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An update on my engine block at piston 8.
It has been a really fun and challenging few weeks...
I've had to wait until I saved enough money to buy some tools for measurement,
and then I took some time with a friend to learn the do's and don'ts of micrometer and bore gauge use.
I have started to develop a consistent feel for the instruments, and it's actually rewarding to begin learning a skill so under-appreciated as "measuring something".
WSM designates piston measurement at 62 mm from crown 90 deg offset to pin axis...
(Noooo, I didn't use those calipers to actually measure anything other than the 62mm depth LOL ! !)
I went ahead and made a "stand" to allow a more stable measurement of the piston...
This is a 1981 Euro 928S engine, and I double-checked my piston and block tolerance groups...
The piston measures 96.963mm (at 62mm from crown).
Spec is (96.970 +/- 0.005) for tolerance group 1.
The bore measures out at 97.089mm. (at 62 mm transverse to the direction of the block)
Spec is (97.010 +/- 0.005) for tolerance group 1.
I repeated the measurements 4 times, over 2 different days.
The most challenging part was definitely the bore gauge, it was hard to get it in exactly the same spot down in the bore for measurements...
Jerry mentioned the possibility that the cylinders had been incorrectly honed at some point. I am very curious about this possibility.
I tried to get some more photos of the "hatching" down in the bore....
I am uncertain of the best move at this point. My goal for this car is to have a fun driver, I am not trying to restore to showroom stock perfection.
I want to drive the wheels off the car with my wife and kids, and pass it onto them at some point (hopefully a LONG time from now LOL)
What would you do next? Any suggestions are very greatly appreciated.
Thanks so much for your time and patience.
If it was a customer car, in my shop, I'd bore it and go oversize. Too much risk/effort to put it all back together and not have it work properly, only to have to do it again.
Took me almost 20 years to "learn" this:
"Never make a customer's problem your own problem. Once you touch it, you are responsible for it, regardless of what the customer says at the time.....because they never remember having that discussion."
However, if it was my own car and I was down to my last couple thousand dollars.....and didn't want to spend it on boring and oversize pistons (or perhaps more importantly..... didn't think I could find a local machine shop that could do the boring and honing properly) I'd find a replacement piston, minimum.
the last thing was what I did. not 'cause I was down to last thousand but I just didn't think it was worth it to go all in when I could go all in later with a new engine.
I'm starting to think that you are in the "hope mode". Hoping that damage away is not going to happen.
Here's as simple as I can make it:
1. At a minimum, find another piston...preferably a group 2. Make sure the piston and pin weigh the same as your current pistons. If you can find someone to run the felt hone/lapping compound in that cylinder, that would be a great thing to do....if only to make sure there are no "high" spots.
2. Buy the first oversize piston and have that one cylinder bored and lapped. Make sure the piston/pin weighs the same as your other pistons. (Porsche approves of this method.)
3. Search the world and find an Alusil repair sleeve. Have that sleeve fitted, bored, and lapped. Use a different standard piston.
Semi-retired, as of Feb 1, 2023.
The days of free technical advice are over.
Free consultations will no longer be available.
Will still be in the shop, isolated and exclusively working on project cars, developmental work and products, engines and transmissions.
Have fun with your 928's people!
Thanks for the advice.
Greg, you are 100% correct, I *am* in 'hope mode' at this point, but it's more hoping that I do the best thing here with what I have.
Dan, it really did zoom right along ! !
Random thoughts:
Now that I'm an expert with a micrometer , I sorta want to take the whole thing apart and measure EVERYTHING... what else is lurking ???
Should I just seal off this rabbit hole and find a nice 5.0 short block assembly?
Truth be told, I am far less likely to grenade something if I stay in bolt/unbolt mode...
Which repair is the most cost-effective?
I'm really having fun with the 'process' involved, and I'm certainly in zero time concern to get the 928 on the road by a deadline date...
Dan
An update on my engine block at piston 8.
It has been a really fun and challenging few weeks...
