Lightweight gearbox problem

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Steve S
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Joined: Fri Sep 30, 2016 12:03 pm
Location: Hertfordshire United Kingdom

Lightweight gearbox problem

Post by Steve S »

Just finished re-commissioning the lightweight model 8 350 and discovered through a test ride that top gear is jumping out under a light load. I suspected I might have problems as the gearbox was empty of oil when I bought it. There had been a heavy leak from the outer cover dues to a screw not tightened, but clearly the damage had already been done.

The workshop manual lists the replacement parts sleeve gear 044075 and the sliding gear 044076 as the fix for early gearboxes, mine is a 1960 one M 1980. However I cannot find anyone selling these - are they still available? If not what can I do?

Steve S.
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Ozmadman
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Re: Lightweight gearbox problem

Post by Ozmadman »

The real killer for these boxes is a low oil level. It Must be kept up to the level of that oval inspection plate as all of the gears are in the top section of the box use SAE 50 engine oil. Those two gears are not available new as expected so the only option is to try to get a s/h box or parts as most internals are interchangeable bearing in mind I would check the whole box whilst apart. Try eBay for any of the lightweight boxes either G2/model14 or G5/M8 you could possibly pay around £60 for a reasonable one. If you need any help post on here, I have done a few of these so should be able to advise you if you come across any problems or what to check. You could also try http://www.amcclassicspares.com He does a lot of s/h stuff
Paul
1960 Model 8
1974 Yamaha RD250B US Model 6 speed
Steve S
Posts: 123
Joined: Fri Sep 30, 2016 12:03 pm
Location: Hertfordshire United Kingdom

Re: Lightweight gearbox problem

Post by Steve S »

Thanks Paul, I've read a lot on the forum and archive about these boxes, I obviously need to strip mine down to look at what's needed. I guess I'll need the clutch centre puller tool, anything else?

I was going by the workshop manual diagnosis of jumping out of top gear in naming these two parts - is it possible a bearing is worn, which might be a simpler fix?

I've bought stuff from AMC Classic Spares and they are a nice outfit. Kempton Park autojumble is coming up on the 21st and I think I've seen that a lightweight specialist attends?

Steve S
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clanger9
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Re: Lightweight gearbox problem

Post by clanger9 »

Hi Steve,

If you are stuck, you can have the sleeve gear and sliding gear from a Model 8 gearbox that I've got lying around here. You never know, they may be the later version (anyone know how to check?). I think the sliding gear needs new bushes as a minimum and also the bearing race looks shot. PM me if interested.

If you do find a source for the complete sliding gear, do let us know. The bushes on mine are so worn that the gearbox dribbles oil constantly from the gap. I think Andrew Engineering can supply the bushes, so I need to give them a call...
1989 Moto Morini Dart 350
1993 Ducati 750SS
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ajscomboman
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Re: Lightweight gearbox problem

Post by ajscomboman »

[quote="clanger9" (anyone know how to check?).

Simple, the later sleeve and 3rd gear have undercut dogs. Earlier sleeve gears had holes and 3rd gear had pegs.
Steve S
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Joined: Fri Sep 30, 2016 12:03 pm
Location: Hertfordshire United Kingdom

Re: Lightweight gearbox problem

Post by Steve S »

Thanks to you all.

Question; should I remove the whole gearbox to see what work needs to be done, or can I remove the cluster with the housing still in the bike?

Steve S.
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ajscomboman
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Re: Lightweight gearbox problem

Post by ajscomboman »

You can remove everything leaving the housing in place on the bike.
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Ozmadman
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Re: Lightweight gearbox problem

Post by Ozmadman »

Steve S wrote:Thanks Paul, I've read a lot on the forum and archive about these boxes, I obviously need to strip mine down to look at what's needed. I guess I'll need the clutch centre puller tool, anything else?

I was going by the workshop manual diagnosis of jumping out of top gear in naming these two parts - is it possible a bearing is worn, which might be a simpler fix?

I've bought stuff from AMC Classic Spares and they are a nice outfit. Kempton Park autojumble is coming up on the 21st and I think I've seen that a lightweight specialist attends?

Steve S
You shouldn't need any puller for the clutch as it will probably pull off by hand. use loctite on the nut when you re-fit it ... Here are the parts you may need

2 x 041275 bush for main driving gear/sleeve gear(output shaft) May need slight reaming or easing with a bit of wet and dry

1 x 041266 roller bearing for main driving gear/sleeve gear(output shaft) carefully check this, it is the large one in the casing behind the output chain sprocket be warned it is expensive!!

1 x 041267 seal for above

1 x 012545 main-shaft bearing.. change this small ball bearing as a matter or course as they are prone to fall apart and use loctite on the nut as these are also prone to undo and wreck the casing

1 x 042158 gasket

1 x 042159 gasket

1 x 042154 gasket

2 x 040006 o ring/oil seal 1 for the footchange spindle and another for the kick-start shaft (even though this is not mentioned in the parts list). there is an x version (040006x) which is a seal rather than O ring, get these if possible. I had a problem with a leaky kick-start seal even when changed so I used two in there and it worked, kick-start was slow to return a few times but eased itself back to normal after a short while

2 x 040079 o ring/oil seal ratchet spindle

Just another thing, make sure the bearing surface on the main driving gear/sleeve gear(output shaft) is in good condition. The main large bearing 041266 runs on this and it is machined as part of the gear and is not a replaceable sleeve.

Paul
Paul
1960 Model 8
1974 Yamaha RD250B US Model 6 speed
Steve S
Posts: 123
Joined: Fri Sep 30, 2016 12:03 pm
Location: Hertfordshire United Kingdom

Re: Lightweight gearbox problem

Post by Steve S »

Thank you both again for really helpful advice.

I can't find the bushes 041275 in the spares scheme or from AMC Classic Spares, does anyone know where I can get these?

I'm anxious about starting this re-build as although I've re-built bottom ends and other engine jobs, that was over 40 years ago.

I don't have a bike lift and so would be working at ground level if I left the gearbox in the frame; how difficult is it to remove it and work on it on the bench?

Ideally I would like to source a good condition gearbox but I guess that'll be near impossible?

Steve S.
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ajscomboman
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Re: Lightweight gearbox problem

Post by ajscomboman »

If' you're going as far as replacing the roller bearing on the sleeve gear you'll find it easier to remove the complete box as a whole and build it at a comfortable working height. Removal is a doddle once the primary drive has been stripped off and the outer engine cover removed from the timing side. Loosen the 2 strap tightening bolts under the G/box top cover and it should slide out from the timing slide.
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