Piston tolerance group

[mueller944s2]

Anyone know where to find the number on the S2 pistons? I know on the 8 valves the number is on the face and pretty obvious, but none of my S2 pistons have any numbers...?

[John_AZ]

Pistons from 1989:




J_AZ

[John_AZ]

Are you planing a complete rebuild?
Old thread:

944 S2 Engine rebuild info: Doc? - Rennlist Discussion Forums

J_AZ

[mueller944s2]

Not yet but eventually. The tolerance numbers aren't labeled on my pistons though.

[9FF]

iirc these were used during production to make up for the manufacturing tolerances of the blocks. Does it even matter anymore on an engine that inevitably has some wear on both the bore and the pistons?

Plus any engine shop can measure the piston/bore clearance to make sure it's to spec and there are piston coatings compatible to Alusil that can be applied to bring them to spec if needed.

[mueller944s2]

So did they make them different on purpose or did they label them based on the exact size of the block that happened to result? I always thought that everything was the same.

[9FF]

Originally Posted by mueller944s2

So did they make them different on purpose or did they label them based on the exact size of the block that happened to result? I always thought that everything was the same.

From what I have read the blocks had a tolerance in manufacture and were split into groups depending on what that tolerance was. They were then matched with the same tolerance group pistons to give the correct piston to bore clearance.

I'm assuming that getting that final Alusil finishing on the bores chemically changed the structure of the surface metal of the bore so that there was a wider tolerance in the final bore sizes. Probably would not be a concern with traditional cylinder finishing but the Alusil finishing relies on a surface chemical reaction so it probably wasn't as exact. Plus once the Alusil finishing was completed there was no way to take more metal off to bring it to spec, that would require another Alusil refinishing process.

My point is during a rebuild all that matters now is the piston to bore clearance is to factory spec and if it isn't there are piston coatings nowadays that can bring them to spec.

Need an engine builder to chime in to confirm...

[mueller944s2]

Originally Posted by 9FF

From what I have read the blocks had a tolerance in manufacture and were split into groups depending on what that tolerance was. They were then matched with the same tolerance group pistons to give the correct piston to bore clearance.

I'm assuming that getting that final Alusil finishing on the bores chemically changed the structure of the surface metal of the bore so that there was a wider tolerance in the final bore sizes. Probably would not be a concern with traditional cylinder finishing but the Alusil finishing relies on a surface chemical reaction so it probably wasn't as exact. Plus once the Alusil finishing was completed there was no way to take more metal off to bring it to spec, that would require another Alusil refinishing process.

My point is during a rebuild all that matters now is the piston to bore clearance is to factory spec and if it isn't there are piston coatings nowadays that can bring them to spec.

Need an engine builder to chime in to confirm...

I would've thought that if you had a 0 block then 2 pistons wouldn't fit in there, but I can't even find the dang number. Mine are just blank. I'm wondering if the S2 was even labeled. IF I did have a 0 block could I rehone the cylinders to be 2 to clear up the scratching?

[9FF]

Originally Posted by mueller944s2

I would've thought that if you had a 0 block then 2 pistons wouldn't fit in there...

The tolerance groups are very close together and there is likely some wear on the bores in a used block (I would think). Maybe a 0 block will take a 2 piston now, I don't really know.

IF I did have a 0 block could I rehone the cylinders to be 2 to clear up the scratching?

It's a specialized process using a special tool and compound for the final chemical finish. It actually changes the molecules on the top surface of the bore. I doubt a regular engine shop would have the correct tools, some older Mercedes used the process as did the Chevy Vega but probably best to contact an engine builder who knows our engines.

I wonder if you can just turn the pistons and coat the skirts to match, that would seem an easier option, but that's just me thinking out loud.

There are a lot of good 944 engine builders on here and RL, try pm'ing them and see what they say, search for Michael Mount, Pete (rsbarn), Chris White or Mike Lindsey.

...

[mueller944s2]

Originally Posted by 9FF

...

where did you quote that from?

[9FF]

Originally Posted by mueller944s2

where did you quote that from?

KS made the block/pistons and I have the KS factory information manual for refinishing the 944 block. I can pdf the relevant pages and email it to you if you pm me your email address. Sorry, it's too big to post up on this thread.