Author Topic: S/arm gearbox main and layshaft dimentions.  (Read 956 times)

Online Topdad

  • bob hebdon
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S/arm gearbox main and layshaft dimentions.
« on: 22.05. 2013 14:47 »
Good afternoon oh knowledgeable one's ,  yes you're right I need some help. Could anyone give me some gearbox mainshaft and layshaft dimensions ie what are new and what acceptable wear at the sleve sprocket runs on the mainshaft  and layshaft end  bushes.  Many thanks in anticipation. BobH
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Online chaterlea25

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Online KiwiGF

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Re: S/arm gearbox main and layshaft dimentions.
« Reply #2 on: 23.05. 2013 03:21 »
Hi Topdad, I can't directly answer your question on "acceptable wear" and I'm not sure the info on acceptable wear is on the service sheets - it looks like the figures are "new" tolerances?

Anyway, the sleeve gear on my bike had over 010" clearance which I thought far too much, so I bought new bushes for the sleeve gear from Draganfly, fitted them and then honed them out myself to 002 clearance.

The hone I used was a cheap 18-50mm brake cylinder hone for use with an electric drill and worked a treat.

I'm pretty sure the mainshaft was slightly worn but that did not matter as I was honing the bushes to suit whatever diameter it was anyway. The shaft where it fits in the sleeve gear was maybe just 001 under OEM size.

My guess the mainshaft as standard would be 001 to 002 less than 13/16 but I have not seen this published anywhere - my theory being that the bushes would have been made to size by BSA using a standard 13/16 reamer, and the shaft was made slightly smaller to give oil clearance.

My first attempt at the sleeve gear re-bushing was a dismal failure  *problem* i'm not sure if that was because I had too close a clearance or the bushes were supplied in the wrong material (brass)  *angry*. In any event the sleeve gear bushes welded themselves to the mainshaft within a few miles. *rant*

The layshaft tends to wear quite a bit where it fits inside the drive side bush. Non std 014 undersize bushes are available to get around this problem, if you buy a std bush it might be of no use if the shaft is worn.

In my case I got the machinest to grind the layshaft back to round, it was around 008 under standard size after that, then make up a special undersize bush slighly undersize of the shaft, and then line ream this new bush to suit the layshaft, he then fitted a "cap" to the bush - he did it this way as the bush is a "blind' as standard so is difficult to ream to size in situ.

The clearance was around 002 on the new layshaft bush/shaft, maybe a bit less.

Just my opinion but I would replace any gearbox bush where the clearance is 005 or more.

I also replaced the gear change side layshaft bush on the basis To NOT do so would cause the drive side to wear rapidly, in other words they should be replaced in pairs.

I also replaced the 2 ball bearings on the mainshaft.

All that and it STILL whines in 3rd......but I wear ear plugs to fix THAT problem  *smile*
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Online Topdad

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Re: S/arm gearbox main and layshaft dimentions.
« Reply #3 on: 23.05. 2013 10:51 »
Many thanks John and Kiwi.
 You certainly reminded me of all the problems I found when doing my last box kiwi, particularly about the std bushes ,when I tried them on my shaft they rattled rather than fitted ,had to have done what you did by SRM ,they ground the shaft and made up bushes to suit. Think I have a local guy who can do this for my mate who's box it is . Since then I've found another problem which may have turned the mainshaft to scrap , some clown has tried to push the shaft through the inner casing by hitting directly on the end of the shaft so its mushroomed quite badly infact he must have jammed  the fixed gear pinion on the m/shaft against something and then proceeded to beat the living daylights out of it cause that cog has moved about half an inch down from the circlip!,shame as the other end and keyway are quite good.So I think it depends how badly worn the shaft is will dictate if viable to get the other end done. Any thoughts please pitch in ,regards BobH.
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