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TallBoysWRF

400 head on a 426 cylinder

5 posts in this topic

I know you can bolt the 426 cyinder onto the 400 cases but can you still use the 400 head on the 426 cylinder or do you need to get the 426 head too? Thinking about bumping my 400 up to the 426 and this though crossed my mind. Also, aren't the wrist pins diff sizes between the bikes too? Meaning I'd have to change out the piston rod?

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im not sure if 400 to 426 is bore or stroke enlargement...if stroke, i dont see any problems in using 400 head

there could be other finesses like diffrent valve adjustment specs

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I know you can bolt the 426 cyinder onto the 400 cases but can you still use the 400 head on the 426 cylinder or do you need to get the 426 head too? Thinking about bumping my 400 up to the 426 and this though crossed my mind. Also, aren't the wrist pins diff sizes between the bikes too? Meaning I'd have to change out the piston rod?

On one of my WR's I have a 95 mm cylinder (426 cc) bolted on with the stock 400 head. On the other I have a 97 mm cylinder (444 cc) bolted on with the stock 400 head. So, I believe the answer to your question is yes it will work. I think you will need the right size head gasket though - not too sure as everything came as a kit and I was not paying attention to it.

The stock con-rod small-end is 18 mm on the 400s. Thumper Racing sells a "special" 95 mm (426 cc) Wiseco piston with an 18 mm wrist pin. I don't believe you can buy this piston from anybody but ThumperRacing. The readily availble commercial pistons with an 18 mm wrist pin stop at 94 mm (420 cc). The readily available 19 mm wrist pin pistons range from 95 mm to 97 mm. As an FYI, I rebuilt both connecting rods (a good idea) with the 19 mm rods before doing the bump-up in ccs.

As for the other question, I have a YZ450 exhaust cam in one bike and a WR450 exhaust cam in the other bike. No issues with fitment.

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Any idea how well a pro-x connecting rod would hold up to the 95mm piston? I rebuilt the rod last winter so it has a new pro-x unit in it. I've heard the stockers don't hold up too well, but haven't researched the matter much.

I have looked into the 450 can change though. The cam will work, the only thing is that the tooth profile on the gear is different on the 450 from the 400. I can't remember which is slightly bigger, the chain or gear, but you will have to keep a closer eye on the chain as you will wear it out faster with the 450 cam. How much faster is another issue. I've heard of people that already notice a difference and some that don't notice any yet. My guess is because the cams have not had enough hours in the older bikes to really tell how much faster they are wearing. Other than that I've heard the switch works awesome.

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