I've had to wait until I saved enough money to buy some tools for measurement,
and then I took some time with a friend to learn the do's and don'ts of micrometer and bore gauge use.
I have started to develop a consistent feel for the instruments, and it's actually rewarding to begin learning a skill so under-appreciated as "measuring something".
This is huge, well worth the price of admission. Most gearheads go through life and never understand this.
Originally Posted by Daniel5691
The piston measures 96.963mm (at 62mm from crown).
Spec is (96.970 +/- 0.005) for tolerance group 1.
The bore measures out at 97.089mm. (at 62 mm transverse to the direction of the block)
Spec is (97.010 +/- 0.005) for tolerance group 1. ...
So clearance is 0.126mm or 0.005", right?
The numbers you quoted are specs are for new pistons/bores, not wear limits. Wear-limits are in the tech spec booklet, under cylinder-piston clearance. The wear-limit is listed as "approx 0.080 mm", 0.003". You are certainly over that, but not so much that the engine won't run OK (as it did). But now that you are in there, "what to do?" is the right question.
On a budget, Greg's "door #2" sounds like the best deal and will get the car back on the road for summer touring with the family. Everything beyond that is a slippery slope...
Measuring things is hard. I used to interact with companies that made complicated investment castings. The most important people in those shops - nearly god-like - had the title "Dimensional Engineer". Sounds mundane; isn't.
A final update on this "phase" of my engine rebuild project.
I inspected and measured the remaining pistons and bores.
On piston 7 and cylinder 7, I found the following:
These gouges are remarkably sharp and deep.
Similar finding were noted on piston and bore 5.
Measurements for pistons and bores at 62mm depth
were also similar for 8,7, and 5.
Interestingly to me, all the damage is limited to this side of the short block assembly.
The opposite side pistons and bores were all clean and measured significantly less wear.
My EuroS heads are completed and waiting for pick up at the machinist,
he says they look great. His business does not handle AluSil,
and he did not even know of a place that does...
His best advice was to give me the name of the fellow
he buys supplies from and ask him if he sells Sunnen/ AluSil stuff somewhere lol ! ! !
Fortunately, Fred has a S3 short block assembly for sale.
Thanks for all your input and guidance so far to everyone.
No, posting pics of my slow progress is daunting, and I have been procrastinating for the past 8 months. I have thousands of pics for my '78 project, from hood off, to the original engine out, then euro engine disassembly and now euro heads refinishing.
I bought a m28/44 short block to be shipped to GB, along with my newly finished heads for an engine rebuild. Reading your thread really pushes me to start my own thread for '78 #413.
Keep us posted about your progress, Dan. Keep up the great work.
what we did was just bore the thing out to 100mm and use S3 pistons and build a 5 liter 3 valve.
its easy... the only thing you need to do is bore the block , and it will be perfect. . the used pistons will probably be great too. yours has way too much damage to risk assembly... and you know i get in "hope mode" more than anyone.
rebore, 5 liter, used pistons from a 85....... you are going to be REALLY happy!
OR, the better way, is to buy a used short block from anderson from an 85. its the best $1000 you can spend on these cars!! after you are done, you will have a new motor!!
Thanks for the encouragement, guys.
FBIII (Fred) has hooked me up with a sweet S3 short block assembly,
and I am hyper-enthusiastic to get this job DONE RIGHT ONCE.
I don't care if it takes me 5 years, I am having a blast learning all the knowledge.
The Porsche 928 is fascinating and incredibly cool.
Every simple discovery is new and fun to me, and I love working and thinking about this poor old car. I just imagine starting her up someday and ZOOOOMMMMM down the road ! !
I'm sure I'll have many many more questions, and I really rely on the wisdom here. But how will I ever "pay it back" to you guys?
Maybe a stack of Dairy Queen gift cards..... hmmm.... there's an idea
02-28-2016, 09:43 PM
, I sorta want to take the whole thing apart and measure EVERYTHING... what else is lurking ???
