JE Pistons

fmangiero · 03-14-2003, 10:47 AM

I am rebuilding my 82 911SC Euro engine. I had a ring failure on one of my pistons and have purchased JE replacements. I showed an automotive machine shop guy and he was saying I should take and lightly emory cloth the sharp edge of the valve clearance holes on the dome of the piston because any sharp edge can be a potential detonation point inside the combustion chamber. Is this hogwash? Could you elaborate?

ChrisBennet · ChrisBennet's Garage

It makes sense to me. Hot sharp edges can be the initiators for precombustion. I do it to mine JE's but I can't point to anyplace where it is written that you should do this. Maybe it will be in Waynes book when it comes out?
-Chris

snowman · 03-14-2003, 01:30 PM

Its true, remove all sharp edges. The reason is that a very sharp edge will get very hot very quickly, and cool slowly. The reason it heats quickly is the heat is all around it when combustion occours. The reason it cools slowly is that it presents a very long and narrow thermal resistance path, ie high resistance, to the back of the piston and rings, where the heat is removed, consequently it takes longer for the heat to be removed. The sharp edge can get so hot that it will ignite the gas instead of the spark plug. The ignition will be way before it should be, ie like too much timing advance and you get knock or preignition.

How much to round it? Well feel it with your finger, if it feels sharp, it is. Round until you no longer get this feeling when you touch it. Don't get carried away and do all them the same way. Do not do anything to the ring lands. Why dosen't the factory do this? Keep track of how long it takes you to sand off the sharp edges. Multiply that by $100 per hour, multiply by 5 for retail price increase. Thats why.

TimT · 03-14-2003, 04:58 PM

This an old engine builders trick, like Jack has said, and others have surmised, the sharp edges will get glowing red hot. This doesnt help things one bit.

I have built 911 engine with JE, Arias, Venolia, and Mahle pistons. You can just assemble your engine with the pistons as recived from the manufacturer. Or you can spend a little time and "prep" them.

After the piston is forged and shaped, an end mill is used to create the valve reliefs. The manufacturer will scrape off the edge of the relief with a deburring tool. Thats not enough. Like Jack said.... its a "feel" if you drag your fingernail over it and it scrapes, you need to get at it again with emory or crocus cloth.

fmangiero · 03-14-2003, 05:15 PM

Thanks a lot guys! I thought that the pistons would need no "finesse" work on them....they look so good from the factory. If I had the $$ I would buy an extra one for a decoration/conversation piece or use it as a very expensive ashtray. I will round out the sharp edge slightly. Around the radius of the milling is sharp enough to scrape off a little fingernail.