Replacing gears

htown16 · Jun 24, 2020

Pulling my 73 850 gearbox apart. Third gear layshaft has a lot of spalling and looks in bad shape but no missing teeth. All the others including third mainshaft look pristine. Just replace the one? Defective from manufacture?

kommando · Jun 24, 2020

You are supposed to replace pairs but I never have and not had issues. The spelling is common, you are lucky it's only on one gear.

Stephen Hill · Jun 24, 2020

Depending how bad it looks, some folks would just throw it back in, unless it is noisy or jumping out of gear.

Fast Eddie · Jun 24, 2020

Replace the bad gear and carry on...

baz · Jun 25, 2020

Yep just change the bad gear

jseng1 · Jun 25, 2020

kommando said:
You are supposed to replace pairs but I never have and not had issues. The spelling is common, you are lucky it's only on one gear.

When replacing only one gear of a pair always check the diameter of the new gear and compare it with the old one. If its larger then mock it up and make sure the new combo spins free without binding - if its the same diameter or bigger and it doesn't bind then all is good. If its too small then there might not be enough tooth engagement. I'll replace one of a pair but not unless I've checked the diameters and made a judgement. Gears from different makers do indeed have different diameters.

acotrel · Jun 25, 2020

jseng1 said:
When replacing only one gear of a pair always check the diameter of the new gear and compare it with the old one. If its larger then mock it up and make sure the new combo spins free without binding - if its the same diameter or bigger and it doesn't bind then all is good. If its too small then there might not be enough tooth engagement. I'll replace one of a pair but not unless I've checked the diameters and made a judgement. Gears from different makers do indeed have different diameters.

What if a PAIR of gears is replaced ? I would not replace one of a pair if one was spalled. If a gearbox lets go and locks up, it is usually a certain crash for you.

Onder · Jun 25, 2020

I always change in pairs. It costs more but there is that reduction of the what if factor.

Torontonian · Jun 25, 2020

Obviously if you have the money to burn on the mating gear then do it. But I've had one bad gear myself full of pitting yet the other mate was fine so I simply replaced the one. Your money.

Madnorton · Jun 27, 2020

You don't know if the worn gear was culprit or victim, is it made wrong or is the mating gear made wrong. If it is marked RS, don't use it.

concours · Jun 27, 2020

I went In to do a starter pawl

northern750 · Jun 27, 2020

I've never changed gears as pairs if only one had issues and the other was still perfect and no other mechanics or engine builders I've ever spoken to have either, including guys who built national level superbike engines. Replace the spalled gear and carry on. Or change the pair if you prefer...

Replacing gears

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