Oil leaking from head gasket

Information relating to the Matchless G11 or AJS Model 30 600cc twin
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Oil leaking from head gasket

Postby silverspur » Fri Oct 20, 2017 11:19 am

After a hard week's riding on holiday my M30 is leaking serious quantities of oil from the head-barrel join and the barrel-crankcase. This 500 miles after a so-called engine rebuild after which the head nuts had been re-torqued.
I've had the head off and the the oilways are clearly leaking although the head gasket itself is sound around the combustion chamber. Is there a known fix? Anyone ever tried small O-rings?

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Re: Oil leaking from head gasket

Postby dave16mct » Fri Oct 20, 2017 12:33 pm

Are the head gaskets solid copper or composite? I've fitted 'O' rings with solid copper. It worked very well.
Dave.

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Re: Oil leaking from head gasket

Postby silverspur » Fri Oct 20, 2017 9:22 pm

Thanks Dave. They're composite but I think I have found the problem: the gaskets are slightly oversize so they overlap each other between the heads by a couple of millimetres. This meant that they rode up to form a ridge which had prevented the heads bolting down flat, leaving a nice gap for the oil to squirt out. Rebuilt engine! Hah!

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Re: Oil leaking from head gasket

Postby Groily » Sat Oct 21, 2017 7:14 am

Worth checking at the same time that the holes for the pushrods correspond precisely with the castings. If not the pushrods can be rubbed severely, and weakened. There will be witness marks on the alloy tubes if there has been contact. (And the oil holes for rocker lube need to be aligned too, for that matter).
There have been a lot of gaskets that aren't quite correct in all these areas, also in terms of thickness. Worth checking that the spigot/recess on the barrels/heads doesn't bottom out before the gaskets are compressed, as that will also cause a flood and/or a loss of compression. Some gaskets are too thin. Some people - me for one - have milled a few thou off the spigots before now to be sure on this point. Just from memory, you need about 70 thou thickness on the gaskets, give or take.

A lot of folk have worn this particular T shirt over the years. Someone not used to assembling these engines won't always spot the issue until it is too late beacuse they assume the new parts will fit. It doesn't (necessarily) mean the rest of the mechanical work hasn't been done properly although it's not a great indication, of course!

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Re: Oil leaking from head gasket

Postby silverspur » Tue Oct 24, 2017 5:45 pm

Thanks Groily. I checked the head clearance at 50 thou and the gaskets are 150 thou so, hopefully, that will be enough compression.

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