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 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

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've had other bushings with 1.5 thous clearance and I was able to feel & see side to side movement.  My opinion is nearly valueless compared to the collective experience of those here guiding me but wouldn't I want something closer to .5 thous ?  Keep it nice and snug but still able to slide in and out and rotate without binding ?
The guys in the shop are fully capable of holding a tight tolerance.

The OD 0.685 of layshaft is 2.5 thou down on 11/16 inch which I would guess as being the original diameter
Even 2.5 thou is probably the maximum clearance you would want on a new bush, so as you are doing, a purpose made bush is in order with 1 to 1.5 thou interference on OD... imo
Graham