Hi All, The gearbox on my recently acquired M21 howls in the first three gears. I replaced both mainshaft bearings and the layshaft bushes all to no avail. The gears seem to be in good condition and 4th gear is quiet, other than the noise the gearbox works perfectly. I am wondering whether there is a component that is under load when any of the first three gears are engaged but isn't when in top and presumably the drive is direct.I currently run a 50,50 mix of straight 50 oil and lucas oil stabilizer which helps for the first two or three miles but as soon as it warms up the noise returns. Any ideas please. Mike.

email (option): michaelggilbert@btinternet.com

...definitely coming from there? as a mag drive pinion meshed too tight ( Mag not shimmed enough) will whine.

If it twer me, (and perhaps you have already done so) I would place the bike on its rear stand and with the bike running and in the various gears, really get down next to it to try and pinpoint the noise you are hearing.

Does the bike make the sound on the stand with no load? On the stand does the sound get louder under load from applying the rear brake, do you experience the same sound with wheel spinning, with the clutch pulled in, does sound change under acell, decel, or specific gear and engine rpm, which gear loudest, etc.

What changes under what conditions?

Does the box feel like it is binding as it turns, in any particular gear? Any vibration associated with sound? Any metal in gear oil or primary oil, clutch action normal, primary chain tension correct, drive chain tension correct?

email (option): teladelujo@ msn.com

Hi Mike, I know what you mean as having the same 'problem', my gearbox also howls a bit when accelerating, especially in the third gear and more than my other M20. Also wondered what could cause it....

I think a bit of howling in the first three speeds is normal as the power then actually goes through the gears. The top is indeed a direct drive, so no power on the gears then.

But when the howling becomes excessive I suspect that the teeth are not meshing correctly, as you already have replaced the layshaft bushes it could be worn bushes inside the loose gears. Or worn teeth; note that the teeth are only wear on one side so at a quick glance on the gears they can look OK but could be not.

The gearset closest to the chain is under power in all the first three speeds, so get the most working hours..and would be the first one to suspect.

But can't give you a clear answer as I haven't solved the howling in my own gearbox yet, so also very interested in other reactions..

Good luck, Michiel

email (option): m.wijbenga [ at ] hotmail.com

Straight cut gears are strong gear design with no axial load but they do tend to be noisy & worse when worn. trouble is, we are used to quiet helical cut gears on modern vehicles, i think any particularly well silenced M20 will suffer some gear whine.

Thanks, Fred, Mag recently been rebuilt, had to cut out 10 thou metal shim so I know that's OK

Thanks, John. I hadn't thought of experimenting with the bike on the stand I will run through all your suggestions in morning.

Thanks, Michiel. Its comforting to know that I'm not the only one. I too have a M20 with a more or less quiet gearbox, maybe the added grunt of the 21 is more punishing for the 'box. If I come across a definite cure I'll let you know.

Mike, what condition is main shaft in? Often over looked is the main shaft spline at the drive end where it runs up against the sleeve gear, the end of the spline can wear and give excessive end float on the main shaft, this might produce a whine in the first three gears.

email (option): spacemonkeym@gmail.com

As a gear tooth wears flat and looses its rolling action against it mating tooth, it starts to rub instead of rolling over the mating gear tooth. This can give the whining noise. You can see if the tooth has lost its profile by a wide mark across the middle of the tooth as its wearing flat. Not much you can do about it if this is the case.
Just an idea that may help.
Tim W

email (option): t.j.walker@btinternet.com

Hi, Tim, This could be it or at least a contributing factor, some of my teeth have this line running across the centre, also there is some wear in the brass bushes so I think its new gear time.

Hi, Matty, I have decided that I am going to have to replace at least some of the gears because of bush/tooth wear I will inspect the main shaft then. Presumably the gears need to be replaced as a matched pair so I'll probably end up replacing most of them.

email (option): m.wijbenga [ at ] hotmail.com

Michiel, I'm of the same opinion you do the first nut up finger tight then fit the tab washer & lock up with the 2nd nut, or so I'm led to believe. I'm thinking maybe the pinion gear bush.

email (option): michaelggilbert@btinternet.com

email (option): spacemonkeym@gmail.com