400 to 426 conversion, the easy way



48 replies to this topic
  • umu24013

Posted January 16, 2012 - 01:17 PM

#1

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.

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Edited by umu24013, January 16, 2012 - 01:19 PM.


  • MrBlahh

Posted January 16, 2012 - 01:25 PM

#2

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

  • crf-500

Posted January 16, 2012 - 03:10 PM

#3

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

  • RasmusDK

Posted January 16, 2012 - 03:38 PM

#4

Good one! :smirk:

  • grayracer513

Posted January 16, 2012 - 04:34 PM

#5

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.

  • umu24013

Posted January 17, 2012 - 01:20 AM

#6

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.

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

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

  • yzf1999

Posted January 17, 2012 - 03:40 PM

#7

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

  • umu24013

Posted January 17, 2012 - 07:29 PM

#8

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[color=#282828][font=helvetica, arial, sans-serif]# 4649M09500[/font][/color]

Edited by umu24013, January 17, 2012 - 07:29 PM.


  • brucegregory

Posted January 19, 2012 - 05:43 PM

#9

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

  • grayracer513

Posted January 19, 2012 - 07:34 PM

#10

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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  • brucegregory

Posted January 20, 2012 - 06:40 PM

#11

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

  • grayracer513

Posted January 20, 2012 - 07:59 PM

#12

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.

  • crf-500

Posted January 21, 2012 - 03:04 AM

#13

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?

  • crf-500

Posted January 21, 2012 - 03:08 AM

#14

i meen to say bore dictates eficient chamber size or diameter

  • grayracer513

Posted January 21, 2012 - 09:15 AM

#15

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?

  • crf-500

Posted January 22, 2012 - 12:35 AM

#16

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

  • toyotanick

Posted January 26, 2012 - 11:11 AM

#17

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!

  • grayracer513

Posted January 26, 2012 - 02:51 PM

#18

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.

  • toyotanick

Posted January 26, 2012 - 05:17 PM

#19

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.

  • grayracer513

Posted January 26, 2012 - 06:21 PM

#20

I would stay with standard compression.





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