Necessary to replace gears as a set?

All the "experts" (Mick Hemmings, Haynes, etc.) recommend that you replace gearbox gears as a set (mainshaft/layshaft). Is this strictly necessary? My 2nd gear mainshaft is looking a bit worn, but the 2nd gear layshaft looks fine. Same for 1st gear. I'd rather spend the money on other parts that actually need replacing. These gears are pricey! Any other opinions?

Huffer said:

All the "experts" (Mick Hemmings, Haynes, etc.) recommend that you replace gearbox gears as a set (mainshaft/layshaft). Is this strictly necessary? My 2nd gear mainshaft is looking a bit worn, but the 2nd gear layshaft looks fine. Same for 1st gear. I'd rather spend the money on other parts that actually need replacing. These gears are pricey! Any other opinions?

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It's a recommendation, not a law. But the rule is that if the non-replaced gear starts to go back you can't complain about it. :mrgreen:

Never followed this advice, never had a problem. Its not as if many of use are doing high milages enough to justify.

Watch that you get the correct 2nd gear set, 74 the dogs should have a groove machined on both.

Also, the 2nd gear ratio was changed in around '74. So if you have an earlier box, the 2nd gear would have to be changed as a pair. Count your no. of teeth & check first. I got caught out & had to buy the mating gear.


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Don't marry a Centipede, they take too long putting their shoes on.

I have the newer modified 2nd gear with the layshaft gear having one less tooth than the older style and the scored lines along the dogs. Thanks for the opinions. I'm just going to order the ones that are actually worn. I agree with kommando--this bike won't get ridden enough after I'm done restoing it to worry about replacing the matching gears that seem to be fine.

I replaced both of mine, but apart from the financial impact it was an easy decision - as they were both totally knackered!



I suspect that the Maney mainshaft support behind the clutch would help reduce wear, but I can't help thinking that if the gears had spent more time in heat treatment in the first place (along with the cam), the gearbox whine (from me) would be less!


I'm sure it is 'best practice' to change them as a pair, as it is to replace chains and sprockets as a set, but I wouldn't lose sleep over just changing what needs changing.