G9 crankcase "catastrophic" failure
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G9 crankcase "catastrophic" failure
A friend brought me his recently purchased G9, asking me to fix an oil leak from the primary chaincase.
The picture explains the source of that. It's clear that the engine was very carelessly assembled (can you
believe NO rollers in the clutch basket bearing ?), but I'm curious to know what could have caused the crankcase
to fail in this way. Is this a known weak point in the cases (both halves have matching numbers so misalignment
is probably out) ? The bike probably hasn't covered more than a couple dozen miles since the rebuild.
Any specific assembly error that could have caused this ? Thanks for sharing any ideas or experience.
JP
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- robcurrie
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Re: G9 crankcase "catastrophic" failure
It could have been caused by pressing or driving the main (roller) bearing outer into the casing. It should be dropped in by heating the casing and cooling the bearing.
Rob C
Rob C
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Re: G9 crankcase "catastrophic" failure
Yes, that's one possibility, although hardly believable someone would pound the bearing into the unsupported bottom
of the case... Is it certain that all G9 crankshafts fit all G9 cases lengthwise, or is there a need to check for endplay when
fitting a new crank ? The search is on for new cases, so the question will come up. It's a vital precaution for some engines,
what about this one ?
of the case... Is it certain that all G9 crankshafts fit all G9 cases lengthwise, or is there a need to check for endplay when
fitting a new crank ? The search is on for new cases, so the question will come up. It's a vital precaution for some engines,
what about this one ?
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Re: G9 crankcase "catastrophic" failure
Hi jP,
My 350's original crankcases are like this - see the picture here http://www.jampot.com/forum/viewtopic.php?f=17&t=18013
I haven't come up with a solution yet but I know that that part of the crankcase is incredibly thin - the diameter of the bearing is almost as big as that part which has sheared off I can't remember if the wall thickness is 1/8" or 1/16" there.
My theory at the moment is that the dynamo chain has been slack and "machined" through the metal there. I have another set of crankcases where the process is partially complete but it hasn't broken through yet.
I now have 3 sets of crankcases but all have their problems e.g. oil pipe tapped holes stripped or burst and oil pump pinion housing scored etc.
Still looking for a solution.
Good luck
John
My 350's original crankcases are like this - see the picture here http://www.jampot.com/forum/viewtopic.php?f=17&t=18013
I haven't come up with a solution yet but I know that that part of the crankcase is incredibly thin - the diameter of the bearing is almost as big as that part which has sheared off I can't remember if the wall thickness is 1/8" or 1/16" there.
My theory at the moment is that the dynamo chain has been slack and "machined" through the metal there. I have another set of crankcases where the process is partially complete but it hasn't broken through yet.
I now have 3 sets of crankcases but all have their problems e.g. oil pipe tapped holes stripped or burst and oil pump pinion housing scored etc.
Still looking for a solution.
Good luck
John
- clive
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Re: G9 crankcase "catastrophic" failure
Or that they could assemble the clutch without any rollers, the basket would flap all over the place as would the chain, perhaps causing damage to the crankcase. Some evidence of chain damage of the face of the broken piece.jp wrote:Yes, that's one possibility, although hardly believable someone would pound the bearing into the unsupported bottom ..........?
clive
if it ain't broke don't fix
if it ain't broke don't fix
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Re: G9 crankcase "catastrophic" failure
Well, there is some trace of (light) contact between the chain and the boss face, but the break is a clean fracture with
matching surfaces, not sawn through, and we didn't find any of the alloy filings it would have left behind.
As John says, the case wall is very thin at that point, clearly not meant to take any axial load. That's why I was wondering
if it was routine to make a trial assembly and check for side clearance. I haven't done one of these engines before, and with the
center web and three-bearing support, it might not be so simple ! So far I haven't found a tech article on crankcase reassembly.
matching surfaces, not sawn through, and we didn't find any of the alloy filings it would have left behind.
As John says, the case wall is very thin at that point, clearly not meant to take any axial load. That's why I was wondering
if it was routine to make a trial assembly and check for side clearance. I haven't done one of these engines before, and with the
center web and three-bearing support, it might not be so simple ! So far I haven't found a tech article on crankcase reassembly.
- 1608
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Re: G9 crankcase "catastrophic" failure
Is the bearing the correct width ( not too wide). Are the centre web thust washers in place, which would prevent the crank from being pulled too far to the drive side. Is there a spacer in place btn drive sprocket and inner bearing race. All these factors could cause the crank to be pulled too far to the drive side. All seems a bit obvious I know, but if the clutch rollers were omitted then who knows!
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Re: G9 crankcase "catastrophic" failure
Hi
My money's on the sprocket spacer not being installed / being an incorrect item - when the rotor nut was tightened the crank was pulled to the left fracturing the crank case bearing housing.
Regards Mick
My money's on the sprocket spacer not being installed / being an incorrect item - when the rotor nut was tightened the crank was pulled to the left fracturing the crank case bearing housing.
Regards Mick
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Re: G9 crankcase "catastrophic" failure
I agree with Mick. And now, how is the centre web ?
- 1608
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Re: G9 crankcase "catastrophic" failure
Think you are probably right Mick.