Porous Head ?

Information relating to the Matchless G3 or AJS Model 16 350cc Heavyweight
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Re: Porous Head ?

Postby superbike62 » Tue Sep 26, 2017 8:01 pm

The con rod moves radially at the little end , it's not a linear sideways movement it's a arcing motion but no lift up or down . I am only asking because all the engines I have rebuilt had white metal bearings not rollers as this one has . I have a 40 year career as a maintenance fitter so I do know a little bit . The last crank i had rebuilt was a Suzuki gt750 which cost me. £800 , if this needs replacing I will do it , was hoping you guys might know .

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Re: Porous Head ?

Postby clive » Wed Sep 27, 2017 4:14 am

It's always difficult to diagnose on the Internet but more than one member has said if there is no up and down movement in the big end its probably good for some miles yet. You do need to test it at TDC as this is where the wear will first start to show. If there is some radial movement I would check there is no similar rock in the small end bush. If there is replace it and all will probably last a long time. Did you check the ring gaps in the bore as well as their alignment? Finally what year is the head, is it possible the oil leak is creeping round from the behind the pushrod tubes at the top, or leaking from a slightly short tappet adjustment cover rubber (had this one myself recently)
clive
if it ain't broke don't fix

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Re: Porous Head ?

Postby superbike62 » Wed Sep 27, 2017 6:22 am

Hi Clive ,
the ring gaps in the bore are .006" so no problem there . The head is the alloy one as fitted to a 56 . This oil staining on the fins is mainly on the opposite side to the pushrods , it's in between any gasket face and no path from either above or below , hence my original thought on a porous head . We have a dye pen test at work which shows up anything porous , so when I have stripped the valves out and degreased the casting I am taking it in.
The inlet valve is leaking very bad through the seat so maybe this staining was from the carb spitting vapour back out through the inlet .

I would normally replace a bearing at work if it moved like this , I guess it would last a long time but I just hate knowing I should of changed it when I had chance .

Thanks to all for their help and opinions.

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Re: Porous Head ?

Postby Peter Morris » Sun Oct 01, 2017 9:03 pm

I have just had a brand new big end, purchased at Russell Motors, fitted to my 1959 G3C...... There is some rocking movement in the con rod.

Peter

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Re: Porous Head ?

Postby 56G80S » Mon Oct 02, 2017 9:09 pm

Have to say, I have built up the crank on my G80S although a long time ago and with someone who not only knew his stuff but had a massive peice of kit for lining up the flywheels. Unfortunately it was back on the day of the rubbish one piece pins and failed after a few thousand miles; was very smooth up to that point!

On the rock. mine has had this for the last 8,000 miles and probably before that but there is no up and down movement at all.

As mentioned above, I have just replaced the small end bush and all good. Is some noise from the camwheel end float that I'll shim after the Alternative.

Johnny B

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Re: Porous Head ?

Postby 1608 » Tue Oct 03, 2017 9:47 am

Have to say, 6thou ring gap sounds a bit tight. I'd have thought 12thou was nearer the mark. But, if it has been running like that then lucky you!

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