The BSA A7-A10 Forum
Technical (Descriptive Topic Titles - Stay on Topic) => Gearbox, Clutch, Primary => Topic started by: Swarfcut on 10.10. 2020 08:42
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Having just removed a well scored blind bush and matching screwed layshaft from plunger box, I'm not surprised they wear given the small oilway and plain shaft. The swing arm layshaft has a scroll, so with the correct fixed pinion and layshaft third gear is a possible swap.
Noticed there is a fairly thick web in the casting below the bush, so to improve the oilflow to the bush another oilway could be drilled vertically from below, into the bush and meeting a new horizontal drilling from above the " sediment level" but well below the resting oil level. The only downside I can see is another external hole to plug.
I'm sure someone else has thought of similar modification to improve the oil flow to the bush. Looks easy to me, but there must be a reason why the design feeds the bush from above.
Any thoughts or experience? Modifying the bush with an internal oilway or a scrolled blind bush from another model?
Swarfy.
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You can cut an oil groove at the top of the bush ( opposite the thrust side) inside the full length about 1.5mm wide x 1.5mm deep, then radius the exit point at the flange ( where it buts up to 4th gear L/S ). Also cut 2x slots across the flange face. ( 1 of them coinciding with the slot)
Far simpler than trying to cut a scroll in a blind bush unless you have the correct machinery & works fine.
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Yes, but how many have a mill with a chuck on the bed to do that? Cutting a scroll is a piece of cake in a lathe - after all, it's only a coarse thread. Just do it very slowly with a blind bush or, better still, cut it on the way out. Another reason not to use thick oil in the gearbox, as with little clearance that bush area is not easily penetrated with lubricant. It was because of the bushes I used to run R40 in my hillclimb bike, as that stuff will stay in places other oils won't.
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Never done one with a mill & chuck. Left the bush in the lathe chuck, ground a tool bit to fit in my boring bar (45 degree angle to get to the end) turn it sideways & use the carriage as a hand shaper. Then use a small square ( or 1/8" 5/32" round ) file to carefully file the slots accross the face. Done it many times & dosen't take long. However you do need a lathe, a couple of files & a boring bar that takes HSS tool bits.
PS I do have an old clunker lathe set up only to cut oil groove scrolls ( over geared carriage) but find the other way is quicker (safer) with blind bushes.
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With almost 150 views to date, looks as if this conundrum has stumped most of you. So thanks to the fellas that gave it some thought.
The easy way would be to fit a scrolled type S/A Layshaft and matching third gear. Does this have any benefit, or do Swing Arm Scrolled Layshafts fail in the same way as plain shaft Plunger Types?
Just thought that a drilling to form an additional oilway would be worth a try, but if it were that simple, someone else would have done it.
Swarfy.
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Have, in recent times, rebuilt both a SA & Plunger box. Both layshafts were in the same state of excessive wear. Mileages unknown of course.
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Hi All,
The 650's extra power seem to wear the bush and shaft more so than the 500's
About a year ago I stripped a Star twin box for inspection and it did not need new bushes, As far as I could make out it had never been apart before
The swing arm 650 gearboxes wear the bushes in spite of the "improvements"
I suppose that is why they fitted needle roller bearings on the competition "T" gearboxes
John