Porous Head ?

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

Postby superbike62 » Mon Sep 25, 2017 9:58 am

Hi ,
I have a 56 G3LS which has developed a strange fault . Basically the engine started to not return to tickover , was running on when trying to change gear , and will not tickover as before either dies or builds revs . I fitted a new Amal carb 2 years ago without issue , but just to be sure I refitted the old carb and it ran the same.
I noticed a oily residue on the cylinder head fins near the inlet tract . There is no leak from the head gasket or rocker chest so could it be the head is porous ?.
I do know my bike has had the valves converted to coil spring , also the engine breather is doing a fine job of stopping the rear wheel going rusty ! . I am going to strip it down and maybe do a dye pen test on the casting and see what the bore and piston condition is in .

Any thoughts please , am I missing the obvious ?.

Thanks Paul

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Re: Porous Head ?

Postby Rob Harknett » Mon Sep 25, 2017 12:43 pm

I assume the cables not sticking and AR unit not sticking, in need of a good flushing of petrol/white spirit to clean it out. I think your problem will more likely be head gasket blowing than porous head. I had oil leak from the gasket some time ago on my 55 350. I never even considered a porous head, just a gasket joint problem. I gave the head and barrel surfaces a good clean. I then ground the two surfaces with grinding paste. Just circular a movement holding the head. After very little work I had clean dull grey surfaces. Re annealed the gasket and put it back together again. Years later, still no oil seeping out the gasket joint. I have done the same with little 2 strokes like BSA Bantam. Then put the head back on without a gasket, just a very thin sear of red hermatite. It must have upped the CR a bit, went like a rocket after that. I actually beat a 7R AJS to a local fishing lake along country lanes. My mate could not believe he could not keep up with me on his 7R. Even though he tried to, just using low gears to get revs up round all the country road bends.

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Re: Porous Head ?

Postby superbike62 » Mon Sep 25, 2017 2:54 pm

Hi Rob,
thanks for info , I don't have this residue anywhere on the gasket fin area it's in between head and rocker box . Cables are ok AR unit ok . Still doesn't explain the heavy breathing from bottom of engine which makes me think there is some blow past into crankcase . The poor idling etc is only a recent thing , the breather issue has always been there during my ownership.
Anyway the heads coming off tonight so we shall see.

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Re: Porous Head ?

Postby Rob Harknett » Mon Sep 25, 2017 3:14 pm

Sorry, I missed it was cylinder HEAD fins. I would not be too concerned with the top end oil leak. That may easy be cured. If you are happy with the carb and feel that's all ok, having tried two. Perhaps the piston may show you something sinister. Does it back fire trying to start it now?

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Re: Porous Head ?

Postby superbike62 » Tue Sep 26, 2017 9:39 am

This is what I found , inlet valve is leaking through the seat which explains the poor running , so this will need looking into.

I removed the barrel and the bore looks ok ,but all 3 piston ring gaps were in line with each other !, this is obviously the cause of the excess crankcase pressure .

However , the big end bearing has a fair amount of sideways rock on it , so looks like a total strip down and do it all properly .

It's testament to the build quality that it ran as well as it did , it deserves to be rebuilt to how it should be ,will last me a lifetime .

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Re: Porous Head ?

Postby Rob Harknett » Tue Sep 26, 2017 11:48 am

Wow, how did the rings all line up. You do get side play on the con rod. You are probably holding the rod at the top to rock it. So it feels a lot. That would not worry me too much if I have found no up & down.

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Re: Porous Head ?

Postby superbike62 » Tue Sep 26, 2017 3:21 pm

first thing I noticed as I pulled the barrel off , all 3 ring gaps were lined up , so whoever was in there before obviously got it wrong. Why would the big end rock if it was ok ? , I don't mind stripping it down !!.

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Re: Porous Head ?

Postby 1608 » Tue Sep 26, 2017 4:44 pm

Are you sure it is rocking and not just side to side play. A little s to s doesn't matter, there should be no up and down movement as Rob has said. A hone and new rings may be all you need. Have the barrel measured.

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Re: Porous Head ?

Postby Pharisee » Tue Sep 26, 2017 4:52 pm

superbike62 wrote:first thing I noticed as I pulled the barrel off , all 3 ring gaps were lined up , so whoever was in there before obviously got it wrong. Why would the big end rock if it was ok ? , I don't mind stripping it down !!.


But you might mind the price of a new big end. :D
Back in the day, I bought a second hand 1957 G9 for less than 1/10th of the price I had to pay for a new crankpin assembly earlier this year!!

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Re: Porous Head ?

Postby Rob Harknett » Tue Sep 26, 2017 5:19 pm

1608 wrote:Are you sure it is rocking and not just side to side play. A little s to s doesn't matter, there should be no up and down movement as Rob has said. A hone and new rings may be all you need. Have the barrel measured.

Don't forget you are feeling at the top end of the conrod. Like adjusting wheel bearings you tighten then back off a bit. The play fell by grabbing the tyre. Feels a lot , but OK. Also remember the engine is cold. There should be all most no up and down. Are you skilled at rebuilding the bottom end. I'm not and would not attempt to do one of my bikes to discover if I could get it right. Read the book, thought no, I have not even got all the tools for the job. If you are concerned take the bottom end to some one for an expert opinion. Unless of cause you've done the job mant times in the past. Judging from your questions, I do not think you have. This book not for you bike but good for reference re tolerances.
http://archives.jampot.dk/book/Workshop ... ndards.pdf

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