Broken compression ring
 

Hi,

I'm racing a 2.0 litre 911 in the SVRA with 81-mm Mahle pistons.

I took the engine out last week for a routine rebuild and noticed that on cylinders #1 and #2 the middle compression ring is broken. All of the other rings on the rest of the pistons look good. Does anyone have any idea what might have caused this? I would also be interested in any techniques people use to install the pistons into the cylinders. I don't think it happened there but I'm curious how other people do it.

On a separate note I've got an old set of 81-mm JE pistons that have never been used. They are probably at least 10-years old. What piston to cylinder clearance is recommended and can I use stock 80-mm Porsche cylinders bored out?

Thanks for all your help.

Rob hurley

2 things come to mind ....rings not gapped enough when installed (too tight in bore)...and ring "flutter" in the groove.
If the ring has too much up and down side play it can get damaged.
Good luck
Bob

Rob,

Detonation is what usually breaks rings, barring installation issues.

Detailed forensics will reveal the cause and you should be looking VERY closely at the piston and spark plug for signs of detonation. Fuel mixture, ignition timing, deck height, CR should all be reviewed as potential causes, especially in 2.0 litre engines.

I would run those old JE's at .025" to .030" and bore your cylinders to suit.

Thanks Steve and Bob. I'll be checking for the things you guys mentioned.

Rob