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i was wondering if a rebuilt 636 front end would help when racing,would it fit the h model,

A few people have done it. Ballsout from here and bemsse racer did it with success. Steve porter from our club, w100, has had it done ,and he goes well.
i reckon if you can get the whole front end, brakes wheel disc, everything, cheap enough then it would be worth it.
im not sure if you would be able to race everywhere with it tho, under f400 rules....you would have to check that out with the clubs your racing with.
havent seen ity done to a h, but cant see any problems

do a search tho, its been covered before not so long ago mate

It could help, but lap records are still being set on stock forks

Also, as far as I'm aware, the only club that allows this conversion in the UK is Bemsee so if you're planning to race with other clubs, check their regs!

As for fitting to an H, I don'y know, it all slots into the L yokes, so if they fit the H, then that's the way to go. The 636 Yokes do fit but have different offset so you could have clearance problems, as well as different geometry (longer trail) which IMO is not what you want

EDIT: If you're starting with a road bike to convert for racing, I think that the cost difference between fitting this and getting a stock front end modified is very little, if anything, bearing in mind that the standard 636 brakes and discs will be more than adequate and shouldn't be too worn (as they are not that old bikes yet, whereas you'll more than likely need new 400 discs and either a caliper rebuild that costs £80+ in parts alone or alternative calipers. The 400 forks do work, but if you go down the cartridge kit and new spring route, it'll cost more than the s/h 636 forks that already have 8.5kg springs and all the adjustment that you need

PS, I've heard that the 636 forks have different valving than the ZX6r forks that are more track focussed. I think I had both, set up by the same person, and the adjustments were completely different, the ZX6r forks IMO were much better.

to fit to an H you'd need to open out the bottom yoke 1 or 2 mm as its smaller due to the different "waisting" on the fork legs

cant use L yokes, the stem is too short.

I'm not familiar with the H model frame but if the headstock is the same as the L model you can have a new stem made to fit. The 6rr stem is longer and can be spaced to work as well with the 6rr triples. The 03/04 models have the least amount of trail for the 600's. They increased the trail for 05/05 and then again for 07/08. More trail isn't necessarily a bad thing, but you have to balance it with everything else. I found that with the 03 triples the bike was better balanced for trailbraking and it was less likely to tuck the front. I could still turn as quickly as with the stock front end. What I really liked is that the newer forks were more supple, no matter how much I worked on the stock SP forks they were still harsher than the 6rr's. Also I can get the best pads available for the radial brakes. Here in the state I could order Carbon Lorraines but they didn't last long and the others, EBC, Galfer, etc aren't available here. Vesrah's last a really long time and have great feel too. One finger braking with a brembo MC Vesrah pads and radial calipers.