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G: Gearbox Cracked G4 final drive shaft

TouringComet

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The bike lost the ability to shift into fourth gear. I opened up the gearbox, and the double gear G11 won't slide off the G4 final drive shaft. I see 5 cracks.

G4Crack.JPG
 

Bill Thomas

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I cut notches in mine many years ago, I think it came from Clev Trev , But not sure,
I was going too fast in third once, Wrong side of the road, Narrow road,
Overtaking too many cars and it locked up,
Lack of oil, My fault,
= 2 gears at the same time !!, Don't want to do that again,
I stayed on and didn't hit anything, Pure luck that day.
 

oexing

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What puzzles me is the trace of brass-like remains on the chamfer of the G 4. There should not be any chance for this except there was a lipped/tophat bush that crushed after hammering the chamfer. The remains of that lip could have been gone down with oil changes possibly. Anyone with ideas about these brass traces on chamfer ?
In any case I would not want more than 2 thou play bush- G 3 fit. Same with long bush in output shaft carrying the sprocket. Excessive play there will kill the output bearing by force of chain pull.

Vic
 

Bill Thomas

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I have seen sprockets that are a bit tight from new,
If knocked too much with a hammer, Could start to weaken it there,
You would be knocking those two things together,
Better to push the sprocket on with the nut ?.
As Greg says there should be a space between the two,
Without the clutch on, I like to feel the play by pulling and pushing the main shaft.
You should be able to hear the clearance.
 

Bill Thomas

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But the main shaft sits in a ball bearing tightly (hopefully) so you can only feel minimal play in the bearing ?

Vic
The shaft the clutch is on , Is not a tight fit, In the bearing, Never has been,
Doing up the nut with the slots in , keeps it all together,
But if too loose, The thrust washer wears out,
That's why they say , Do it up to the next slot , never go back to the slot before.

If the bearing was a tight fit, It would be a begger to put together,
You would have a job to know if the alloy plate the bearings are in has gone all the way home,
Same with the layshaft bearing.
 

Cyborg

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I wonder if part of the story on why the other bushing materials tend to fail is because they were installed without sufficient clearance. Oilite would be more tolerant of that sort of thing? If the general consensus is that oilite is the recommended material (unless going with Viny4’s recommendation), why does the Spares Company offer a cheaper lead bronze option? Maybe they shouldn’t?

I have some NOS bushings, including a G16 that I would like to use. Is there a sure fired way to tell if it’s Oilite? It looks like it..... warm it up and see what oozes out?
 

oexing

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Oilite/porous bronce seems a bit absurd to me in a gearbox with oil filling. This type is allright for places like girdraulics with infrequent lubricating. But in a gearbox a good phosphor /tinbronce would be my choice, also easily available . Basically it would be a good idea to have some sort of circulation within gearshaft assemblies by having some scroll grooves to get oil inside and an exit as well. I tried to do so but am not quite sure about suitable philosophies to achieve this. In our case here I´d have max. 2 thou play in the bush.

Vic
 

Cyborg

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Oilite/porous bronce seems a bit absurd to me in a gearbox with oil filling.

Vic

I don’t disagree, but if absurd really bothers you, then why own a Vincent?
Same theory goes for cam bushings.... oilite seems to survive a lot better. I still think it’s a better alternative especially for the ones that don’t end up getting sufficient clearance or have questionable oil supply. There are ways to get around these things, but then you need to totally re-engineer the entire motorcycle. I think in a lot of cases, the bushings don’t get sufficient oil and some folks installing them don’t have the tools to install them properly and measure the clearance.... assuming they know that the clearance should be checked and know how to correct it. Probably lots of bushings getting the ends mushroomed over when installing, so that helps to keep oil out. Oilite is more tolerant?
 

greg brillus

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It is a clearance issue in most all cases.........The gearbox bush would fail due to either insufficient clearance and/or too tight a rear chain.........If longer rear springs are used,it is very common for the chain to hang loose..........if this is not done then the chain will be too tight in service. The gearbox oil level can drop, bleeding into the primary unnoticed.........this could be a culprit as well. I have cut a scroll into the large bush to feed oil into this cavity, but I feel it probably causes a leak from the outer bush due to the pumping action like a mini oil pump. i have heard of folk who drill a small hole though the output shaft to feed oil between the 2 bushes..........It is hard to say what is a total fix, but the modern 5 speed box tends to have small needle rollers in place of these bushes and a tiny seal between the shafts at the sprocket end.
 

john998

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Hello, Also too little clearance between the hollow shafts and the constant mesh pinion cuts off the oil. 1/16th holes drilled in the taper at the end of the hollow shafts should fix that problem.
I used phosphor bronze once but it soon failed, I think this contributed to the failure of the cam plate spindle boss in the crank case.
Best to stick to oilite to be safe, although lead bronze is an excellent bearing material for many applications. John
 

Bill Thomas

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Last time I put mine together, As well as V notches, I packed it with Copper slip,
Something we were told to do with the Mini Car, British Leyland, Primary Gear,
I was never happy with the oil supply there, As well as a lube' it was to act as an oil seal ?.
Back in the 70s.
 

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