Yz250 cylinder head

I have seen this when only one of the head O-rings fails and then the coolant becomes pressurized by the cylinder combustion pressures and causes the head to crack. This issue has been around for some time.
What does the other side of the head and o rings look like?

Paw Paw

You should replace that head with a VHM race head. Not only will you get a performance gain, but the quality is second to none and you wont have the same issue. It has better cooling jackets with removable tuning domes for fine tuning. check them out at www.langstonmotorsports.com if your interested in one you can email me at brad@langstonmotorsports.com

This happened to us and I talked to yamaha and they said it only happens when the upper head mount is not tourqed to specs, always tourqed after that and never had another problem.

I've never had that problem in several years with several YZs.

The engine mount torque is probably a good suggestion on where to start.

Sometimes, those o-rings want to jump out of the groove when you're trying to assemble the engine. It might start and run fine for a while, but then start leaking coolant, or something like this could happen after a few minutes or hours when it gets hot and you're really on the gas.

A thin smear of silicon grease (grease, not sealant) on the o-rings seems to help them settle into place and helps prevent pinching them during assembly.

Is your rod bent?