Anybody know if a Yz450 top end will fit a 426 case?

I floated a valve on my sm 426 the other night. So far I have only been able to locate a 450 top end. Anybody know?

The cylinder off a 450 will not work as it is longer than the 426 cylinder. If I remember correctly it is longer by 0.100". The head is also different, I have not compared these, but I know the part #'s are different. And the valve train components are not interchangeable, with the exception of the cams.

Which makes me think that if the mounting surface on the base of the cylinder then I could take an entire cyl/head from a 450 and use it. I wonder if I could use a 450 connecting rod on the 426 crank to make up for the stroke difference. I blew up my 426 and I can find a 450 top end for very cheap. So far I have not had any luck finding a 426 head assy used, although there is one on ebay at the moment.

the 450 cylinder and head work on the 426 bottom end. Trust me im running it right now. But you must run the 450 cylinder to run the 450 head

Yes the cylinder and head will fit as the bolt hole pattern is the same. So technically it will work, you will just loose alot of compression in doing so. That is why I said it will not work. Im always thinking of optimum performance. If you want to run the 450 rod you would have to make sure the throw of both cranks are the same along with the inside diameter of the big end on the rod. If these dimentions are the same then I would recomend using it along with the 450 cylinder, head, and piston. There are many things to consider when making modifications such as these. Unless you enjoy doing this, I would spend the extra money and buy the 426 parts.

Kyled: Did you run into any issues using the 450 top end? Did you use the stock connecting rod and crank? Increase in power from the 426 cylinder? How long have you had yours running and what kind of machine is it in?

The 426 parts are crazy expensive and I could get the 450 head at a fraction of the cost. I will probably sell you the head if you really want it. Cylinder is still in great shape as well. I am curious if I could swap out the crank and connecting rod to the 450 parts avoid the stroke difference.

Nobody enjoys doing motor work on their toys and I love this bike, but it is not worth dropping 2k into to get it running again with stock parts. Its a 426 with a sumo conversion. At this point I would probably sell as is for 2k, entire bike. Plastics are in good condition and the bottom end is strong.

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