Author Topic: Gearbox bush clearance  (Read 565 times)

Offline Russ

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Gearbox bush clearance
« on: 15.10. 2016 01:38 »
I am in the process of rebuilding my gearbox for my 51 A10 Plunger and can have new bushes made at work. I have service sheet 702, showing original specs, however, they may no longer be releverent for my box.
I was going to allow 1 thou over on the O.D and also 1 thou clearance on the I.D. for the Layshaft bushes. Does that sound right?
Next, their seems to me to be excessive play in the constant mesh gear on the main shaft, however, I don't know if this is O.K or not. Any thoughts?
I can have the two bushes in the gear machined / pressed out and new ones made. Is this a good idea and if I do what clearances should be allowed?
Why are their two bushes and not just one, and do they but up against each other with a square edge on the bush(edge not rounded off)?
Finally, I am going to get a kick start stop made. Is it just plain mild steel or should we use something else?
Cheers
Russ
1951 A10 Plunger.
Australia

Online KiwiGF

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Re: Gearbox bush clearance
« Reply #1 on: 15.10. 2016 03:50 »
A subject dear to my heart, see this thread  *smile* around 2 thou clearance I think I ended up with, I used a car brake slave cylinder sprung hone tool to get the clearance right.

http://www.a7a10.net/forum/index.php?topic=5202.msg45789#msg45789
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Offline Russ

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Re: Gearbox bush clearance
« Reply #2 on: 16.10. 2016 02:17 »
Thanks KiwiGF
Interseting reading. I will take it all on board before deciding whether to replace mine or not.
Russ
1951 A10 Plunger.
Australia

Online groily

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Re: Gearbox bush clearance
« Reply #3 on: 16.10. 2016 18:15 »
Dear to my heart a while back too.
I was faced with having to replace the direct drive bits in mine owing to worn dogs (jumping out of top all the time etc), and with a worn mainshaft as well  . . . 
I did the best I could to get the shaft round (and then took quite a bit of extra metal off the outboard end so it would pass a new sleeve gear bushing maybe 10 thou smaller id) and then made a one piece bush from leaded bronze. Can't remember how tight a fit I made the od - probably copied the original of what was in the old gear and pressed it in and reamed a tad after.
The bore of the bush was always going to have a wee bit of slop due to the shaft's state and my paranoia about going through the hardening if I took too much off it - plus the fact it is mighty hard on a small lathe to get a dead parallel bore on a one piece item of that length, owing to flex in the boring bar. Why I went 'one piece' I can't remember - maybe because of something someone said on here!
Anyway, the point is this: the one piece job has done about 30K miles and is still in there. The noises and whines are still no worse than before, and mostly to do with third gear anyway, as so often. There is a little bit of play - I know this because I had to dig around the other week when replacing sprockets and chain after a lump of something metal or granite-like wedged itself somewhere, stripped half the rear sprocket and snapped the chain clean in two miles from any-bloody-where. As happens. But I was frankly surprised how little wiggle there was on the mainshaft considering what I know of the state of the internals - so I am presuming all is reasonably well.

Because it's held together no problem, I've not bothered yet to do what I should, which is do the whole 'box properly!
Bill

Offline Russ

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Re: Gearbox bush clearance
« Reply #4 on: 16.10. 2016 22:12 »
Thanks Groily
Trevor has used a one piece bush with success as well so I might go down that path too
1951 A10 Plunger.
Australia

Online chaterlea25

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Re: Gearbox bush clearance
« Reply #5 on: 16.10. 2016 22:37 »
Hi Russ,
If you fit a one piece bush I would recommend an enlarged bore section in the centre where the gap between the bushes was originally
 Reasons to do this, the mainshaft will have an unworn section where the gap was originally
This will give a tighter clearance on that part,
I tried a one piece bush without relief and it seized in short order, I machined out a relief in the middle and
increased the clearance a touch and it lasted ok
The gap between the 2 bushes also holds a drop of oil which I believe helps
Make sure to add the 3 oil  holes at the inner end

John
1961 Super Rocket
1963 RGS (ongoing)

Offline duTch

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Re: Gearbox bush clearance
« Reply #6 on: 16.10. 2016 23:15 »

 Fairly sure I've seen a 'Bushing size chart' somewhere in the service sheets
Started building in about 1977/8 a on average '52 A10 -built from bits 'n pieces never resto intended -maybe 'personalised'
Have a '74 850T Moto Guzzi since '92-best thing I ever bought doesn't need a kickstart 'cos it bump starts sooooooooo(mostly) easy
Australia

Offline Russ

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Re: Gearbox bush clearance
« Reply #7 on: 23.10. 2016 09:21 »
Thanks for that info John I have copied it for reference.
DuTch I have that service sheet with bush sizing but as my shaft will be well and truly worn I will need to have bushes made to suit.

Cheers Russ.
1951 A10 Plunger.
Australia