The first question is at what point of the piston are you measuring the piston diameter? The correct measurement point for a GT/S4 piston is 62mm from the top, perpedicular to the pin (minor and major thrust faces). That's far down the skirt, the piston has a smaller diameter higher up. Unless the piston skirt is visibly scuffed low down there, my money would be with the piston to bore clearance being close to what it was when the car left the factory.

The factory spec is +/-0.005mm on the piston and +/-0.007mm on the bore, so by my math it could come out of the factory with a 0.032mm piston to bore clearance and still be A-OK for the hundreds of thousands of miles that it's supposed to last.

By the way, I am not an expert and so don't believe any of this.

 

I have changed my mechanictoday and found a new engine rebuild shop with proper Sunnen machinery and also understands Alusil.

The untouched cylinders and their pistons were checked. I have been to told that the difference between the cylinder diamater and the piston diameter is 0.08mm. I guess this means I will need new pistons and al cylinders will be processed.

 

I would get a second opinion on the measurements.
FWIW the 928 engine block and piston assembly has been made to cover 250K miles or more, Seeing wear at this low mileage doesn't sound possible,
unless the engine was run in an overheated state or low on oil

 

LFA, do you have the ability to take and post pictures? There is plenty of collective experience (re-)using pistons or not based on the appearance of their skirts. (Which of course does not obviate the need to actually measure clearances before reassembly, but is a good start.....)

If you can email but not host or post pics, send them to roberte01 at sbcglobal.net and I would be happy to post them for you.

 

LFA - Wrap it up on a crate and send it the Greg Brown at Precision Motorwerks and save yourself a fortune.

 

Thanks for your replies. I will definitely take pictures tomorrow and post them and also email to Rob. Thanks guys for all your help, I very much appreciate it.

 

In addition to the links above, you also need this one:

It has the link to the Tech Specs book for 90-95, which includes all the wear limit info and measurements for bores/pistons/rings/bearings/everything.

https://rennlist.com/forums/928-foru...ks-online.html

From what you've said you already have the Workshop Manuals, so I won't bother linking them too.

The people recommending you ship your engine to Greg Brown probably didn't read the part where you're in Istanbul. Hopefully you've got a good shop this time.

If you're worried about the local selection of 928-shops, one of the members here (Cheburator) uses a shop in Eastern Europe (possibly Bulgaria?) to do his 928 racecar's engine work - he may be able to give you a referral which is closer to home than the USA

 

Thanks Hilton I totally missed that - however it still may be cheaper to ship it to Greg using sea freight.

 

Roger - do you really think it's not cheaper to get an engine from a rear-ended European car and just swap that in? In the US, low miles great condition fully dressed engine goes for usd 2500. That's about round trip shipping to CA by the cheapest method.

 

I agree - anything has to be cheaper than letting inexperienced mechanics loose on such a special engine.

There are two low mileage GT engines on sale in the UK at Steve Strange's yard for about $4k. One is a 90 and the other a 91. I am sure other examples can be found closer maybe.

 

Plus he has intact heads and cams that can be swapped on to any late-model engine.
So any late S4 bottom end would do just the same, and those are cheaper than complete GT engines.

 

Matching number projects being a whole another difficulty level.

 

From what I've been told, good condition GT and GTS engines are really very rare, in Europe.

So many of these engines failed under high rpm use on the faster highways in Europe, that an intact one, in any condition, is difficult to find.

 

89 and later bottom ends are identical between GT and S4, aren't they?

 

'89 and early '90 can still have different parts but changes were so small that its meaningless. Bore and piston condition is much more important than few tenths compression ratio change.