G9 crankcase "catastrophic" failure

Information relating to the Matchless G9 or AJS Model 20 500cc twin
jp
Posts: 4
Joined: Tue Aug 16, 2016 12:04 pm
Location: BELGIUM BELGIUM

G9 crankcase "catastrophic" failure

Post by jp »

rsz_dsc_0391.jpg
Hi everyone,
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
You do not have the required permissions to view the files attached to this post.
User avatar
robcurrie
Member
Posts: 957
Joined: Fri Dec 09, 2011 5:33 pm
Location: Limpopo SOUTH AFRICA
Contact:

Re: G9 crankcase "catastrophic" failure

Post by robcurrie »

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
jp
Posts: 4
Joined: Tue Aug 16, 2016 12:04 pm
Location: BELGIUM BELGIUM

Re: G9 crankcase "catastrophic" failure

Post by jp »

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 ?
Johnobirches
Member
Posts: 155
Joined: Mon Jul 08, 2013 10:19 am
Location: Lancs. UK

Re: G9 crankcase "catastrophic" failure

Post by Johnobirches »

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
User avatar
clive
Member
Posts: 5657
Joined: Mon Jan 01, 1990 12:00 am
Location: LONDON UK

Re: G9 crankcase "catastrophic" failure

Post by clive »

jp wrote:Yes, that's one possibility, although hardly believable someone would pound the bearing into the unsupported bottom ..........?
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.
clive
if it ain't broke don't fix
jp
Posts: 4
Joined: Tue Aug 16, 2016 12:04 pm
Location: BELGIUM BELGIUM

Re: G9 crankcase "catastrophic" failure

Post by jp »

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.
User avatar
1608
Member
Posts: 2493
Joined: Mon Oct 05, 2009 1:00 am
Location: ESSEX UK

Re: G9 crankcase "catastrophic" failure

Post by 1608 »

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!
Mick D
Posts: 2886
Joined: Mon Mar 24, 2014 5:44 pm
Location: Aberdeenshire

Re: G9 crankcase "catastrophic" failure

Post by Mick D »

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
JEAN-NOEL
Member
Posts: 601
Joined: Wed Oct 12, 2005 10:45 pm
Location: FRANCE

Re: G9 crankcase "catastrophic" failure

Post by JEAN-NOEL »

I agree with Mick. And now, how is the centre web ?
User avatar
1608
Member
Posts: 2493
Joined: Mon Oct 05, 2009 1:00 am
Location: ESSEX UK

Re: G9 crankcase "catastrophic" failure

Post by 1608 »

Think you are probably right Mick.
Locked