I love my 351… but, I’ve had problems with leaking head gaskets. As you can see in the pics the 250 original head gasket has about 1/4 of inch to seal. However on my 351 there is only 50 thousands of gasket material the coolant has to make it by to get into the cylinder. Which mine was doing. I recently made a solid copper head gasket. No leaks yet. Most 351’s won’t have this problem but there seems to be a small amount of us that do. I would love to see comparisons of the 331 vs the original 250. I’ll post back after I’ve put a few hundred miles on her.

 

You probably have a cylinder head from an early model klx250 that has the larger coolant passages into the head. It's been documented on YouTube from other klx351 builds. It only affects certain model years as Kawasaki never intended to have such a large cylinder, but I know that my 06 has never had an issue. I put my 351 kit in 12 years ago and over 10,000 miles an no coolant loss issue.

 

Thanks 94. That looks exactly like what I’m facing.

 

It’s taken a while to get her going again. I’ve rebuilt engines my whole life (63 yrs). Diesel’s. Gasoline, airplanes. You name it but this just about stumped me. I checked the head with a machinist straight edge and couldn’t find any warping or cracks. Upon closer examination My head is not like the one in the video. The water ports are indeed close but enough to seal. It wasn’t until I started milling the head that the warp showed up. For future big bore installations I would recommend a head resurface every time. Here are some pics of my home made resurface machine🤣

You’re allowed .002 thousandths warpage on the head to cylinder mating surface. I had four thousandths. Check inside the cylinder all the way around and try to get it perfect.

 

How did you resurface the cylinder head? Did you get it perfectly flat?