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CraigW

YZ 420 conversion

6 posts in this topic

Just how reliable are the 420 conversions?

The only conversion example I know about made me very happy not to be the owner of the bike.

I was racing in the Toyota Desert 1000(km) race in Botswana in 1999 on my 1998 KTM 300 when I heard a YZ 400 catch me up. This was after about 100km on the second day (500km each day) and I though well the race is on. Unfortunately this was not to be. The first time we hit a flat out 5th gear sandy whooped river bed he came past me like I was looking for parking. Now a KTM 300 is no slow coach but I had absolutely no answer to his attack. I went back to 4th gear to no avail, he just disappeared into his own dust.

On a long race like this you've got lots of time to think about things and I started trying to figure out what was wrong with my bike, it couldnt be making maximum power as I had had other races with 400s and the contests had been faily equal.

About 100 km's later I came across him again but could see he had a blown motor (oil all over the place)- I had got back my race position.

I finished the race late that afternoon and attended the prize giving that evening. Lo and behold this same rider walks in at about 21h00 having been picked up by a sweep vehicle. It turns out he had been riding a YZ420 not 400 and this was obviously why he had so much more power than me but obviously a lot less reliability. Of course, I felt a lot better about my 300, knowing I didnt have to strip the motor down to find my power deficit.

Now I have a YZ426. I hope its a lot more reliable than a 420 conversion.

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The word is the "hop-ups" on the 400...416,420,426 are time bombs. There are some who are willing to do regular work on the motor in exchange for the extra performace, I like the fact that I don't have to work on the stocker...to each his own :-)

The 426's were beefed up in the "important" places ;-) The guys I know with them (10 or 15) have had NO problems. I've seen some posts here about clutch problems but I've not seen any in my circle of riding friends.

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Chris in the Mojave

'98 YZ400F

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My local dealer was talking to Weisco in the US after a 331 kit for a KLX.. He was told that Weisco have stopped making pistons for the and the YZ450 due to reliability problems.

Some he mentioned were, stretching of head studs, blowen head gaskets (many!) and odd bottom end failure.

They said it really wasn't worth it in the long (and not so long) run.

If you're looking at internal performance increases have a think about flowing the head and adding oversize stainless valves and heavier springs.

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--

**Ride it like you Stole it!**

Matt Porritt

99 YZ400F

Rubber Chicken Racing

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Wiseco stopped only making 97mm pistons to fit the standard YZ400 rod. They are currently building 97mm pistons for the 426 rod. If a 94mm "420" kit is done "properly" it's as reliable as the stocker.

There is lots of money to be spent, and little return to be had when looking for performance from the YZ cylinder head.

The 94mm kit is the best price/performance mod I've seen.

Rich

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Craig,

Rich knows of what he speaks, I'd follow his lead, if it was my motor. :)

You should check out the dirtrider.net forums there is one that Rich moderates on 4 stroke performance mods. If you look back through it, you'll find lots of discussion of big bore YZ's.

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Chris in the Mojave

'98 YZ400F

[This message has been edited by Chris in the Mojave (edited 04-18-2000).]

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Wiseco stopped only making 97mm pistons to fit the standard YZ400 rod. They are currently building 97mm pistons for the 426 rod

Thanks for clearing that up Rich..

Does the 400 have reliability probs with the kit and not the 426 as I've been quoted?

------------------

--

**Ride it like you Stole it!**

Matt Porritt

99 YZ400F

Rubber Chicken Racing

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