motobecane help

I got a motobecane from a friend who couldn't figure out what was wrong with it, and i found that the head gasket had a hole in the side of it. I replaced it and the same thing happened after half an hour of riding. I thought it blew because i tightened the nutd too much, but then i replaced it again with the proper torque

and it blew again. why does it keep happening?

Re: motobecane help

Ron Brown /

Sam,

The head and cylinder have to have matching, flat surfaces to allow the gasket to stay in place.

Look at the parts in your hand, you should be able see why the gasket blew in that spot.

Ron

i dont think that's the problem

Ron, I dont think that the problem is an uneven head, because the first gasket blew on the right side, the second time on the left side and the third time back on the right side, and each time the other side looked fine. ????????

Re: i dont think that's the problem

It dont take much to blow out the head gasket. I have a thick piece of glass that I put #400 wet sand paper on and then sand the head and top of cylinder. Use a figure 8 pattern when sanding. This should smooth out the matching surfaces enough to hold the gasket.

Scott

Re: i dont think that's the problem

use a 500 to 1000 grit sand paper on a belt sander in a vise.so it shaves it smooth and evenly across the whole head.. a level and a straight edge helps.

Umm, what's the gasket made out of ?

Reeperette /

Just wondering...

Should be aluminum, right ?

-R

Re: i dont think that's the problem

Ron Brown /

Sam,

The major problem here is that you don't think and we can't possibly know since we do not have access to the parts. : )

Ignore Duck, take Scott's advice and it will be obvious as soon as you begin, if the head or cylinder surface are warped. If they look good, then the only other reasons I can think of are that you are running way too hot, this usually siezes the piston in the bore, or you have bad cylinder studs and you are not really tightening the head. You can check this by running the nuts down the threads with the head removed. If you are running hot, either the timing is off or your mixture is too lean.

Ron

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