Knackered con rod

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

Postby JEAN-NOEL » Fri Oct 04, 2019 5:58 pm

I think about a lack of lubrication : in this case the conrod on the drive side fails always before the other. If your bike has an oïl pressure discharge valve (it was discarted some years) it could have been stuck open by a crap, causing a drop of pressure.

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Re: Knackered con rod

Postby 1608 » Fri Oct 04, 2019 8:42 pm

I don't believe a filter is a good idea on the feed side, although arguably fine on the return! These engines have been good for thousands of miles on just a regular oil changes every 1000 miles or thereabouts with the original filters in use. The problem is due to low oil pressure due to a filter on the feed line. The chamfering of the oil hole on the b/end journal is only a mod to encourage a spread of oil btn journal and shell,
it won't compensate for any crap that has built up in the sludge trap.

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Re: Knackered con rod

Postby McBob » Sat Oct 05, 2019 10:00 am

Can't agree that a modern filter on delivery side is inferior to a piece of felt, but I do appreciate the discussion.

Will add a pic of the bearings and cap.

I tested the relief valve with a calibrated gauge and it started to open at 60psi

Any views on the centre web bearing clearance? For future reference as I have committed to the above.
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Re: Knackered con rod

Postby JEAN-NOEL » Sat Oct 05, 2019 11:00 am

Hi McBob,
you have two spin on filters (cannister type), on delivery side, and on return side.
I agree with you that a modern filter is far better than the felt one.
But how did you fitted the new filter on the delivrery side ? On a special made device to drive oïl Under pressure in the spin on filter ?


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Re: Knackered con rod

Postby JEAN-NOEL » Sat Oct 05, 2019 4:02 pm

What a good job ! Now the mystery is thickening! The filtration is perfect, the pumps in good condition, the discharge valve opens to the desired pressure. I really do not see what could have caused this damage...Opening the return side filter with a spécial tool (to avoid filings) will permit to see what craps are there.

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Re: Knackered con rod

Postby McBob » Sun Oct 06, 2019 10:01 am

Don't think the bearings have spun, picture below

Debris from the bearing in the filter, but the filter wasn't blocked up
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Re: Knackered con rod

Postby JEAN-NOEL » Tue Oct 08, 2019 7:41 am

Obviously the shells ran into the conrod : a shell tang has made groove in it. Generaly the rod breaks and makes a big hole in the drive side crankcase... Regarding the hole in the bottom shell, the white metal has melted and has filled the hole.
The breakdown happened during a long ride, or just after starting ?
Did you fit an anti sumping valve in the feed line ?

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Re: Knackered con rod

Postby McBob » Tue Oct 08, 2019 10:58 am

Hi Jean noel, no anti sumping valve, the pump is modified and prevents this.

I was 10km in to the ride doing 80kph on a flat road. Yes very lucky with the crank case.
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Re: Knackered con rod

Postby Duncan » Tue Oct 08, 2019 8:08 pm

in the "journal.jpg" it looks like the conrod has been modified to take 60 thou mini shells, it might be an optical illusion what does the good rod look like ?

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