once the old ones have been removed? And do I bring the valves along with the heads? Sorry for the stupid questions, but I'm sure others out there have the same or will when they decide to go down this path.
WHat about the guides themselves? Is there a preferred brand to use? Porsche, SI, someone else? And would I normally supply said guides with the head, or do we source them once the old ones have been removed? And do I bring the valves along with the heads? Sorry for the stupid questions, but I'm sure others out there have the same or will when they decide to go down this path.
01-13-2014, 10:05 PM
#180
Nordschleife Master
My guess is that this kinds of jobs are not hard to do right, but they are hard to do right enough at acceptable cost. To do the right enough at an acceptable cost, purely based on logic with no experience I would say that tooling is necessary. Threading the seal side of the valve guide is reasonable quick, and screwing on the bolt too. However, I think one needs a specialized tool to quickly align a drill bit relative to the seat and/or valve guide bore and drill the most of the inside of the valve guide out. After that is done, it'll be cheap to push the guides out and heat the head and cool the guides to push them in.
To an inexperienced person, this whole valve guide / valve seat business is a bit of a chicken or egg problem. If the seat is right, then one could index any valve guide operation of the seat. If the guide is right, then one could index any seat operation from the guide. Again, purely based on logic, everything will be the easiest the least violence is done to any part, guide bore or seat, as a part of the replacement operation. With sufficient volume, I am not sure that the method I described would actually have a much higher variable cost overall than the simplest possible method.
As for your measurements of the bearing shells, that is a very tricky measurement with people who know what they are doing. The proper way to do it is to stick the shell into the bore and measure the bore after. That in itself can be tricky. The best way is non-contact with an air gauge. Most people don't want to spend $3000.00 to measure just one size with a range of around .100mm. For every different size add another $1000.00.
What you meed to do is what is called a GR&R with your mic. It will tell you the reproducibility of your measurements. Generally you need 10% or less on your GR&R to have an effective measurement. On a bore of +/- 0.012mm that means you need to be able to take 90 measurements and cannot vary more than .0006mm (which is the limit of the best technology available today). Those numbers are not exact as the calculation in incredibly complex, but that is a good starting point. So in short, your mic measurement is a best guess as well. At least to about +/- .0025mm
I do not understand the quotation units of your "GR&R" measures. Percentage of what? Since I have a specific application, I think I can just quote the standard deviation units in mm. Maybe I should go and learn the quotation conventions of this field.
(A digression: As a philosophical point, I am a Bayesian so I don't believe that there's a set number of measurements that one needs to understand the measurement error dispersion. More is of course better! And if there are independent experimental dimensions, such as standard, mike, person, etc., the third measurement per dimension seems worth a lot. But I digress in areas that are of no interest to anyone except possibly armchair statisticians...)
A point relating to measurement error that I found, at the time, profound: I was taught to use the flat anvil/flat spindle mike to measure the rod journal first. Then, use that mike to zero the dial bore gauge which is used to measure the installed bearing. A lot of the individual measurement errors cancel in this process, which I found fascinating.
I just did my GT engine and the guides were at .5mm as well. I just put it back in. If I had rebuilt the engine (bored the block, etc. I would have replaced the guides. As Greg Brown says that is wear limit and not rebuild limit. If I was blueprinting the engine I would replace the guides. If I just wanted a couple more years out of it I might be inclined to leave them. In reality, to me it would depend on how much they charge to do the job.