Adamant1971

Hi All,

My machine shop is about to build my short block for the 89.

The factory specs are 0.024 - 0.048 mm with a wear limit of 0.080 mm.

They want to do 2 tho clearance (0.058 mm), which is on the loose side, is that ok? Or do you recommend I have them go tighter at 1.5 tho (0.0381 mm)

(using new OEM oversized pistons @ 100.50)

Thanks in advance

GregBBRD

Just looked at the website of 928 Autohaus.

Shouldn't the people that specialize in the restoration of 928's know the answer to this?

Mongo

Why do they want to exceed the factory limit for clearances? Is there wear that exceeds the threshold to which they are trying to compensate for?

Perhaps crating your short block over to Greg would be more practical in the long run despite freight costs. Especially since his rebuilds are backed by knowledge and success.

AllMine

These decisions are made based on experience of the machinist/builder. Conversations with the owner about usage. All within the piston manufacturers specification range. A little concerned about the decision being up to you. I appologize if I am wrong I know neither you or your builder/machinist. The question could show either the machinists lack of confidence in themselves, or lack of confidence in your machinist.
Now, just for conversation I ask these kinds of questions all the time...
To make a better contribution would require complete piston specs, including metalurgy, and part number (if not custom made)
If nothing else, here is a bump to the top
Curt

Adamant1971

Greg, that's why I'm using a machinist to build the short block vs doing it myself.

Mongo, the wear was not enough that they need to compensate. The block is tolerance group one with 100 mm pistons. We are rebuilding this block with all stock internals and new 100.50 mm Porsche pistons/rings/pins.

Curt, you are 100% correct.

I have had several conversations with the shop and they understand what we are trying to accomplish with this build. In my conversation with them today I stated that I trust them and want them to do all that is necessary with this block. They are recommending 2 tho for the clearance.

This shop has a solid rep with the local Porsche and Ferrari crowd, so perhaps I was wrong in doubting them and posting the question here. It's just that this machine shop is new to me, and I may have been over thinking. ( I do that sometimes)

AllMine

No worries.. I ask all kinds of questions here and definately overthink MOST things about my ride. Sounds like you have done your homework. I am always very nervous when having machine work done, soo many ways for it to go wrong.
Enjoy your build, keep us posted
Curt

GregBBRD

It will certainly run at 2 thou and might be a decent "target" for them to "shoot" at....depending on an almost overwhelming amount of factors that can only be contolled by knowledge and labor.

The biggest issue with boring and honing these engines is the inability of machine shop level equipment holding factory tolerances for both circumferential and vertical size. The very best equipment and operators will hold individual cylinders to about .0005". It is much more common to see variances of .001". And this is for individual cylinders! The variance from one one cylinder to the next can also very more that .001"

The pistons that I've been receiving from Porsche inventory are like fingerprints....no two are the same. For what they currently have in inventory, the different size groups (0, 1, and 2) are a joke. I've been sent 8 pistons that are more than .001" different.

At any rate, carefully measuring each bore and matching each individual piston with each bore is a virtual requirement to getting a great result.

Getting a machine shop to do all.of this labor is the really tough thing....it adds hours of additional time to accomplish! Additional labor costs money!

Here's the reality....if their equipment is really good and the operator is really good and they spend the time to measure and fit each piston to each bore, .00125" of clearance will last the longest and give the best results.

If their equipment is isn't, they aren't....or if they don't: Even a "target" of .002" can be a disaster.

And here's the real problem: Since they are doing the machining and doing the assembly, you won't know until it doesn't function correctly.

This is usually a mistake.

My machine shop is very good and they know what a crazy perfectionist I am. And they know I'm going to measure and check everything they have done. I think this "inspires" them to do their very best.

Adamant1971

Yes they are measuring each piston and will bore accordingly, 1 piston was a 1/2 tho off from the the rest. As far as labour they have my full approval to go all the way.

I should have her back by the end of the week. That's if Porsche gets me the one missing piston pin clip.

GregBBRD

Sounds like you've found the correct shop, if they already know this.

So what is their logic for shooting at .002" piston to wall clearance?

James Bailey

That could have multiple meanings

SeanR

Who is 928 Autohaus?

Speedtoys

Imo000

928 AutoIgloo!

Kevin in Atlanta

The coating that was already applied to my stroke pistons was applied too thick. My shop is sending them out to be re-done properly. Same folks that coated my bearings.

Given that 100.5 piston is smaller than my 104mm pistons and from what I read the larger the piston the larger the clearance, should the .00125 be greater?

Thanks.

EDIT: According the 968 WSM the clearance looks like .020mm

worf928

@Adamant1971 Epilogue to this? What was the final target clearance? What was the final measured clearance? Did the engine run?