Daniel5691

Hello!

I've been reading through the WSM and studied the brief section about piston tolerance groups.

I understand that these were standardized pistons of uniformly differing diameters.

I also read in the WSM that different PTGs can be found in the same engine.

So my questions,
Were the PTGs originally a normal part of engine assembly for the 928? For instance, would differences be found in cylinder sizes prior to final assembly at the factory, thus requiring a different PTG installation?

Were different PTGs only meant as an anticipation for possible need to slightly repair/resize the bore due to some damage or imperfection?

I know that this was a minor issue on the 5.0L block which I just received back from the machinists, but I'd really like more details on the full story of tolerance groups if possible.
Just trying to learn. Thanks.
Dan

GlenL

Nothing is perfect except Porsche engineers.

The tolerance groups a due to manufacturing variances in excess of the goals of the engineers. What they did was make all the pistons to be perfect and then sorted them by actual sizes.

The engines are, similarly, bored as accurately as possible. Then they measured and got a piston (0,1,2) that'll have the right fitment. They can also bore to fit the pistons they have. In the last step of fine honing, the bore can be adjusted in very small (0.0001") increments.

The etching compound doesn't move the silicon back and doesn't make the bores bigger. There's no reason to use it if the cylinders aren't freshly bored.

Oversized pistons come a full 0.5mm and 1.0mm bigger than stock. That allows taking out deep scratches and scrapes.

Red Flash

To be fair, pairing of mechanical parts is not new in any area of precision manufacturing/assembly. Methinks it had nothing to do with the perfect Porsche engineers!

extanker

in the 80,s GM called it selective fit .....main bearings on some 4 bangers were A,B,C half a thousandth each. in the 40,s WW2 British called it file to fit or machine to finish

GregBBRD

The difference between tolerance groups is .0004". Less than half of one thousanth. That means if you measure the piston on a 90 degree day and again on a 70 degree day, it is in a different "group"..

Me thinks not many manufacturers split hairs this thin....and if they do, they probably speak German.

Keep in mind that if Tiger Tanks didn't take so long to build, because of German perfectionism, we would be having this exchange in German....

FredR

If you want to see a Royal Tiger [Konigstiger] from one of the Panzer Divisions take a visit to the Tank Regiment museum at Bovington Camp on Salisbury Plain, Wiltshire. Compared to what our boys had those things look like they came from the future. Shells from our tanks bounced off the Royal Tiger's frontal armour and apparently the only way they could be taken out was with a "shell up the ***" so to speak which is a bit difficult when you are running away from them!

Fortunately they were non too reliable, swallowed huge amounts of fuel and only 500 of them were built- their engineering capability was no fluke- you only have to look at the detailing of their kit to understand this.

Rgds

Fred