1999 wr400f to wr426 rebuild

This is my first four stroke rebuild and could benefit from any advice out there. Crank went out and seized the motor in my '99 wr400. From what i've read the 426 rebuild is the way to go. I have an oem 426 crank and 426 cylinder along with splinned gear for the clutch side of the crank. Looking to see what parts I will need to rebuild my engine to a wr426. Will my wr400 head (still good) work on a 426 cylinder. Also will I have to jet up for the extra displacement of the 426? Getting parts together for my machinist so any advice from someone that has done this will help.

Thanks, that answered a lot of questions. The only points it didn't seem to cover was the parts needed for the splinned side of the wr426 crank, re-jetting (wr400 carb) for 426 cylinder and any possible difference in the hot start (mine seemed to have an aftermarket hot start). Going for stock compression and reliability on the rebuild. My WR400 is CA plated and want to spend more time on the street than the shop. From what i've read on here the 400 crank is a weak point in the engine so the 426 seems to be the way to go and i've scored an oem 426 crank really cheap (new).

Iam not sure any rejetting has to be done, but you will find out when you get it running. I got a stock 450 carb on mine i just adjusted the fuel screw, else plug n' play. I dont think these old yama's got any weak spots really, all have been running 10 years +, remember that.

My 400 (yz) is 13 years old, lets say it has done approx. 35 hours a year, i think that is low set even, that gives 455 hours (90 oil changes, approx. 700 liters of gas burned) i dont think the engine has been opened on mine, none of the screw heads looks like it, even though it has, it hold up damn good i think. And for now it has no signs of dead at all..

Do what you think is best, I would go for stock parts only..

Update on rebuild. Cases are split, stock crank/rod is frozen leaving metal shavings from the bearings in the cases. OEM 426 crank purchased and cylinder is getting prepped for sleving to 426cc cylinder. Top end was in good shape along with cams although hot cams with auto decompression wouldn't be a bad idea at this point. Nothing but time and money at this point to put this dual sport back on the road.

Does anyone know if there is a complete valve set you can buy or do you have to buy each valve individually? Already ordered all the main bearings, gasket and seal set, piston kit and timing chain. It would be nice to put fresh valves in it while the engine is torn down.

Instead of starting a new thread I thought i'd try here first. Looks like my 2000 426 crank is installed and bottom end is together. The problem i'm running into is the head gasket size. Had a LA Sleeve installed to fit the 426 piston and the gasket set I ordered for the wr 400 has a head gasket that is too small. My machinist says I need a 95mm head gasket. Am I wrong in assuming the 95mm diameter head gasket is the 426-450 head gasket? And before anyone brings it up, the machinist gave me a great price on labor if I ordered all the parts and delivered them to him. Seeing as the parts shop deals in part numbers and not head gasket diameter I wanted to see if anyone else had been down this road before. Valve set should be delivered today and would really like to get the correct head gasket ordered to get this rebuild finished and back in the frame and on the road again.

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