400ex head bolt problems

[Locknload82]

Ive got a 400ex, last spring had it overhauled, bored .20 over and 11:1 piston installed. ran great for 6 months then all of a sudden had a head bolt that pulled the threads in the cylinder. When I say head bolts I am referring to the studs that hold the head down to the cylinder, NOT the 6 mm valve cover bolts.

We took it apart and ordered heavy duty head studs from GT Thunder. 2 of the stock head bolts were tight in the threads, infact, so tight neither I nor the machine shop could get them out.

So we left the stock 2 bolts in, had the machine shop drill out the other 2 and retap them with roll form tap. when putting things back together they torqued down great. Quad ran great for about 5 hours after that, then one of the orginal stock bolts that we couldnt budge before pulled its threads.

Tore it back down, got the 2 original studs out and was going to put the heavy duty studs in them. Found out they had been helicoiled at some time. Can we still drill them out and roll form tap them and get them to hold?

Wonder if we have something else going on. My son has a 400ex bored out to 426 with 11:1 piston that is doing the exact same thing. we are using the stock all metal head gaskets. We are also using 93 octane gas and adding octaine booster.

Its odd that they are both doing the same thing at the same time. I had just started using the octaine booster last summer, about the time the problems started. Could that be causing the problems? Seems like the bolts torque down good, but then the threads pull after several hours riding (several heating and cooling cycles)

Any Ideas? I am tired of beating my head against a wall.....

[zrpilot]

Originally posted by Locknload82
Ive got a 400ex, last spring had it overhauled, bored .20 over and 11:1 piston installed. ran great for 6 months then all of a sudden had a head bolt that pulled the threads in the cylinder. When I say head bolts I am referring to the studs that hold the head down to the cylinder, NOT the 6 mm valve cover bolts.

We took it apart and ordered heavy duty head studs from GT Thunder. 2 of the stock head bolts were tight in the threads, infact, so tight neither I nor the machine shop could get them out.

So we left the stock 2 bolts in, had the machine shop drill out the other 2 and retap them with roll form tap. when putting things back together they torqued down great. Quad ran great for about 5 hours after that, then one of the orginal stock bolts that we couldnt budge before pulled its threads.

Tore it back down, got the 2 original studs out and was going to put the heavy duty studs in them. Found out they had been helicoiled at some time. Can we still drill them out and roll form tap them and get them to hold?

Wonder if we have something else going on. My son has a 400ex bored out to 426 with 11:1 piston that is doing the exact same thing. we are using the stock all metal head gaskets. We are also using 93 octane gas and adding octaine booster.

Its odd that they are both doing the same thing at the same time. I had just started using the octaine booster last summer, about the time the problems started. Could that be causing the problems? Seems like the bolts torque down good, but then the threads pull after several hours riding (several heating and cooling cycles)

Any Ideas? I am tired of beating my head against a wall.....

Well, wouldn't a .2 overbore be bigger then a 440? I know that to bore to a 440 requires a new sleeve.

As far as the threads pulling unfortunately it may be time to get a new cylinder as I think once they have been heli coiled the HD stud kit from GT can't be used..

[F-16Guy]

Octane booster doesn't work. Your engines are probably getting too hot due to detonation, causing the aluminum of the cylinder to weaken, and allowing the studs to pull out. If you can't find race gas, go to your local municipal airport and get some 100LL AVGAS and mix it 50/50 with 93 octane. If the other studs had been heli-coiled, the holes are probably already too far oversized to use the roll form tap and HD studs. You may have to break down and get another cylinder, or bore the stud holes bigger and use a couple of oversized studs. Either way will probably be a pain.