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M20 gearbox wear, wisdom required!!

Hi,
I have taken my gearbox apart as part of this eebuild most/all bushes are shot, and most of the gears have some pitting yo a lessor or greater extent.
Do I apply the aerospace standards I would use at work and ditch the lot or is that to fussy?
TIA
Scott

email (option): Larkesr@geeeemail.com

Re: M20 gearbox wear, wisdom required!!

Hi Scott,
Worn bushes and shafts will need to be replaced, the gear teeth will stand some pitting but if there is wear on the teeth then replace (maybe post some photos).

Re: M20 gearbox wear, wisdom required!!

How do you judge wear on the teeth, then ?
I Always find that hard to decide.

Re: M20 gearbox wear, wisdom required!!

Mike, A flattening of the curved part on the tooth and / or a step worn into the face of the tooth usually about 1/3 the way up from the root of the tooth.

Re: M20 gearbox wear, wisdom required!!

Can't resist but perhaps a visit to the dentist:upside_down_face:

email (option): jsaft1951@xtra.co.nz

Re: M20 gearbox wear, wisdom required!!

Wear on the main shaft where it passes through the sleeve gear bushes can be cleaned up by using an oilstone with the shaft mounted in a lathe, or by regrinding on a cylindrical grinder at a suitably equipped workshop...The shafts are through hardened so removal of say .005" -.010" is fine...Undersize bushes will then have to be made of course..The same applies to the lay shaft though exact replication of the fixed bushes that fit into the case is not so easy as they have oil scrolls machined into them...That was originally done on a machine designed for that purpose...An alternative scroll pattern applied on the lathe using a boring tool and a very fast feed is the only practical alternative but works OK...

Bush to shaft tolerances for the bushes fitted to the single gears that run on the main and lay shafts are different to those for the sleeve gear bushes...If the shafts are not altered dimensionally and standard replacement bushes are employed the fixed bronze bushes in the cases may need reaming to size after fitting, depending on how heavy the fit to the case is for each bush...Gear bushes may also need reaming but as an alternative the bushes in the gears can be bored to finished size if an accurate lathe is available..(and internal micrometers, a vernier is not accurate enough for this work)....

Some fracturing of the surfaces of the gear teeth, particularly on the thrust side of the teeth is usual in these boxes due to their age and often, erratic levels of lubricant...The fracture lines run horizontally along the tooth. This looks alarming but in more than forty years I have yet to experience, or see, a tooth break...Wear of gear teeth that alters their basic form, as described in a previous post, should be eliminated by replacement...To help the retention of lubricant fit a 6207 2RS main bearing...That's a rubber sealed metric bearing and readily available...Remove the seal from the side of the bearing facing into the box before fitment...The seal can easily be prised out with a small screw driver or similar..

Later tele fork rigid and plunger gearboxes, though visually different externally, share most components with the WD types so are a good source of spares and being a little newer are often less worn. Avoid the very early versions which do not have a seal fitted at the sprocket end, they suffered from a frequent lack of lubrication due to leakage just like the WD boxes....These later boxes also have the later tooth form on the engagement dogs and their mating gears and these can be retro fitted to the earlier boxes to improve selection...Parts can be mixed selectively as they are all compatible...Also, the later boxes have the selector fork shaft that was modified to improve its action (the radii of the cam tracks were increased to smooth out the action)..This is also a straight retro fit...

Accurate shimming of the shafts within the case and the elimination of wear at the relevant points in the gear change mechanism and all related selector parts are the key to getting these boxes to function as well as they can..'Lost movement' caused by such wear results in more erratic and less accurate gear selection...

The BSA box was essentially a pre war hand change design that had the foot change added to it and it displays all the bad features and difficulties of such a design...Though a BSA fan I wouldn't argue for the merits of BSA boxes of this era (other than their inherent strength and durability)...The 'cam plate' box introduced by Triumph in about 1937 was far superior and BSA failed to match it until he introduction of the rigid A7 a decade or so later...Ian

email (option): ian@wright52.plus.com

Re: M20 gearbox wear, wisdom required!!

Gents,
Thank-you for your detailed replies, Ian you’ve gone above and beyond, much appreciated. I am out of the country for work this week but will take and up load detailed pictures, maybe even video, next weekend. I think a lot of us could learn from a team assessment, even if it is “yep, they’re all toast!”😂
Speak soon guys, now get back in your garage!!
Scott

email (option): Larkesr@geeeemail.com

Re: M20 gearbox wear, wisdom required!!

Hi Scott,

I've sent you an e-mail, let me know if it hasn't got to you.

Cheers
Pete

email (option): petercomley@web.de

Re: M20 gearbox wear, wisdom required!!

Thanks Pete,
Got the email, away with work will read on the plane back.
Cheers!!
Scott

email (option): Larkesr@geeeemail.com

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