Vilhuer

There might be something buried in archives but I couldn't find anything useful with quick search. I'm very quickly puting together GTS engine for automatic. Pistons going in it are from three different GTS engine. None of 24 GTS pistons I have have visible arrow which points to front of the engine so that pistons are installed in correct way. Only one option is correct due to 2mm wrist pin offset. Anyone know for sure if wide and narrow side is pointing left or right when looking from front of the engine for example. I quess I could try to dig up some S4 pistons and see if they still have arrows visible or use 968 piston valve cut size to point out correct orientation but its easier just to ask here as engine build has really tight timetable and every second counts in garage.

blown 87

I did not think my S4 had marks on the pistons, but after a quick trip in the carb cleaner there they were.

Vilhuer

Cleaner doesn't help. Most of them do not even have tolerance group number in them at top center even thought they all are group 1.

mark kibort

arrows point to the front as the valve reliefs are the same size and depth . do you mean you cant see the arrow?
just read it again.
I think the narrow side is on the bottom (exhaust side) of the piston for a more optimal angle of rod to crank on the down stroke. So, narrow side is on the right as you look at the engine from the front on the driver side and on the RIGHT also for the passenger side.

I would wait for a second motion on that, as it has been a while since i went through it.
Greg brown made sure I had it right on the 85 5 liter for scots engine.

Vilhuer

Can't see arrow anywhere in any of them. As all pistons are same it means narrow side must be on exhaust side on one side of the block and on intake side on another. Question is on which side (intake or exhaust) it is at 1-4 cylinders for example. Answer to that will tell its on other side at 5-8 side of the block. Quess I'll have to check from 968 pistons. They have different size valve pockets and thus will tell correct orientation without visible arrow. Wish I knew where I put those.

mark kibort

narrow side on exhaust on driver side and intake side on passenger side. It provides a more optimal rod piston angle on the power stroke.

toofast928

The exhaust valve pockets are little smaller than the intakes. Look for that.

mark kibort

Not on the S4, so it can take advantage of the proper offset. if there is a difference with intake and exhaust valve pockets, it means that one side of the engine will not be optimal. (porsche design for convenience issues). (unless they made a left side and right side piston part number )

This is why we took the S3 pistons and carved out valve reliefs on the opposite side of the piston for one side of the engine. now, we truely have a left and right side pistons.

I suppose the 968 pistons have an issue like this , if the valve reliefs are different exhuaust to intake.

mk

Vilhuer

On 968 pistons narrow side is on exhaust side which is same as narrow side on exhaust side in passenger side and narrow side on intake side on drivers side. This means if they are used as guide it would be totally opposite to what Mark says if Mark meant looking from the front of the engine. Marks way would make more sense though. I quess I'll have to dig up some clean S4 pistons just to make sure. Pistons are ready to go in.

Also GTS pistons have equal size valve cuts. Now all four are 39mm as heads have 968 intake valves.

toofast928

This topic Mark I disagree. Follow the manual. Arrows point to the front. Small valve pockets point towards the exhaust manifolds.
Erkka, When I assembled my 5.0L I had the pistons ceramic coated. The coating covered the arrows. Luckily I had 9th junk piston to follow. Small exhaust pocket toward the exhaust manifold.
Tony

Vilhuer

Tony,

On S4 etc. '87+ pistons have same four size valve pockets. As there is only one piston style all four pockets must be same or offset would be wrong way round on one side of the engine.

Just checked one S4 piston which arrow is visible. Based on it this is correct setup. Looking from the front of the engine.

GregBBRD

This discussion just causes too much brain damage. The whole offset piston thing is just silly. Porsche screwed this up for so many years and on so many cars...it's just stupid.

How you install GTS pistons depends on your goals:

Install the GTS pistons so that you have the "best" rod angle to the crankshaft, when the combustion process occurs...this makes for the best power and the least amount of wear.

If your desires are to have the engine be as quiet as possible, install them the other way....like Porsche did.

By the time they got to the GT engines, they had figured this out and made all the reliefs the same...so that they could at least install the pistons with the offsets going in the same direction...even if it was wrong. All the early engines are screwed up. 1/2 the pistons sit one way and 1/2 sit the other way...sometimes it amazes me that those engineers could walk upright.

GregBBRD

....And this gives you the worst possible rod angle to the crankshaft, when the power stroke occurs...but makes them quieter!

ptuomov

Which way would you / do you install them?

I agree with you that one half one way and the other half the other way is a curious solution.

6.0-928S

Porsche wasn't the only one that believes in this crap. The Chrysler Factory Race Hemi manual says to reverse the pistons to pick up some free ft.lbs. of torque. Put the narrow side outboard on the driver's side & inboard on the passenger side. I've done that on every engine I ever built & I was never able to hear any piston slap.

Hammer