The oil has to be somewhere so will fill half a thou' gap between shaft and bush and leave nil clearance. If you start off with less than half a thou', it's likely to nip up.

You can feel this running clearance when it's dry but, after a squirt of clean oil, the clearance disappears.

Ray

Room for the oil makes perfect sense.  Just so that I'm perfectly clear on this.... no less than 1/2 thous?  Meaning 1/2 thous is acceptable?  Or not even as tight as 1//2 thous.... keep it closer to a full one thous.?

I agree with John - I'd give it at least a thou' and a half for safety's sake. Which means I'd probably have left it running with the original bush still in there. I feel it would have been a waste of time mentioning this at the outset because folk mostly do what they want to do and not what I think may be best.

Also, I'm very surprised you can't obtain 7/16" UNF plain nuts - they were introduced to us by the USA decades ago and are still easily available here despite being replaced by Metric a long time ago. Have you tried at a tractor or truck spares supplier? Do you have trade motor factors on your industrial estates?

Ray

Hi fellas

At some point I need to make decisions based on guidance gathered.

When originally looking for info on the layshaft measurements I saw nothing about tolerances.  That doesn't mean it's not there and I have time to re-check.  I don't need to catch a ferry like John did.  LOL
But given the experience of Ray, John, etc. I'd be very surprised if anything were to be found that's contradictory.

Regarding the blind layshaft bushing, I had 4.5 thous clearance between shaft and bushing.  Allowing for a wanted 1 - 1.5 thous design clearance for the oil would leave 3 - 3.5 thous of wear space.  I'd be surprised if that were acceptable in any piece of machinery.  And even if "usable" it certainly puts it closer to the end of it's useful life than it does a fresh start.  Ditto the other "end of a shaft" bushings I replaced.

On the 7/16-20 UNF nut, I didn't say it was impossible to get - just inconvenient after two trips to two different hardware stores.  I only even mentioned it because I found it surprising.  Eventually I know I'd find them at a hardware store, tractor supply, etc. but the running around is time and money better spent.  For $5 shipping I can place an internet order with a distributor and get additional product for the same amount of $$$ the hardware store would charge, will not be driving place to place and can stay home and do something else with my time.  If I lived in a rural area where I would enjoy the drives I'd be happily running all over the place.  I generally like drives/rides.  But unfortunately for me I live in a suburban hell (urban sprawl) where driving 4 miles involves 15 traffic lights and takes 20 minutes. 

The fact that two of the most knowledgeable fellas say 1.5 thous that is what it will be!  I like a consensus... removes doubt and second guessing.   So, I've got a plan now regarding the new bushing which is 1.5 thous between the layshaft (after dressing it up) and the bushing.

John - that running in of a rebuilt gearbox with an electric motor was genius and is a great story!!!!

ALL of this input is valued & appreciated and I thank each of you!!!!!!

I fully agree, and have said so in the past. These bushes have an easy life and will likely never wear out no matter how many years the bike lasts.
It's rather like Triumph twin cam bushes that "restorers" always felt they had to replace years back, and they removed perfectly good bushes to replace them with after-market Taiwan parts, and then proceeded to amateurishly hand ream them (usually without the benefit of the Triumph piloted reamer) leaving the bushes in a worse state than when they'd started the job.

As a blind internal bush on a 11/16 shaft, running in constant oil supply, I'd think up to 5 or 6 thou clearance would be perfectly ok....but like you Pete i am interested in others opinions.
Graham

Hi All
I have found that the inner layshaft bush wears the most in the gearbox , The same bush is fitted to the heavyweight C series 4 speed as used on the A and B series with 350, 500 and 650
So my experience tells me that the least wear will happen on the lowest power bike and the most on the 650's
For some reason these bushes do not seem to wear evenly and expect to see scoring and ridges on the shafts especially on the bigger bikes. Its the roughness that needs addressing before worrying about a couple of thou wear.

This bush takes the highest load in the gearbox as the final reduction gears next to it are under load in first, second and third gears

John

Have a look at BSA service sheet 702 as it gives bush and shaft tolerances. I had a heavyweight gear box with the same type of wear as you have which was caused by low oil level. That gearbox kept giving me issues changing between 1st and 2nd gear.