Gearbox rebuild questions

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I'm going through the gearbox on my 850 project. I have a couple of questions I'd like advice on:

2nd main and 3rd main gears both have bad dogs. Can the dogs be repaired by welding and milling, or do I need new gears? If I replace the gears do they have to be replaced as a set? The teeth look fine and the matching lay gears look good too.

I measured .015" runout on the mainshaft using my trusty Harbor Freight dial indicator. Seems like quite a bit to me. Do I need a new mainshaft or are they all like that? The layshaft only had about .002" runout.

thanks,
Debby
 
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Debby, if you have a heat treat in town, they may have a person there who buy using heat and pressure can save your main shaft and also make the lay shaft better. This is common on items that are warped during heat treat. No more that 400 degrees F will be used to maintain the Rockwell hardness. Straighteners are getting harder to find too much whiskey. Yes they need fixing. The dogs can be fixed but both tradesman have to very good at their jobs hard face welding on a preheated part with very precise machining to get all the dogs hitting together not so easy. If they are proud of their work you could face a bill as high as the new gears . You just have to build your network and know your prices.
 
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You are supposed to renew in pairs but I never have and there have been no ill effects, make sure you get the right gears as there was a change at that time to the teeth numbers on 2nd to reduce the noise levels.

Welding and milling will work only for a short time as the weld will be soft and the mating holes still hard so wear will be quick, you can't harden the gear as it will warp, orginally they were hardened before final machining to remove the warp but you can't.

The mainshaft has a much harder time than the layshaft and being longer is even more disadvantaged to bending forces so no suprise is run out is greater but 15 thou is too much to put back in. Either get a good alternative one or get the old one mounted in a lathe between centers and bent back to shape, as long as the bend is in the center this will work.
 
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How about 7 thou? I was digging through a friend's parts stash and came up with a mainshaft, and that's what I measured for it. New mainshafts are currently $232 from OldBritts. :shock:

Should I toss this one back in the box and cough up the money? (assuming I'm unable to find someone able to straighten it, that is)

Debby
 
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Im rebuilding my gearbox as well. I watched the Mick Hemmings gear box rebuild DVD and never thought about run out. I have put in a roller bearing in the layshaft ( the layshaft Superblend) and noted that the inner race was very tight on the layshaft. I felt that would be a good thing as I didn't believe it need too much float. I reassembled the gearbox ensuring I could shift through the gears which was O.K. But after assembly, the gear changes ( in the Vice) tends to lock up occasionally. Ive pulled it apart but cant determine whats causing it. Im wondering if it the new type of bearing? and is me turning the mainshaft is a real test anyway. Any suggestions?

Regards Bryan
 
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Debby, You can buy a whole 2nd hand gearbox for the price you have been quoted for a new shaft. I recently bought a couple of shafts on e-bay for not much money and they are perfectly serviceable. I would also support what Kommando says about replacing gears in pairs. We all know it is the correct engineering practice, but in the real world of trying to keep our 40-year-old bikes on the road it doesn't seem to make any difference.
 
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It's a crapshoot buying used parts, especially with mainshafts. With my luck I'd just end up with more bent ones. :?

I did find some good used gears though, so at least that's taken care of.
 
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Gearbox mainshaft & layshaft journal tolerances new and

Evening all.

I have a question about what the tolerances (and acceptable worn tolerances) are on the mainshaft and layshaft where the various gears rotate.

For the layshaft I don't have a lot of concern but would like to know what the 'as new' diameter for the area that the 1st gear rotates and where the layshaft rotates in the kickstart bush.

Gearbox rebuild questions


The mainshaft is a bit different. The area where the second gear rotates on the mainshaft looks fine (and is nearly new except for polishing by the rotating bushing).
Where the sleeve gear rotates is what I am wondering about.

Gearbox rebuild questions


You will notice that two areas that are free from wear (as no gear runs on these areas) are the original diameter at 0.8100".

Where the drive side sleeve gear bushing rides, the shaft has been worn to 0.8091" which is not bad - about 0.001".

However, where the timing side sleeve gear bush rides, the bushing has worn the drive shaft down to 0.8072" or near 0.003" undersize.

Time to get a new drive shaft?
Has anyone tried hard chroming a gearbox shaft and then grinding to size?
What clearance between the shaft journals and bushings is acceptable?
:? ?
 
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Hi Bryan,

Could be that you have too much endfloat in the layshaft. With the roller bearing you will likely need to shim the layshaft. Good instructions on how to measure endfloat and how to shim on the Old Britts technical site. Also, i think it's pretty hard to tell how well the gearbox will shift with the box out of the bike. You can adjust the endfloat pretty easily with the gearbox in the frame.

Tobin

bryan_woods said:
Im rebuilding my gearbox as well. I watched the Mick Hemmings gear box rebuild DVD and never thought about run out. I have put in a roller bearing in the layshaft ( the layshaft Superblend) and noted that the inner race was very tight on the layshaft. I felt that would be a good thing as I didn't believe it need too much float. I reassembled the gearbox ensuring I could shift through the gears which was O.K. But after assembly, the gear changes ( in the Vice) tends to lock up occasionally. Ive pulled it apart but cant determine whats causing it. Im wondering if it the new type of bearing? and is me turning the mainshaft is a real test anyway. Any suggestions?

Regards Bryan
 
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