Head gasket

Information relating to the Matchless G12 or AJS Model 31 650cc twin
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Head gasket

Postby bob121 » Fri Feb 14, 2020 2:41 pm

Took the aj out for a few of hours this morning. On the way home, less than 10 miles away I heard a concerning noise. Pulled over and had a quick inspection. Sounded like it was blowing from somewhere, so I carefully carried on home. Just had another look now it's cooled down. One of the head gaskets has blown..... OUT😱.
Can see it hanging out the side of the head, almost an inch of it is visible.

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Re: Head gasket

Postby les ward » Fri Feb 14, 2020 10:06 pm

I have gone with the copper head gaskets from Steven at AMC Classic Spares. They have worked well and don't need such frequent torquing down as the composite ones.

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Re: Head gasket

Postby bob121 » Sat Feb 15, 2020 3:46 pm

Looks like that may have to be considered. I've got to pull the motor down anyway. It's been leaking from the filter tunnel for a while. I've bought some high energy grey sealant. I used hylomar black originally. Oh well.

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Re: Head gasket

Postby ajscomboman » Sat Feb 15, 2020 4:05 pm

Well by the time you get to pulling the motor down hopefully the parts service will have the correct copper ones back in stock, we're just waiting for the viton o-rings to be delivered and then they'll be listed.

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Re: Head gasket

Postby 1608 » Sat Feb 15, 2020 5:13 pm

The use of doubling up on head gaskets was an old bodge of over coming warped heads, before simply re-newing gaskets ensure the heads are flat. A decompression plate under the barrels is the correct way of lowering compression. I know its a matter of choice, but I've never had any problems with the standard composite head gaskets, but its important to enlarge the push-rod holes to prevent the gaskets chafing on the pushrods.
I tried copper gaskets but had no luck preventing leakage. I believe too higher torquing of the head bolts is required. Just my observations, others may have had better luck.

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Re: Head gasket

Postby bob121 » Mon Feb 17, 2020 8:22 am

I don't remember modifying the holes for the push rods when originally assembling. I'll make a decision of which way to go after pulling it down and inspecting. My preference would be composite, but we'll see. Got a few things to do before I can start on the bike.

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Re: Head gasket

Postby raffles » Mon Feb 17, 2020 6:50 pm

I have never had a head gasket blow have always used composite gaskets from the club.
However I always resurface the head every time. Just spread fine grinding paste on a good
thick sheet of glass lubricate with 3in One oil press down firmly and move the head back and
forward then check for overall light grey colour on the face.It is what I was taught as an
apprentice and has always worked for me. Plus I torque the head to 28 ft lbs
Tony

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Re: Head gasket

Postby Groily » Tue Feb 18, 2020 7:18 am

1608 wrote:I tried copper gaskets but had no luck preventing leakage. I believe too higher torquing of the head bolts is required. Just my observations, others may have had better luck.


I also found I had a very small oil weep on one side I couldn't get rid of on my Mod 31, using well-annealed copper and viton seals with heads and barrels 'T&L flat'. I am just reassembling the top end with composites this week to see how they do, after replacing a rattly pushrod end cup.

Tony, when you go to 28ft/lb, is that on 3/8 studs, or on 5/16ths? (Seems quite a lot if the latter?) I've never gone beyond 20ft / lb on my 500, always on composites and nary a problem, but am wondering for the larger motor (also 5/16th) what's ideal. I was always taught that evenness (and a rigorous re-torquing regime in the early days) was more important than actual tightness - but somewhere there must be a happy medium. It would be nice to find it!

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Re: Head gasket

Postby MalcW » Tue Feb 18, 2020 9:34 am

Page 25 of the 1957 - 1964 Al Models Workshop Manual states that you should torque the 5/16 bolts to 22ft lbs, and I was told that this is the generally accepted figure. That's what I use on my 650 with composite gaskets, and it's been fine so far.

Malc

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Re: Head gasket

Postby Groily » Tue Feb 18, 2020 11:15 am

That's reassuring to read Malc.
Neill quotes 18 for the earlier 500s, but 'tis because of the 22 in the later info for same size fasteners that I go 20 on mine.
22 is what I've done the 650 to (with copper hitherto).
After the normal re-torques and valve adjustments, hopefully all will be well for a good long time at that on composites.

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