Head Gasket Problem

Information relating to the Matchless G9 or AJS Model 20 500cc twin
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Head Gasket Problem

Postby MalcW » Mon Mar 25, 2019 10:28 am

A friend of mine has just bought a 1959 Model 20 - very nice looking bike that's obviously had time and money spent on it. Yesterday was the first day he was in a position to ride it, as he works away in the week, so we decided to go over to my bike club's breakfast meet, then have a longer ride back so he can get used to it; it's his first classic bike.
Well, we only got about 4 miles and one of the head gaskets went. The 3rd member of our small group rode back to get is bike trailer, and we took it back to my garage so I can work on it in the week. I made a start yesterday afternoon, just by removing the seat and tank, and I see that the heads are held on with domed nuts. Is this normal? On my 650 they are normal nuts. Surely a domed nut could bottom out, giving the impression that it is torqued up correctly, whereas it is anything but? The PO did tell him that the gaskets are composite, and that he had done the usual torque routine with these, and the next step was to get some miles in and then torque them again. I have composite gaskets on my 650 and have followed the same torque/retorque sequence with no problems.

As an aside, after getting the bike back to my place and taking its owner home, I decided to take mine back out. The kickstart broke! Do you ever get the impression that the Gods are trying to tell you something?

Malc

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Re: Head Gasket Problem

Postby g5wqian » Tue Mar 26, 2019 6:38 am

my model 30 has ordinary 5/16 nuts with washers underneath and thats all .

i have seen domed nuts being sold but i guess it all depends on how long the studs are physically on the engine as to whether they could botom out or not .

i did have problems with solid copper head gaskets in the past but that was down to the copper being too thin and not as advertised and the barrel spigot bottomed out in the head recess .

composite gaskets are quite thick and thats what i use now without any trouble .

best put some normal 5/16 nuts on and hope you sort it easily to satisfaction .

all the best
ian

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Re: Head Gasket Problem

Postby Mick D » Tue Mar 26, 2019 7:57 am

Hi

The parts list for the 59 Model 20 lists the cylinder head nuts as part number 000004 which is a standard 5/16" x 26 TPI nut - personally I'd replace the dome nuts with standard and remove the possibility of the threads becoming bound. If the owner likes the 'look' of the dome nuts you could ensure clearance by fitting extra washers.

Regards Mick

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Re: Head Gasket Problem

Postby Stuoyb » Tue Mar 26, 2019 4:52 pm

You don’t state the age of the bike - the early twins with 4 bolt rocker covers (pre 1953) had an acorn cylinder head nut.

Oops missed the 1959 - acorn nuts were used pre ‘53
Last edited by Stuoyb on Tue Mar 26, 2019 8:09 pm, edited 1 time in total.

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Re: Head Gasket Problem

Postby Mick D » Tue Mar 26, 2019 5:14 pm

MalcW wrote:A friend of mine has just bought a 1959 Model 20


Thought he was talking about this one and head nuts :roll: ;)

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Re: Head Gasket Problem

Postby MalcW » Tue Mar 26, 2019 5:24 pm

Mick D wrote:
MalcW wrote:A friend of mine has just bought a 1959 Model 20


Thought he was talking about this one and head nuts :roll: ;)


Indeed I was Mick.

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