can the swap work

i have the frame of a yz400f that is in excellent cond. but the engine is blown. My buddy has a 426 that i can have. Has anyone ever done this with a 400 and a 426 motor before. and if u have what where the problems and what if anything had to be changed to get the bike to run right and be operational.

Why are you looking to swap a 426 into a 400 frame? Isn't the 426 complete?

no its not. he has been racing and stuff since he was little and i dont know what he did with the frame and stuff but he just has the engine and some plastic for the bike and thats it. and im going to buy the motor off him and put it in my bike if it will work. i just want to know if anyone has done this and if so i want to know like what all had to be done to it to get it to work well and stuff

426 should bolt right into the 400 frame with little or no problem.

yeah i thought that it would but um do u know what would have to be done to make the engine run right

like im pretty sure that i would have to rejet the carb but im not sure

just trying to find out what i have to do to make it work i just want my bike back running again

I don't see why you would need to rejet the carb just because you put it in a different frame. Do you ride at high elevations a lot?

I have a customer wondering if a wr426 motor can be put in his 98 yz400 frame. I have no clue what to tell him. His 400 broke the wrist pin and pretty much wasted everything so a used 426 that's available would be cost effective if it can be done. Any help out there?

You might need to rejet the carb in the 400 bike if that's the one your using for the 426. AFAIK the CDI box is the same so it's basicly a bolt it in and tune it process.

400 and 426 had slightly different carbs...the 400 had an external hot start circuit commonly referred to as the "octopus" (a bunch of rubber hoses, tee fittings, and a frame mounted hot start plunger). The 426 had the hot start circuit contained within the carb. If you get the 426 carb, you will be "in fat city". If you have to use the 400 carb, you will have to drill a hole in the head on the intake side (there's a boss cast intot he head for it) tap it and install a vacuum nipple to connect the octopus to (I can post a pic of the port plugged with a bolt if you need to know where it is). Other than that, it would be a straight drop in. I would get the 426 header, too, if you can. The 400 header will work, but blocks access to the oil filter cover for oil changes (you have to remove the header to change the filter). On the 426 the bend was changed slightly to clear. 426's have a bigger small end connecting rod bearing and some clutch hub changes to make things a little more durable as well.

I just assumed that he would have the electronics and carb to go with the 426 engine. If that is the case then he shouldn't have to do any jetting changes unless he rides at high altitudes.

so all the things needed to drill the hole and stuff where can u get them at and is that all that i would need to make the 426 eingine work with the 400 carb. o and i guess if u want to u can send me the pics and stuff that u where talking about- my email is


The red circle shows where the boss is for the vacuum port. You need to measure the Inside Diameter of the hose that goes to the octopus, and go to a plumbing shop and get a brass nipple that size. You want iron pipe threads on one end, or a press in nipple. If you get an IP threaded nipple, tell the guy at the plumbing shop that you need a drill and tap to tap the hole it goes in to. He should be able to hook you up.

If you are lucky enough to find a press in nipple, measure the shank diameter then go to a machine tool store, look in a "Machinery's handbook" to find out what size hole you need for a "Class IV or Class V" press fit, and get a reamer that size. Also get a drill bit that is appropriate for the reamer. The sales guy at the machine tool store should be able to help you out with that.

If it were me, I would pull the head off the motor and pull the intake valves to be sure I didn't get crap down the ports and into the barrel, but everyone's not as anal as I am. You could make sure the intake valves are closed and stuff the ports full of rags, or wipe a bunch of grease on the port wall to catch the chips...

Anyway, drill the hole and if you have an IP nipple, tap the hole. Clean all the chips out of the port (use a shop vac, compressed air, solvent wash, can't be too careful here) Put some teflon tape or teflon putty on the threads and thread it into the hole. You're done.

If you have a press in nipple, drill the hole in the head and ream it to size. Clean out all the chips as above. Get some dry ice or freon and freeze the nipple to shrink it (be sure to use pliers or heavy gloves...stuff that cold can burn the crap out of your skin!!!) and stuff it in the hole. If it's cold enough, it should slip right in, then, as it warms up it will expand to seal. I would put a fillet of permatex No 2 or other thick gasket goop around the base of the nipple for good measure as well.

Kinda sounds like a lot of will be 5 bucks or so, the drills and tap or reamer will probably be another 20-25, and the hassle of trying to find it may be worth a lot more if you are not familiar with that kind of work...of course, your local machine shop will probably do it for 20 if you have the head off the bike.

Or, you could just pick up a 426 carb on e-bay...

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