Head gaskets

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

Post by MalcW »

I've taken the heads off to change the head gaskets. I was under the impression that they only came in copper or composite, but these are alloy. Were they ever available, or has someone made them? They do seem a bit amateur. Also, there is evidence of all four pushrods having rubbed against something, and one of them is slightly bent.

Lastly, the pistons don't have any '+' numbers, so I assume they're standard size, but they do both have a 73 on the crown. Is that significant?

Cheers,

Malc
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ajscomboman
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Re: Head gaskets

Post by ajscomboman »

72mm is standard. The pistons I would assume are GPM.
g5wqian
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Re: Head gaskets

Post by g5wqian »

the pistons i have are GPM from the club and do have 73 on them although i dont know if that relates to bore size exactly , i am using +040 pistons with a 2.880" finished bore size .

i have used copper and composite gaskets but not seen alloy ones , you need to make sure they are thick enough though if made from copper or alloy , should be minimum of 1.5mm .

i did buy some copper ones from club a few years ago and they were incorrect thickness , being only .092mm thick instead of the advertised 1.5mm .

the composite gaskets come out at 1.7mm thick .

if push rod is bent id check the clearance between piston and valve , also check the spigot on the cylinder barrels hasnt been machined down , its at least 2mm depth normally , someone might have attempted to raise the compression ratio but in process could have made contact between piston and valve in one area , so measure thickness of alloy gasket while your doing things .

compression pressure would normally be around 130-140psi for a standard compression engine , 10 kicks wide open throttle .

i use thick base gaskets and composite head gaskets on my twin and it runs just fine on unleaded fuel but when i used it with the thin copper head gaskets and thin base gaskets the compression ratio was a bit higher and i would get "pinking" occur going up hill .

all the best .
ian

,
Groily
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Re: Head gaskets

Post by Groily »

Pretty sure the 73 means +40. (What with 1mm being all bar 40 thou.)
Just fitted some GPM '68.5' ones to a Notrun turned Nowrun - that is +20 for them.
Never seen alloy gaskets though, I must say. Composites and copper, but not alloy.
Almost all gaskets need a bit of clean up round the pushrod tunnels to avoid scuffing.
Loads of pushrods have quite deep witness marks worn into them, in some cases with a serious risk of weakening as the tubes aren't that thick-walled. I've replaced a few over the years
MalcW
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Re: Head gaskets

Post by MalcW »

Hi

Thanks for all of the information. Once ajscomboman said that 72mm is standard I did a quick sum and came to the same conclusion; i.e. my pistons are most likely +40 thou. The bores are good though, with no appreciable step at the top, so they will hopefully be good for a while yet.

As far as the slightly bent rod is concerned, there is no evidence of valve/piston contact, so quite why it is like that I don't know. I think for peace of mind I'll replace all 4 of them, and I think I'll go for composite head gaskets. I haven't checked the sealing of the valves yet; I'll do the petrol test first, reseat if necessary, and then do a compression test once reassembled.

At the moment I don't think I'll lift the barrels, I'll just do the top end. But when I do, what, if any, sealant should I use with the bottom gaskets? I want it to be sealed, but I don't want to risk gluing the barrels in place!. Similarly, do you use sealant with the head/inlet manifold gaskets, or assemble it dry?

Malc
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dave16mct
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Re: Head gaskets

Post by dave16mct »

My first twin had aluminium gaskets. I never ran it with them and fitted composite gaskets when I rebuilt it. I gave them to a friend who used them in his G12 with no problems. I've been using the solid copper gaskets for quite a few years now, but I enlarge the oil holes and fit O rings. I use Wellseal on both sides of the (thick) base gaskets. Just grease on the manifold gaskets to hold them in place. Don't forget to lightly nip up the manifold before you torque the heads to 22 lbs ft.
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1608
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Re: Head gaskets

Post by 1608 »

Most importantly, as already mentioned, make sure the holes in the new gaskets ( whichever type you use) do not encroach on the push-rod holes in the barrels or the gasket will wear away the alloy pushrod.
MalcW
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Re: Head gaskets

Post by MalcW »

I'll certainly check that out, and alter as necessary. Dave, thanks for the advice re. the manifold when refitting the heads. Also, am I right in thinking that O rings aren't necessary with composite head gaskets.

Malc
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dave16mct
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Re: Head gaskets

Post by dave16mct »

No but some members put a smear of Wellseal around the oilholes, not too close. I've never done that but you do need to follow a rigid torqueing down proceedure with these. That is a) initial torque down. b) with rocker caps removed run for a few minutes to check oil delivery to rockers. Leave to cool overnight. c) re-torque and go for a short run say 20 miles.Leave to cool overnight. d) Re-torque and run for say 500 miles and re-torque again. They shouldn't move after that but re-torque say every year or two. I've always followed this advice with composite gaskets.
With solid copper gasketsyou only need to follow the first couple of stages then every couple of years re-torque. They hardly ever move.
I've only ever blown one twin head gasket and that was when I forgot about re-torqueing the solid copper ones for several years. I was camping in Scotland and the bike (1960 Model 20) hadn't missed a beat. On the final morning it didn't start easily and ran rough all the way home, missing when going under 50mph. So I kept the revs up and got home. The next morning I realized the gasket had blown. I didn't strip it down, just re-torqued them and they're still ok now over 4 years later.
Dave.
MalcW
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Re: Head gaskets

Post by MalcW »

I must admit I prefer the idea of copper gaskets. I'm just a bit concerned about comments in this thread and elsewhere in the forum about the gaskets supplied being too thin. I have a decent torque wrench, so following the procedure wouldn't be an issue.

Malc
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