My gearbox is now ready to go into the frame .
Frame is on it's wheels and the engine is fitted.
No problem you say. Wrong!.The width of the mounting lugs of the box is 63mm. The width of the space between the mounting plates of the gearbox is only 58mm.
Simple solution would be to grind 2.5mm from each side of the mounting lugs of the box but before I resort to such drastic measures I'd like to know if this is a known problem and if so if there is an other solution then grinding the lugs .
Many thanks in advance .
Maxim Jurgens
Phuket, Thailand

email (option): siamsailing@gmail.com

Max I think we should discuss this a bit before you grind anything. There is something obviously wrong.....Like wrong box/frame?? Can you post pictures of what you have (or send them to me if you want). Close ups are good but some overall pictures of your bike also. Ron

PS I just measured the lugs on a WM20 box and they are indeed about 63mm. So it would appear there is something amiss with your frame or plates. The plates should be parallel from the back of your engine right through to mudguard.

email (option): ronpier@talk21.com

Hello Ron
Boy, you are quick !
I fear I do not have a working camera handy ,so I'll describe as well as I can in words .Frame nr of the bike is 6 digit :WM20 104037 . There is no nr on the engine ... Box is stamped 66-3141 . I think this means I have an early bike with a late box ? Gearbox mounting plates are what came with the large pile of bits which constituted the bike I bought, they are about 5mm thick.
I know they are straight as I checked them for straightness .
The left(drive side)gear box mounting plate sits without any washers or spacers between the central vertical frame tube and the bottom horizontal frame rail.
The right (timing side ) plate sits against the vertical central down tube, then there is a 37 mm long spacer between it and the bottom horizontal frame rail.
If I want to avoid grinding the lugs on the gearbox I suppose I could shorten the 37 mm spacer and place 5 mm spacers between the vertical down tube and the right side mounting plate but that would mean having to place similar spacers between the
plate and the 3 bolts on the engine as well as inviting issues re the alignment with both primary and secondary chain I fear ?

email (option): siamsailing@gmail.com

I had the same problem with my Egyptian Military Museum quality M20
The plates were neatly recessed in the form of the frame lugs, and the frame lugs were worn from 63 to 60 mm. First I tried to fill the gaps with washers, but then a NOS front frame was found and that solved the problem. But I fear that you will have to use the washer method; check for symmetry ( 2 x 31,5mm)
Good luck....

email (option): viaconsu # planet dot nl

I can't really get at much on my M20 as it's all together and on the ground. But My special which is on the bench and includes standard M20 frame and box is sort of getatable. I have made these very rough measurements from underneath the gearbox plates. Spacer between drive side frame rail and plate=46mm. Between plates =64mm. spacer on timing side=39mm. As I say these are just a guide as I had a job to get straight on with measuring stick. Maybe Ian will pick this up as his gearbox was out. But it's probably back in by now.
I will still keep looking so don't grind anything yet. Ron

email (option): ronpier@talk21.com

Well there's a problem I hadn't thought about! The other problem with grinding the lugs would be doing it at the right place to keep the sprocket lines correct. Ron

email (option): ronpier@talk21.com

This might help; the different chainstay bridge collars are as follows:
8: 66-4602 45,7mm left hand
9: 15-6240 63,5mm centre
10: 66-4601 40,6mm right hand
bridge distance piece front 66-4581 39,8mm

email (option): viaconsu # planet dot nl

If the bridging collar (spacer) in Hans' drawing is 63.5 mm then the mounting lugs for the gearbox plates on the vertical frame tube should also be 63.5 mm in length. Mine are not, they are only 58 mm,both of them. The engine went in without problems though. I havent' measured the back side of the engine and i can't measure mine without extensive surgery but are the rear mounting lugs on the engine then also 63.5 mm long ?

I do not want to grind the gearbox lugs but if I was to I'd take 2.5 mm from both sides of each lug to avoid chain alignment problems .

What I think I will do ,unless someone has a better plan, is make up 2.5 mm thick washers and shorten the forward righthand side spacer(bridge collar) by 2.5 mm.

I'd place the washers on the vertical frame lugs and between the engine and the plates. Only issue would be placing the spacer between the lower frame lug and the left hand mounting plate but forcing the bottom rail by 2.5 mm should not pose too much of a problem , I think .

email (option): siamsailing@gmail.com

Max I have a NOS frame and that measures within a thou or two of 2.5" across the frame mount points (63.5mm for those of the metric persuasion).

This also has new engine and gearbox plates and spacers etc so if you need a specific measure let me know.

I just posted this while you were posting so in answer to your suggestion about spacers then that should work and you can adjust sideways by the thickness - figure it out with washers and then you could get specific spacers machined.

I would be very reluctant to go cutting anything that is original.

If you do work out you need a shorter spacer (and on initial impression I can't see why if you are essentially restoring the width of the frame lugs) then that would be easy to knock up on the lathe and your precious original part would remain unmolested.

I just emailed you a couple of photos.

email (option): rays54@hotmail.com

Maxim, as mentioned, the answer is NOT to remove metal from the gearbox.
The two mountings on your saddle tube need to be 2.5" wide, if not, you must add shims (washers) its easy to see how much to add each side because the lugs are exactly central to that down tube so you can use a straight edge & measure distance from the down tube both sides.

If you were to cut metal from the gearbox mount, how would you know how much to cut off which side? One side could be a mil under and the other 3mm, so if you just cut an equal amount of both sides then the chain line would be out.
Spacing washers, every time..

Yes, There is no way you should alter the box.. The gearbox mounting bosses are the correct width and the problem obviously lies elsewhere. That problem should be remedied and bought within spec. and the gearbox will then fit as intended...Ian

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