atb

Sometimes, too much information is just that, too much information.
I've been assembling a nikasiled S4 motor keeping an extreme eye on attention to detail on assembly. I really wanted to see how perfect Porsche got it with these motors. I was having fits with the crank/bearing clearances being too tight. The crank mic'd in at factory spec, but when I measured the crank bearings under torque with a dial bore gauge, I was only reading a 4 to 8 ten thousandths difference from the crank spec. Factory spec calls for a clearance between the main bearings and crank from 11 to 28 ten thousandths. So, my search began for that extra 7 ten thousandths clearance. Cleaned the bearing saddles, no change. Torqued the cradle with the crank installed to seat the bearings, removed crank, retorqued crank and measured, no change. I applied loctite 574 to the cradle, thinking that would provide the extra clearance at the main bearings, but there was no detectable change in the main bearing spec. I talked to my machinest about hitting the crank with a light polish to see if I could remove about a thousandth of material and pull the crank into spec. He said no way. If I was working with a SBC, he be willing to work with the main caps and get the extra spec that way, but with the crank cradle set up of the 928, he said he wouldn't be willing to mess with it. He suggested plastigauging it just for kicks and see what happens. I checked in with Garrity and Dave at Motorsport, and Louie as well, just to see if they've come across this situation on a 928 build. No one had, but they all suggested plastigauging just for a general reading, and torquing the cradle with the crank loaded and see if it spins freely. I guess since the WSM shows plastigauging as the correct way to measure spec, it must be right? So, I plastiguaged the crank, and sure enough it was in spec, I'd estimate about .0013". Oil the main journals, torqued it down, and hung a wrench on the crank bolt. Crank spun freely from just the weight of the ratchet handle. Nice. Moral to the story? Use the plastigauge. Dave Lomas had mentioned its the only method that measures both components (crank and bearing) when they are both interacting with each other under the stress of the torqued main studs. Makes sense. Plastigauged the Oliver rods, and they are also in spec. Went to check ring gap, woah, need to invest in a ring end gap file. These total seal standard rings are going to need a lot of work.

So, the build goes on, and we're just a ring end gap file from short blocking this beast.

Garth S

That's a valuable bit of knowledge to tuck away . ..and a better cure for a major headache than asprin ...

69gaugeman

Being in the auto industry and in the gauging end of it, geometry has a huge effect on what size you read. .0001" is size change due to temperature. To accurately measure clearance is VERY difficult. Platigauge is one of the only few ways (without spending 10's of thousands of dollars) to do this.

Rod

John Veninger

Adam,
Just and FYI.
Use the ring end gap spec from the 968 spec book, not the 928 spec book.
A bit different for 104mm pistons vs 100mm pistons.

Tom. M

This is JT's rear turbo motor build...so he's running 101.5 Ross pistons...and nicasiled bores...Adam's motor is next in line..once he gets those gold plated platinum centered Mahle's he ordered like 5 years ago hahaha..

later,
Tom
89GT

JEC_31

Rod just wrote my entire post for me, down to the word. Thanks!

All that I can add are these two:
A) Something told to every new kid learning how to machine metal: Remember, steel is just stiff silly putty.

B) Your measurement contains all the error of the measuring system. So the indicator has it's allowable error, the indicator stand has vibration, the steel has surface oil and temperature changes from your hands, etc etc.
The plastigauge simply has less variables and as noted is very real world as it's "in" the interaction between the assembled components.

atb

Rod wrote:

I agree, but I think 6 hours with a micrometer and dial bore gauge is about as accurate as I can humanly get. That's over an hour a hole of getting consistent measurements.

JV wrote:

As Tom noted this isn't for the stroker, but thanks for keeping on eye out for me. I'm going to need all the help I can get when I build that beast.

JEC_31 wrote:

Never would have considered this interaction as a variable, it is good to know.

I was very suprised that the loctite didn't give any additional clearance on the crank bearing spec. I have a magazine article somewhere that espouses this as a critical spec in crank clearancing, and got down to the point of stating that a particular nap roller has to be used to spread the loctite to ensure the correct pre-torque thickness. What a load of crap. (with regard to 928's anyway)

JEC_31

The loctite thing must be specific to a certain engine design, or perhaps specific to one mentally deranged engine builder who successfully B.S'd a magazine writer. Nap roller? Like the kind I'm using to paint my kitchen? That's a lot of loctite...

Fabio421

Nap'd roller for loctite? Where are you putting this loc-tite? On the bearings? I wouldn't see where it would make a difference in clearance if it were on the bolts.

atb

Hey Fabio,

The loctite is rolled onto the top surface of the crank girdle, which is the mating surface to the bottom of the block. Sincle liquids theoretically don't compress, and since some amount of the loctite must remain in the seam between the block and crank girdle, it would seem that the parting line in the main bearing bore would be wider than just plain metal to metal contact, resulting in a looser bearing tolerance in the main line. While at some nominal level this has to be true (every crank I've split I've had to scrape dried loctite off the joined surfaces) it appears that the thickness of the loctite gets compressed enough under the torque of the crank studs to truly be nominal. My dial bore gauge has .0005" increments, the space between the hash lines on the gauge is large enough to see differences of .0002", and I didn't see any difference in the bore size with or w/o the loctite. If it makes any difference it's got to be less then .0001".

a4sfed928

Measure the dial bore gauge with the micrometer you measured the journals with? This may show you the error. Put the micrometer on the dial bore gauge and screw it in until the dial bore gauge reads the same reading you had taken of the I.D.then read the micrometer. This will take a little time to master due to holding everything and getting it centered for proper readings.

atb

4sfed,

Did exactly what you mentioned as far as caliberating the dial bore gauge to the micrometer. You are indeed a master if you can do this by yourself, I had to enlist the help of another set of hands. I was pleasantly surprised when the mic give the same reading as the dial bore gauge.

BC

Adam - I just spent three hours doing this. But on the rod bearings. I am getting some very conflicting info, and I simply have chalked it up to my first usage of a real dial bore gauge (well, if real is 3k, them mine at 300 is still fake, but...)

I have "porsche bearings" which have yellow paint on the side, and I have glycos, which have no paint.

The glycos are larger. Funny that, as its been said repeatedly by one person that the opposite is true. I will pick and choose, I suppose, between these 36 bearing halves to find the proper 2 and a half thousands that I have figured out that I would like on the rod bearings.

Next is plasti-gauge, as you found it gave a different reading.

atb

Hey B.C.,

In reading through this old thread I once again got to live through through the frustration of taking very techinical measurements with cheap tools and inadequate experience. But, I can rest on the knowledge that the motor has been taking a beating without skipping a beat so far, so while many will say plastigauge is for backyard mechanics, its got to be in the WSM's for a reason.

Interesting point on the bearings. Just goes to show that the engine builders mantra is true - take nothing for granted until you verify it for yourself.

I'm not implying that I'm a pro, but I was able to get consistent (although they could've been consistently wrong ) readings by utilizing the same method of "coming at" the measurement with the dial bore gauge. I'd insert the botttom of the gauge first, compress the pin and put it in the opening, and then slide the pin into the bore by lifting up on the gauge end. I would measure past the narrowest distance and then come back to it, kind of like finding TDC with a degree wheel. You can tell when you've got the true measurement because any movement at all- lateral, up/down - increases the measurement.

I'm curious to see how your plastigauging goes.

RyanPerrella

oh man, i dont know, but i dont think gold pistons will last very long!