LFA

Hi,
I have a 1991 GT and my mechanic is rebuilding the engine.
It looks like we need to replace the pistons of my M2847 engine ( part number 928 103 043 79).
Should I buy the original Porsche pistons above or do you recommend any other/ better brands.

Thanks for all your help.

The Forgotten On

Get original, there are no aftermarket pistons in stock sizes as good as the original Porsche ones. Or they are more expensive.

BC

There were some S4 pistons on ebay recently as a set. What is the tolerance group stamp on the block?

Rob Edwards

How many miles on the engine and what's wrong with the original pistons?

namasgt

Why does the mechanic wants to change the pistons? The original ones are likely fine, unless some major failure or wear has happened.
Post some pictures of your pistons.

Mark Anderson

We have several good sets of used pistons and a good short block as well.

LFA

Thanks for all your replies, they are very helpful.
The car has 90K miles on it, and has been sitting for some time. I am also puzzled that my mechanic wants to replace the pistons. His comment over the phone was, he had the bores and pistons measured and the gap was around 0.0080, way above the limit. he wants to replace with 8 pistons with 100.50mm tolerance group.928 103 043 79 parts number. BC mentioned the tolerance group stamp on the engine, where exactly is this stamp located so that I can trace it. Does the tolerance change if the engine was rebuilt before. I will post pictures of the pistons tomorrow, Thanks again for all your help.

BC

The tolerance number is right at the edge of the V where it meets the bottom edge of the heads. Rob - I'll assume you have a picture.

Rob Edwards

No 'real' pictures, but I have a snap from the tech specs book:

BC

Ladies and Gentleman - one of the most reliable men in the surrounding areas.

Red Flash

Without ever having any direct contact with this gentleman, it is my opinion as well just based on many observations.

John

namasgt

If the Alusil bores look good with no wear step on the top and bottom of the bore and the pistons special coating is intact and the tolerance numbers for the piston and bore match (its either going to be 0,1, or 2; 1 being the most common) I would reassemble with new rings and be done with it. And by the way make sure your mechanic doesn't hone the bore like they do with steel bores. Alusil bores requires a special process. If he doesn't have access to this process its best to just clean the bore with a soft plastic brush to clean off the carbon deposits and oil stain.
Who is working on your engine? Has he done a 928 engine before? If he has, did it last? Is he an experienced 928 mechanic? (Notice that I didn't say experienced Porsche mechanic)

And by the way not sure how bad your cams are but its normal for GT cams to be slightly pitted on the cam nose due to soft valve springs. Some people use stiffer Porsche 944 S2 springs and lighter VW lifters to help fix this problem. Search for GT cams and you will see a lot out there with pitting and people just re-use them as long as its not terrible.

This forum is going to be your best friend for your 928 related issues. Use the search function and read as much as you can to learn and be more informed before a irreversible costly damage happens to your car.

ROG100

Rob deserves a "lifetime" membership for all his help - he is up there with Mr. Merlin and Dwayne and Bill Ball and and !!!

LFA

I have checked and unfortunately they have honed one of the cylinders. Is this a disaster or is there a solution?
Can you please give some information on the ''special process to hone Alusil bores''

( If the Alusil bores look good with no wear step on the top and bottom of the bore and the pistons special coating is intact and the tolerance numbers for the piston and bore match (its either going to be 0,1, or 2; 1 being the most common) I would reassemble with new rings and be done with it. And by the way make sure your mechanic doesn't hone the bore like they do with steel bores. Alusil bores requires a special process. If he doesn't have access to this process its best to just clean the bore with a soft plastic brush to clean off the carbon deposits and oil stain. )

GregBBRD

Regular honing turned the existing bore into absolute junk.

You now have no choice but to have the block over bored and correctly honed, by someone who knows this process.

The only pistons that will work need to come from Porsche.

And seriously, if your mechanic doesn't know enough to not run a hone through one of these cylinders, you are in a "world of hurt".