warped head?

[muddzilla94]

ok guys i was working on a buddy's 400ex today put a brand new stock piston in it new gaskets, new cam all the stock goodies. well when i went to start it up the d@#$ thing blew the gasket right under the exhaust, this is the 3rd time its did this any help is appreciated. this thing has just been a pure pain for me. it even squirted me in the eye with gas-my fault-accidently hit the throttle when turning the bars and had the carb laying on the nerfs and it shot me right in the eye.

[F-16Guy]

Click on the "SEARCH" button and enter "studs". The 400ex is notorious for having the head studs (especially the forward one near the exhaust) pull out of the soft aluminum cylinder. The most popular remedy is to send the cylinder to GT Thunder and have them install their heavy duty head studs (about $115 for parts and labor, plus shipping). They're longer, larger in diameter, and have a more course thread pitch. The holes in the cylinder are drilled out and tapped with a roll form tap. A roll form tap compresses the aluminum to "form" threads, instead of cutting material away to cut threads. That process makes the threads and surrounding metal more dense and stronger. Between the larger studs and the roll formed threads, the GT Thunder HD studs are pretty much bullet proof. When you re-assemble it, use an OEM XR400R head gasket or a Cometic MLS head gasket; don't use the ones that are made of fiber.

[tri5ron]

Originally posted by F-16Guy
Click on the "SEARCH" button and enter "studs". The 400ex is notorious for having the head studs (especially the forward one near the exhaust) pull out of the soft aluminum cylinder. The most popular remedy is to send the cylinder to GT Thunder and have them install their heavy duty head studs (about $115 for parts and labor, plus shipping). They're longer, larger in diameter, and have a more course thread pitch. The holes in the cylinder are drilled out and tapped with a roll form tap. A roll form tap compresses the aluminum to "form" threads, instead of cutting material away to cut threads. That process makes the threads and surrounding metal more dense and stronger. Between the larger studs and the roll formed threads, the GT Thunder HD studs are pretty much bullet proof. When you re-assemble it, use an OEM XR400R head gasket or a Cometic MLS head gasket; don't use the ones that are made of fiber. X2,
if you'd like to learn more about how this strengthens the parent material, just goolge "roll forming", "hoop tension", or "coining holes".

[sab24711]

did you retorque the head studs? i just had an oil leak near the head but no blown gasket. id try that before reaching out to the bank

[Bill Martin]

I here a lot about the studs and my builder told me that the biggest mistake everyone makes is to torque the head bolts to the factory specs. I believe he seed to only torque them to 5lbs short of the specs.
That way they don't tend to pull out.
It may or may not be true but every motor he dose holds up.
Including mine.