umu24013

400 to 426 conversion, the easy way

50 posts in this topic

IDK, if this has been covered on this forum. But for those who may not know, there is an easier (and probably cheaper) way to convert a YZ400F (and WR400F) to 426cc.

The magic is in the piston. You can use the piston from the 450 (actually, ’04-’05 YZ450F & ’04-‘06 WR450F piston marked 5TJ) with the 400 connecting rod (5BE). They both have the same pin diameter (18mm). Since the 426 piston pin bore is 19mm, a 426 con rod is needed in order to use the 426 piston and jug with a 400 bottom end.

Parts required for the conversion:

1x 5TJ-11631-00-00 PISTON (STD)

1x 5JG-11603-00-00 PISTON RING SET (STD)

1x 5TA-11633-00-00 PIN, PISTON

2x 93420-18082-00 CIRCLIP

1x 5JG-11181-00-00 GASKET, CYLINDER HEAD 1

1x 5BE-11351-00-00 GASKET, CYLINDER

1x 5JG-11311-00-00 CYLINDER 1

BTW, I've had this setup for a few years with no problems. Today it’s time for a new 5TJ piston and rings.

rod5be.jpg

piston5jg.jpg

piston5jgtop.jpg

Edited by umu24013
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I bet it's a blast with that short stroke setup

I had a 400, with a 426 cylinder bored 2mm over 444cc total, wiesco had a 13:1 piston that fit, I believe it was a special run, I'm not sure if that piston is still available, ran fine on pump gas.

the bike was stupid fast, the short stroke made it rev insanely fast, it was as much fun as a 500 two stroke

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thats nice to know about being able to run a 426 cylinder&different pistion without having to replace the 400 rod

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No one mentioned it, but the 450 piston was made for a completely different engine with a longer stroke. The swap obviously worked, but one thing to look at is the "deck clearance", the distance between the top of the piston and top of the the bore. Too much clearance costs compression, too little costs money.

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No one mentioned it, but the 450 piston was made for a completely different engine with a longer stroke. The swap obviously worked, but one thing to look at is the "deck clearance", the distance between the top of the piston and top of the the bore. Too much clearance costs compression, too little costs money.

Thank you for pointing that out, Gray. I thought about that obvious thing many times before.

I don't have a gauge to measure the clearance, but with the head off, I can tell that the flat surface of the piston top is perfectly flush with the cyl deck at TDC. And the height of the deposit ring at the top of the cylinder wall equals the height of the piston top land.

caliperpiston.jpg

carbonring.jpg

Any suggestions? What else to check/measure? :smirk:

P.S. I've got more than 450 hours on this setup with no problems.

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I did the same thing back in 2001..thumper racing made a 426 piston that fit the 400 rod...i jettedd t their specs, and it ran perfect

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I did the same thing back in 2001..thumper racing made a 426 piston that fit the 400 rod...i jettedd t their specs, and it ran perfect

It was part# 4649M09500 Edited by umu24013

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what is the advantage to using the 450 piston and cylinder on the wr400? will it give me much of a gain? i am buying a 2000 wr400 that has about 10 hours on it and the first thing i am going to do is change the cams to get rid of the manual decompressor hot cams are about 260 for both cams

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Read more carefully the entire post. You cannot use the 450 cylinder. The 450 piston is used with the 426 cylinder, which has the same bore size, and the 400 head. A 426 head will interchange, but there is no significant advantage to doing so that I'm aware of.

You also don't really need both cams. The Hot Cams stage I intake is virtually identical to the YZ400/426 intake, which interestingly is the same cam the WR400/426 uses. The only difference is the exhaust cam.

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so by using the 426 cylinder the 450 piston there is no gain in power? i am looking to get more power from my 2000 wr400 i have a fmf exhaust and i know i am putting a hot cam exhaust cam so i have the auto decompressor on my bike i also have a extra 40mm fcr from my honda crf450r well i have 2 extra carbs and i am going to try one on the 400 i just want to wake the bike up i am going to do the air box mod and the gray wire mod i know it wont run with my honda and i am not trying for that but i want to get it around 50 hp also i am going to put a 13.5;1 piston in the bike

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so by using the 426 cylinder the 450 piston there is no gain in power?

Who said that? Using the 426 jug and 450 piston makes it a 426. Of course there's a gain in power.

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wouldnt a 426 compression chambered head desighn match the 426 cylinder better than a 400 head?and are the 426 cams a little lumpyer than the 400 ones?

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wouldnt a 426 compression chambered head desighn match the 426 cylinder better than a 400 head?and are the 426 cams a little lumpyer than the 400 ones?

To the first question, yes, a little, but there's no significant gain associated with using one or the other.

To the second, the exhaust cams are the same. the intake for the 426 has a different part number, but it's nearly identical. You were going to use Hot Cams anyway, so what's the difference?

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greyracer i can see that you're very knowlegable on these bikes indi really do appreciate all of your input to all the ideas that i have. i bought the 99/yz400 rode it a bit and had 3rd gear go, i got another tranny but thought, while its apart, why not stuff in a wr 5th and what about the older rod and pistion rebuild? and why not change up to a 426rod cyl and piston its an older bike the piston seems whorn and the 400 cylinder is in nice shape and i do have a redone 426 head from the old days pile. i hear these older bigend rod bearings and crank bearings are a weak point so even so a 426 the easy way is just a 450 pistion on a worn 400 bottom end. so guys i,d like to hear some opinions on this screwed up thought process of mine. lol and bye the way i,m kinda cheap and i hate it when my toys blow up. lol.and no i wasnt talking about hot cams i think that was brucegregory

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i read on another forum that you can use the 426 crank aswell? is the stroke longer? is there a difference? before this thread i was going to do 426cylinder, 400 connecting rod, 426 piston and rings... would the 450 piston benefit me? and is the 426 crank doable? and would that also benefit me?

i have the bike in a million pieces right now just looking for a little guidance to get some power out of this already impressive motor.

side not: this is a supermotard and gets used almost daily so i am looking for reliabuility, i am not getting rid of the manual decompression because i love it, i have never had to adjust the valves since 1998...

thanks for anyones help!

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i read on another forum that you can use the 426 crank ?

Yes, you can.
is the stroke longer?
no.
before this thread i was going to do 426cylinder, 400 connecting rod, 426 piston and rings...
Can't do that. The 426 piston uses a larger wrist pin, so you'd need the 426 rod
would the 450 piston benefit me?
That would allow you to use the 400 crank.

All this was already answered.

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I'm sorry, it seemed kinda scattered when i first read it. i will be ordering a new 400crank and will be getting a 450piston and rings. do i go factory compression? or can this crank handle a little more compression? i read the entire thread but do not recall if the 450 piston bumps the compression in the first place.

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