Knackered con rod

Information relating to the Matchless G12 or AJS Model 31 650cc twin
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Knackered con rod

Postby McBob » Tue Aug 27, 2019 10:59 am

I have had an enjoyable 50 odd miles on my new pistons and now the drive side con rod has shat itself. Leading into sorting this next issue out, I would like to understand why.

Has the bearing failed? Seems to have gotten very hot, lack of oil? The hole in one shell is blocked and the journal oil way had a shallow plug of debris in it.

I note in the workshop manual the requirement of re-chamfering the journal oil hole after a regrind, this it seems was not carried out when the engine was rebuilt in the 90's. Could this be the cause of the failure?

Replacement con rods, Ridge crest engineering no longer make them. Thunder engineering make g45 rods, are these suitable? Are there any other options apart from chancing second hand rods?
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Re: Knackered con rod

Postby SPRIDDLER » Tue Aug 27, 2019 11:13 am

You've posted this under 'Buyers Guide' rather than the main forum, e.g. 'Help' or 'G12' so views/assistance may be limited.
I know nothing about twins but I think you've answered your own question since if the oilway hasn't been chamfered the 'sharp' edge of the hole could have removed material from the shells and blocked it. Or it could be debris from elsewhere blocking the oilway causing the bearing to overheat/seize. ...........?
Last edited by SPRIDDLER on Tue Aug 27, 2019 2:59 pm, edited 1 time in total.
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Re: Knackered con rod

Postby McBob » Tue Aug 27, 2019 2:11 pm

Cheers Neville, might not be the best sight in the buyers guide. Would a kind moderator please move this to help section, least I end up littering the forum with posts like so much swarf in the bottom of a crank case?
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Re: Knackered con rod

Postby SPRIDDLER » Tue Aug 27, 2019 3:09 pm

Hmm, now that you've posted the latest pic showing the horrible collection of metal shavings in the c/case I've gone back to the conrod pic and noticed the damage at the side of the big end. In that case, as a singles man I wouldn't like to suggest what could've caused such extreme damage except that as the conrod bent the big end end would've perhaps been displaced laterally on its journal into contact with the flywheel........ :?
If a Mod doesn't move the post redo it in the 'Help' section and don't worry about 'littering' the Forums. This one can always be deleted later.
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Re: Knackered con rod

Postby Mick D » Tue Aug 27, 2019 4:05 pm

Hi and Ouch!!

This looks like a classic case of a blocked crank shaft sludge trap starving the big end of oil. Photos can be deceptive but the bearing shells look like the white metal has been worn away and the crank pin bears evidence of lack of lubrication. Once things went into meltdown the con rod absorbed the energy and distorted, possibly causing the shavings in the sump, (are they aluminium or white metal?).

Regards Mick

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Re: Knackered con rod

Postby ajscomboman » Tue Aug 27, 2019 6:10 pm

On a positive note, the Thunder Engineering rods are fit for purpose, but you should also be ok getting a standard rod and have it checked for roundness by T & L

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Re: Knackered con rod

Postby raffles » Tue Aug 27, 2019 6:19 pm

Rob
the guy is in New Zealand. T&L ?
Tony

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Re: Knackered con rod

Postby ajscomboman » Tue Aug 27, 2019 7:08 pm

Didn't look at location, however the statement stands, a used rod is ok, need checking for roundness though.

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Re: Knackered con rod

Postby McBob » Wed Aug 28, 2019 9:59 am

Cheers guys, the debris is a mix of bearing and rod with some larger shavings off the cases.

I am having the crank crack tested before continuing, the engine re-conditioner said it was difficult to determine the cause due to the amount of damage to the rod but it was not what he would expect to see from oil starvation (i.e. the bearings were not blue from heat).

Strip down has also revealed wear to the cams and followers, any other area worth investigating with the engine in pieces?

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Re: Knackered con rod

Postby SPRIDDLER » Wed Aug 28, 2019 11:22 am

McBob wrote: Strip down has also revealed wear to the cams and followers, any other area worth investigating with the engine in pieces?

Oil pump(s)?
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