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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Re: Oil leaking from head gasket

Postby 57G11 » Tue May 01, 2018 2:16 pm

” My expensive professional 1957 G11 engine rebuild in 2005 gave rise to the new pistons nipping up; ‘run in for 3000 miles as the pistons are slightly different alloy spec’ was the rebuilders advice. By 2013 It would still run hot and nip up. By 2015 I thought drastic action is needed so: hone the barrels, relieve the piston high spots yet again, and fit new rings. The outcome; it was starting to free up, ie 75mph potential. YES, 10 years of regular but short distances due to the nipping up took 10 years of frustration and lack of confidence, an outlook that started after the bottom end rebuild by Joe Francis Motors in the 1970’s was faulty as to the centre web bearing chamfer angle, (A major cause of concern in the 1970’s). 3000 miles of use meant JF accepted no responsibility for the wrecked engine. Cosworth reground the crank and I fitted new barrels and pistons, only gave rise to piston slap due to the n.o.s. wire wound piston skirt collapsing when warm, {found to be the problem when the professional rebuild happened in 2005}

With the improved engine revs came a significant oiling problem: timing case pressure was forcing oil through the dynamo bearing. A new sealed bearing meant the oil filter tunnel then leaked,; new end washer meant the cylinder head gaskets leaked, new copper gaskets = still leaking. New head gaskets this year gave rise to wet sumping and oil as a constant flow out of the Primary chain case when the engine was started. My diagnosis: oil being pumped in but not returning that well, as evidenced by sporadic flow return of minimal pressure. Blocked oil way/ blocked return pipe/ faulty seal on the new in 2005 uprated oil pump / worn pump (brand new 3000 miles). Draining the sump and refilling the oil tank gave rise to about 2 minutes of no oil flowing from the chain case. Cylinder pressure is 100 & 105

Off with the timing chest and check oilways, all OK. New oil filter and check oilways, all OK. Off with return pipe etc, all OK, oil tank breather not blocked. Off with oil pump: return pump in the correct slot, no air leaks, not worn. Replace oil pump and start engine with timing chest cover off. A massive gush of oil from the new (fitted by engine rebuilder 2005) design uprated oil pump relief valve. Strip valve to check if seating is OK, and it was, BUT the spring pressure was only between 5 and 10lb, ie WAY BELOW THE START UP OIL PRESSURE. I have packed out the spring to give 25 to 30lb relief pressure and now the oil return is more consistent and there is a constant flow into the oil tank. The manual talks of start up 110lb pressure if a relief valve is fitted, so SHOULD I UP THE PACKING TO > 25 TO 30LB?

I am not sure if my diagnosis and solution is acceptable, hence I have only done a short run so far: it did not seize and seems more lively, perhaps due to less pressurized oil below the pistons?

Perhaps all along the piston alloy was not the problem, but the oil pump relief valve spring pressure was?

Missing yet another decade of none use during my 50 years ownership!!!”.

So I am hoping to enoy many miles of use as I did in my teens and early 20’s prior to the first rebuild. Without the +60 high compression pistons as fitted when I bought it, I am not expecting the ‘ton plus’ performance I had as a lad, but reliability would be nice as would the speed potential to stay with modern traffic

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