HELP identify Pistons

[drumbraker]

Took engine apart and trying to identify the pistons. Deep Pockets, 81mm
numbers PMC0250 and PA0250/5

Anyone know what I have?

[tom1394racing]

Post some photos to help to identify them

[drumbraker]

They are Cosworth, but that is all I know about them. Here are some pictures.

[tom1394racing]

Looks like a replacement 906 piston.

[Walt Fricke]

SCCA had an allowance for a 1mm overbore for the 2.0 engines, apparently because overboring steel cylinders (in any kind of motor) was a common way of repairing a worn motor, and they wanted their racers to be able to take advantage of that. You'd suppose that led racers to overbore perfectly good cylinders.

[stownsen914]

What they said ...

Sounds like a fun engine. What else is in it?

[Flat6pac]

Except for the size they look like the 9.9 69 S Pistons
Not that I have anything written but the 81mm were used in the 2.0 Carreras racing motors such as the 68 911R. @ 10.3 CR
Bruce

[chris_seven]

I think that they are a typical 'Jam Jar' Piston with a skirt side relief around the pin.

They may have the crown and valve pocket cut outs as per a 69S but the basic design is much more like a 67/68S piston.

The 906 and the 69 S used a Slipper Design with a very different skirt configuration.

Much more like these:



If you search Google for 906 pistons you will find that the 'slipper' style is the most common.

They are probably made from 2618 as is the case with most Cosworth pistons.

There is much confusion about oversize pistons.

FIA Appendix K is quite clear that 1 mm overbore is allowed BUT the engine must still remain within the original capacity class.

This would mean that in many European events 81mm wouldn't really be allowed but I do think most people just look the other way.