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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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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.
 

oexing

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Due to oil starvation the brass bush seized on G 3 gearshaft. Seizure expanded the bush and cracked the thinwalled G 4 drive shaft. How does the G 3 look , brass remains on it ? Or shaft/bush fit extremely sloppy ? Please add more photos .

Vic
 

greg brillus

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Has the gearbox been together for some time or apart recently........i wonder if the input shaft was pressing on the end of the output shaft...........it is showing signs of this. The input shaft should have at least a half a mm of end float with the G 2 cover fully tightened home.
 

greg brillus

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Haven't seen one split like that before..........I know it might sound extreme, but I wonder if the mating gear which would be the G 8 or a G 8/1 is in fact the wrong gear and has been causing bad or incorrect meshing, thus overloading the tips of the teeth.......Fit the layshaft and this mating gear back into the box to see if this might be the case.
 

Alyson

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Haven't seen one split like that before..........I know it might sound extreme, but I wonder if the mating gear which would be the G 8 or a G 8/1 is in fact the wrong gear and has been causing bad or incorrect meshing, thus overloading the tips of the teeth.......Fit the layshaft and this mating gear back into the box to see if this might be the case.
since the '80s ? I'd think if it was the wrong gear trouble would have started some time ago. I don't see any bluing in the shaft/bushing so must have had the oil level correct, and I don't want to start talking about the dip stick :O
 

TouringComet

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Not counting consumables like pistons and valve jobs, this is only the second failure for the Shadow. My father had an Alpha big end installed in the late 50’s but did not put the engine back together until the 80’s.
 

greg brillus

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Maybe it's simply a matter of the shaft being hardened too much, the G16 bush could have too much interference and the shaft has just let go on the weakest points at the base of each spline.
 

Bill Thomas

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I think Steve must have cleaned up the shaft a bit,
I would think it showed a bit of colour !!,
Would a tight rear chain stress it ?.

For me another reason to keep the full 2 UK pints of oil in there ?.
 

Peter Holmes

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Hello, I have had a G4 fail in the same manner, luckily not so catastrophically. The bushing looks like the correct oilite.
Just old age, much like many owners!
John.
A very good point, I suppose even our beloved Vincents are not totally immune from metal fatigue, and like most of their owners they are getting on a bit, and lets be honest, when the clutch has occasionally not functioned as it should who hasn't been a bit rough with the gear lever just to get you home, I certainly have been guilty of that over the last 56 years.
 

vibrac

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The twin gears are tough compared to most classics but perhaps they aint 'Burman tough'. 10,000 DR's boots stamping down throughout WW2 saw to that testing
 

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