Blown head gaskets

Information relating to the Matchless G12 or AJS Model 31 650cc twin
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Blown head gaskets

Postby McBob » Fri Jun 14, 2019 9:27 am

Hi, both head gaskets are gone on my bike. The engine was rebuilt for me in the mid nineties and since then, for all the usual reasons, only covered 1200 miles.
What would you say will cause this, they have gone in the middle and the fire ring is distorted. Can the gaskets interfere with each other or be torqued
incorrectly? I noticed misalignment of the holes, especially the push rod tunnel, can these gaskets be trimmed?

Second issue I am unsure of is the vertical marks on the drive side bore, I can just feel them my finger nail, is this something to be concerned about?
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Re: Blown head gaskets

Postby Rob Harknett » Fri Jun 14, 2019 11:13 am

Your workshop manual will show the correct way and method of tightening head bolts. Also re tightening after approx. 500 miles has been covered. Sounds like this was not done. Twins do tend to blow gaskets if all is not correct. Someone with more knowledge will come along and explain in more detail perhaps. The holes do look a mess. But the correct procedure as stated in your manual may not have been followed, so may also be a consideration..

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Re: Blown head gaskets

Postby jack clegg » Fri Jun 14, 2019 11:17 am

A scored bore is no fun. Possibly nipped up/heat seizure. Piston will also be scored. Head gaskets look awful. Could have been over-torqued. They appear to have been squashed outwards. You should not need to "trim" any correct headgasket & by this question it makes it sound like you are going to reuse the gaskets. I do hope not. Driveside barrel needs to come off for inspection anyway.
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Re: Blown head gaskets

Postby McBob » Fri Jun 14, 2019 11:26 am

Thanks Rob, I did the re-torque as specified, but can't speak for first one. The heads seems flat from what I can tell.

Cheers Jack, I have a replacement gasket set, that ride I hoping to do on Sunday now seems a long off, your right about those marks

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Re: Blown head gaskets

Postby 1608 » Sat Jun 15, 2019 7:24 pm

The composite gaskets can be trimmed. They sometimes need a little trimming to ensure they do not lap where they meet. I would suggest that a little extra is trimmed around the push-rod holes to ensure the push-rods do not chafe on the gasket ( a very common fault ). Be a good idea to check the heads for flat and follow the torquing down procedure carefully.

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Re: Blown head gaskets

Postby McBob » Sun Jun 16, 2019 10:23 am

Cheers John, I will do all of that

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Re: Blown head gaskets

Postby g5wqian » Mon Jun 17, 2019 5:47 am

check thickness of the head gaskets you use or used , they should be at least 1.5mm thick , the clubs composite gaskets are 1.7mm thick and i use them without trouble .

you might have had some gaskets that didnt seal properly and have burned out due to that .

use the club spares thick base gaskets as well , this will give slight less compression ratio but wont affect you .

check that the nuts on the studs are not bottoming out on the threads when you pull the heads down .

if those bores are only scratched and not scored you should just hone them and fit new rings or new pistons and rings .
perhaps youve had JP or GPM pistons fitted which need more clearance than original , so youll be looking for a ballpark of .005" over the specified bore size , ie if its 2.835" bore in the book you need it 2.840" when honed , then will allow piston to expandv , this will also allow you to clean up that bore if its not been honed out to give enough clearance .

.

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Re: Blown head gaskets

Postby McBob » Mon Jun 17, 2019 11:53 am

Good to know the clubs gaskets have a bit of extra meat on them, I will be getting those.
I didn't get away with the piston, plus the bore has gone a bit out of round, re-bore and new pistons. It's a bit of a shame, the pistons markings are
D61 026324.

Reading back through posts on the subject of pistons is not fun when facing this, has anything changed ie good quality piston available
and 5-6 thou clearance doing the job. The 7.5-1 CR seem hard to get as well.
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