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51 mac gearbox component dimensions #731807
04/13/18 5:53 pm
04/13/18 5:53 pm
Joined: Feb 2006
Posts: 625
Ontario , Canada
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RGSROB Offline OP
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working through a MAC gearbox and wonder if anyone has the overall length for the bronze bush that fits in the Kickstart housing and supports the layshaft

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Re: 51 mac gearbox component dimensions [Re: RGSROB] #731836
04/13/18 11:36 pm
04/13/18 11:36 pm
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Joolstacho Offline
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0.784"

Re: 51 mac gearbox component dimensions [Re: RGSROB] #731900
04/14/18 9:05 pm
04/14/18 9:05 pm
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Ontario , Canada
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RGSROB Offline OP
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thanks Jools, Looking for a missing.080" in the layshaft to bush dimension.i.e I seem to have way too much end float.
My floating bush is a little shorter(.771) but if my measurements are correct I still need about .068
How do you go about checking end float on the layshaft?

Re: 51 mac gearbox component dimensions [Re: RGSROB] #731912
04/15/18 12:04 am
04/15/18 12:04 am
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Joolstacho Offline
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Have you got the little hardened BK95 'Layshaft washer' steel ring in there on the end of the layshaft? It is 0.96" thick.
What sort of end-cover gasket are you using? Thin paper one?
Co-incidentally I'm due to visit guru Norm ('Norms Technicalities') on Tuesday as I'm building up /5 a gearbox for my KSS/RS.
I'll see if he has any ideas on your excess end-float.

Re: 51 mac gearbox component dimensions [Re: Joolstacho] #731945
04/15/18 12:21 pm
04/15/18 12:21 pm
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RGSROB Offline OP
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The BK95 on my shaft may have been modified .it measures .085,The end gasket is the thick one and has been used in trial assembly a couple of times , it measures .026" thick. The fixed gears on the layshaft had moved on the splines pinching the two inner floating gears.
The splines in the fixed gear or shaft are obviously worn so I have reassembled and locked them in place with loctite 680.
I set the axial clearance at .0015 with a feeler gauge , the rotating inner gears now spin freely ,hopefully the loctite does the job and keeps the fixed gears fixed.
I have removed all of the springs from the KS assembly as I was not getting consistent measurement .This was caused by the two springs binding and preventing the KS shaft from seating.
Going to check all measurements again .


Last edited by RGSROB; 04/15/18 12:22 pm.
Re: 51 mac gearbox component dimensions [Re: RGSROB] #732402
04/19/18 12:10 pm
04/19/18 12:10 pm
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Joolstacho Offline
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I don't know if this is going to help Rob because the 51 MAC and my /5 KSS / MSS box I think have different width gears.
Anyway just in case it might help anyone else, I've assembled my shafts and the layshaft is as follows:

/5 KSS or iron MSS gearbox:
Layshaft total length OVER THE SPLINED SECTION - from end to end, (but not counting the plain ends which fit into the bearings) is 4.280"
Then working from the drive end I have:
End of spline to 4th gear pinion face .004" (possibly should be flush, but 4 thou is okay by me).
Between 4th gear pinion and 2nd gear pinion I have .007" clearance
Then there's the sliding selector dog in the middle
Between 3rd gear and 1st gear pinions I have set .004" clearance
When the hardened steel washer is fitted it just fits on the splines and is dead flush with the kickstart end of the splined section.

There were quite a few minor variations in the gear box components over the years, but because the parts look very similar, over the years it's quite possible errant P.O.s have fitted
a wrong part. You sure the drive-end layshaft bearing is the correct one? I'm sure you've considered this but if the layshaft bearing is slightly narrower than standard it would account for the discrepancy.

Re: 51 mac gearbox component dimensions [Re: RGSROB] #732549
04/21/18 1:22 am
04/21/18 1:22 am
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Ontario , Canada
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RGSROB Offline OP
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Hi Jools, thank you for going to the trouble of providing the above. You are quite correct about the gear width
These are only 1/2 inch wide. However, the clearances you provided should still apply.
Turns out my layshaft had zero end float, temporarily tried a couple of .006 " feeler gauges between KS housing and gearbox face, will make up a thinner thrust pad in the KS assy.
Everything works reasonably well now. with the exception of selecting first gear consistently.
Will try a shim as there may be some wear On the mainshaft first gear shoulder bush.
Rob

Re: 51 mac gearbox component dimensions [Re: RGSROB] #732558
04/21/18 3:03 am
04/21/18 3:03 am
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Posts: 227
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Joolstacho Offline
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You've got it under control Rob, good on ya. Bear in mind that there are at least a couple of different 1st gear M/S bushes.

Re: 51 mac gearbox component dimensions [Re: Joolstacho] #732921
04/24/18 12:49 pm
04/24/18 12:49 pm
Joined: Feb 2006
Posts: 625
Ontario , Canada
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RGSROB Offline OP
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Jools, thanks for your help on this- bike is running now after a long restoration- just have to sort out reason for a lot of smoke at start up- suspect our old pal the ball valve.
Let you know how it rides when all is sorted

Re: 51 mac gearbox component dimensions [Re: RGSROB] #732925
04/24/18 1:22 pm
04/24/18 1:22 pm
Joined: Jul 2016
Posts: 227
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Joolstacho Offline
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Excellent Rob, post a pic sometime eh?



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