420 kit?

[Shawbridge Husky]

Am going to have my sleeve replated as I have found some vertical scoring caused by either a cold seizure or debris in base. The piston skirt also has scoring so it will be replaced as well. This would obviously be an opportune time for an oversize kit such as the 420. Does anyone out there run one? ( my bike is a '99 400 )I was also wondering how it would affect reving ability and what jetting changes would be needed. I read here that the 400 revs easier than the 426. Is this so?

[motoman393]

Many people run the 420 and they have been proven reliable on the 99' YZ400 but on the 98' YZ400 they are like a time bomb...

This is due to the the piston to waterjacket clearances the 98's waterjacket clearance is too close and frequently the waterjacket will crack which leads to a seized engine (and major bucks to repair) but the 99 cylinders were much thicker so the 420 is a great idea.

Doug Henry was secretly using a 99 YZ400 w/the 426cc displacement in the 98 nationals season this is why the 99 cylinder is different (and can be bored out more reliably) I am not sure of the jetting specs but contact thumper racing and they can give you a good deal on the boring and will give you jetting specs (you should have to go up 2 sizes in the pilot and main similar to the 426's jetting) Hope this helps!

Garrett

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I get my kicks on a 2001' YZ426!

Friendswood, TX

[Shawbridge Husky]

Thanks for the informed tip Montoman! I will feel more confident going this route.

[Rich_Rohrich]

Originally posted by motoman393:

but on the 98' YZ400 they are like a time bomb...

Anyone who has a 98 barrel shouldn't jump to any conclusions. While the 99 barrel is a better candidate for a 94 or 95mm bore I've seen a host of 94mm bores run HARD on 98 cylinders without ANY problems. I have one sitting here in front of me that that ran ~60 hard hours at 13.8:1 compression, and all it needs is a set of rings and a quick pass with a ball hone. The barrel is still round and there is no increase in taper. Motoman393 where do you get your data on 98 model failures from?

BTW, Yamaha ran bores much larger than 95mm (426 CCs) ?

[This message has been edited by Rich Rohrich (edited 01-20-2001).]

[motoman393]

My friend (BBR_Guy) had a 98' YZ420...it was professionally bored and ported and polished by RAT racing (12.5:1 compreesion piston installed) It lasted about 1yr and then suddenly one day radiator fluid was coming out everywhere...and would not fire and was hard to kick through the stroke!! Well he took it to the shop (K's motorsports) and we found out he needed all new valves, piston, cylinder, cams, cam sprockets and chain!! Well the total damage came out to around $2100 ! This is where I got my facts from Rich also look in the March 2000 issue of MXA magazine ( on pages 32-36 ) they do a test on turning your 98/99' YZ 400 into a 420/426 ! I believe the 420 kit show great HP improvement but I also believe in reliability...dont you? I mean come on it sucks when you have to spend 2k on a bike that has had a 420 for a year! But on the other hand I have heard of guys w/ a 99' YZ400 who have had the 420 kit for almost 3 yrs and are loving it! I Hope this helps!

Garrett

Here is a pic of the 98' YZ 420 That I was talking about!

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I get my kicks on a 2001' YZ426!

Friendswood, TX

[This message has been edited by motoman393 (edited 01-20-2001).]

[motoman393]

Oh yeah rich ,

I know that Yamaha ran higher than 95mm actually they went as high as 521cc (but decided it was to much power and settled the best displacement is around 450-460cc for Doug Henry, Button likes around 440) I cant even imagine how much power a YZ 521F would have...I think the 426 is fast enough!

Garrett

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I get my kicks on a 2001' YZ426!

Friendswood, TX

[Boit]

Motoman: Gary Anderson is a friend of yours? I saw him last October at Lake Witney. I really enjoy the 4-stroke racing with the great first moto between Holland, Decker, and Johnson.

[motoman393]

Yeah I know Gary Anderson... I bought my 01' YZ 426 from him (that he was supposed to race)...and my friend bought that 98' YZ400 (pictured above)from him, he is now sponsored by KTM and rides the 520 ! We ride with him almost every weekend since we live fairly close to each other (he is the best 60yr old mxer I have seen)!

Garrett

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I get my kicks on a 2001' YZ426!

Friendswood, TX

[Shawbridge Husky]

Many thanks Guys! ? Being Canadian I will go the conservative route ( great Canadian compromise ) and will opt for the extra 2.0 mm dia bore and not the full 420. Wiseco calls it a P8 which is .080" oversize. Will keep the P12 option for a rainy day!

[Rich_Rohrich]

Hugh - .080" overbore IS 94mm i.e 417cc commonly called a 420 ?

It's still a great choice, if it's done correctly. The 4650-P8 kit is the 94mm (417cc) piston kit, and the 4694-P8 is the 97mm (444cc) piston kit.

[This message has been edited by Rich Rohrich (edited 01-23-2001).]

[Rich_Rohrich]

Originally posted by motoman393:

I believe the 420 kit show great HP improvement but I also believe in reliability...dont you? I mean come on it sucks when you have to spend 2k on a bike that has had a 420 for a year

I'm sorry to hear your friend had bad luck. Nothing in my experience would lead me to believe that a properly done 420 is any less reliable than the oem setup. IMHO MXA's opinion isn't worth the paper it's printed on.

[Shawbridge Husky]

Great info Rich. I appreciate it!