blown motor

[3RD STRIKE]

Hey guys &gals. I just built a motor about 600 miles ago,The motor ran great. But i used this motor in my race car so i am sure that had a lot to do w/it. The motor is an l28e. It will turn 8000 rpm,s very quickly, I know it broke the piston today. BUT WHAT I DONT KNOW IS HOW. Acouple of weeks ago the #3 piston hit the spark plug and closed the gap. I put a new one in it and there was a little bit of a rattle an then it cleared up with no problems. (I did check the cyl. for trash and there was none) well today it started again but this time it was #6. I changed the plug and ran it up the road and it ran fine. but when i got off of it, sure enough it done it againbut this time it moved back to #3.There was no rattles or knocks it just started missing. so i took all of the plugs out because they are the long shank plugs. and i put some short shank plugs in to give them some more clearance. Then i took it up the road, and it ran fine. So i stopped, pulled over and turned around. When i started moving again , it started to miss and then she started smoking real bad, so i am assuming that the piston broke. What i want to know is how come it kept swapping cyl., and how come it did not hit he plugs every single time i cranked the engine if it was a clearance issue? Any help would be greatly appreaciated. I just dont want to make the same mistake again. THANKS Gene.

[JimmyZ]

There is at least one engine builder on the site who could easily answer your question. Haven't seen him post in a while. I'll give your post a bump.

I've only rebuilt a few engines and have always built them for street use. Never got to try anything wild.

My first guess is that you had plug interference on more than #3 cyl. Dome cracked and in time failed. You might find that more than one piston got "stamped". Try using some clay and reassemble head/block to verify this. Stressed parts don't do well when notched.

How much clearance did you have with the deck? I imagine the head was milled to increase CR?? Forged pistons or cast? Forged might tilt just enough when running cold to take one side up a few thous and if the clearances were too close...

Just pulling at straws.

Bump

Jim

[Exdamyankee]

I'm no expert on motor builds, but I suspect a number of factors could be in play. If your engine's clearances were marginal to begin with, thermal expansion & rod stretch at 8000 rpm could create a contact issue where there may be none at idle. Crank deflection at high RPM might be a possibility too. We're only talking a couple thousandths, but like I said, if it was borderline to begin with it may be the case.

You pull the head yet?