YAMAHA FZR400 SPROCKET THREAD PROBLEM

[richs20a]

I AM THINKING ABOUT BUYING A YAMAHA FZR400 (1988) AND THE ONLY PROBLEM IS THE DRIVE SHAFT THAT THE FRONT SPROCKET BOLTS TO, THE THREADS ARE SHOT.

WONDERED HOW INTENSE THIS REPLACEMENT IS (I AM MECHANICALLY INCLINED) AND WHAT THE PART COST ROUGHLY WOULD BE, AND WHERE I COULD GET ONE FROM. ANSWERS TO ANY OF THESE QUESTIONS I WOULD APPRECIATE !!!!

ITS GOT 5 MONTHS MOT AND TAX AND HE SAYS GOOD CONDITION AND ENGINE RUNS GOOD. JUST THE THREAD IS MESSED UP.

[Kickstart]

Hi

Proper fix is a complete engine strip down unfortunatly.

If you are lucky you might be able to clean up and rescue the thread without splitting the cases though.

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[Pete.]

That's the proper fix, but since the retainer only keeps the sprocket from sliding off the shaft and has no torsional dorces applied to it I would be inclined to make an alternative fixing of some kind. If the shaft isn't too hard to be drilled & tapped I would make a retainer that's held onto the shaft with a bolt, or something similar.

You could possibly buy a die to cut a new smaller thread on the shaft over the top of the old one.
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a.k.a 'Geri'

132.9mph off and walked away. Gear is good, gear is good, gear is very very good

[richs20a]

Does anyone have a drawing or diagram of what this atually looks like?

Is it a shaft with a thread (external) on the end? The sprocket slips over this and maybe a keyway or square flats, and a nut holds it on ?

IF its a external thrad on the shaft, i could use a pin going through the nut or something similar, i am in engineering so can get special drills, taps etc....... hmmmmmmmmmm

Or is is a hole, where a bolt goes through? that would actually, be easier..

HELP

[Pete.]

You know, if the bike is cheap enough you could just buy a brand-new top quality chain & sprocket set and weld the front sprocket to the output shaft, or better yet find a really thick washer that fits snug over the thread and tack-weld that to the shaft stopping the sprocket from coming off. Should be good for 10,000 miles of riding if you look after your chain carefully.

Don't use too much heat or you'll melt the output shaft seal. Only a tack-weld is necessary.
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a.k.a 'Geri'

132.9mph off and walked away. Gear is good, gear is good, gear is very very good

[Kickstart]

Hi

Not checked a 400, but it is probably the same as the FZR600 (and loads of other Yamahas), as the 600 is a VERY similar engine (some internal engine parts on the 600 are marked with the model code of the 400).

On the 600 it is a large diameter shaft with splines for the sprocket to sit on and then a large fairly fine thread for the nut. The nut is slightly recessed at one end to allow it to go slightly over the splines and so locate a lock washer.

You might well be able to get an appropriate die to recut the thread.

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[Fagin]

I remember having a knackered old british bike for round the fields in the 70's.

The chain was mashed, but no one ever tried to change it.

When I inheritied it off my brother he had taken out a couple of links from the chain but had made no other changes.

I know we were skint but that chain was a liability. I used to sweat cods everytime I rode it waiting for it to snap!

Decided to have a look at the front sproket, aha! I see the problem....

Someone had welded it on!