1955 AJS 18CS - Fire Ring dimensions

Information relating to the Matchless G80 or AJS Model 18 500cc Heavyweight.
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1955 AJS 18CS - Fire Ring dimensions

Postby Halfwhelk » Thu May 02, 2019 11:22 am

I'm having to get a new liner made for the alloy barrel on my 55 18CS. This is both to correct the error on the rebore supplied to the previous owner, which was .030" out of square (relative to the base) at the top and also to replace the fire ring - the spigot or lip at the top of the barrel liner
Can anyone please give me the fire ring dimensions (or a source of this info)? I need to know how far it extends from the mating surface at the top of the barrel and its thickness or overall diameter.

Information on the alloy barrel would be useful. I'm assuming it was not a factory standard for '55. Who made it? The liner is pressed into a steel or iron sleeve. Was the barrel cast around this sleeve, i.e. do I need to worry about the sleeve coming loose? I'm getting the liner made so it has a lip at the top to prevent it dropping - which I understand is a problem with bonded liners without a lip.

Thanks
Matt

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Re: 1955 AJS 18CS - Fire Ring dimensions

Postby SPRIDDLER » Thu May 02, 2019 2:04 pm

Can't help with the machining info but the alloy cylinder might be one of these which were available for many engines...
Advert from 1958
WellworthyCylinders (640x576).jpg


EDIT: I had wrongly followed up your suspicion that you had identified your cylinder as a special (i.e. non-standard) part. :roll:
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Re: 1955 AJS 18CS - Fire Ring dimensions

Postby Andy51 » Thu May 02, 2019 8:00 pm

Hi Matt, if you have a genuine '55 G80CS then I believe it should be fitted with an 82.5mm bore alloy barrel. With regards to a new liner, Phil Irving in 'Tuning for Speed' recommends the liner should be not less than 1/8 in wall thickness and 1 thou per inch diameter interference fit - around 3 thou for your barrel. A flange is a good idea. Look at Westwood liners range, they probably have something suitable. Hope this helps

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Re: 1955 AJS 18CS - Fire Ring dimensions

Postby Halfwhelk » Thu May 02, 2019 10:19 pm

Thanks Andy, Yes. I'm (or my rebore engineer is) going to Westwood, but its going to be a custom liner, because it needs to be fitted then bored to correct the out of squareness.
We don't have a reference size for the fire ring, hence my question.

I'm hoping that someone with a barrel (not necessarily alloy) on the bench can measure the fire ring height and diameter for me.

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Re: 1955 AJS 18CS - Fire Ring dimensions

Postby Mick D » Fri May 03, 2019 6:06 am

Hi

Excuse me if I've misunderstood the question but why not measure the recess machined into the head, add the dimension of the head gasket and allow a suitable clearance to both? This would cater for the possibility of a non standard head, (possibly skimmed), already being installed.

Regards Mick

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Re: 1955 AJS 18CS - Fire Ring dimensions

Postby Halfwhelk » Fri May 03, 2019 7:03 am

Mick,
Yes of course. In the absence of any other information thats what I'll do. But what is 'suitable clearance'? There are considerations of expansion and probably others I'm unaware of. I'm just hoping someone has a barrel on the bench and a handy pair of calipers, who can verify the information.

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Re: 1955 AJS 18CS - Fire Ring dimensions

Postby Mick D » Fri May 03, 2019 7:40 am

Hi

Aluminium has a greater rate of expansion than steel so a cold clearance will only increase as the engine warms up.
In the absence of a comparative measurement I'd aim for 0.010" on diameter.
The vertical clearance will depend on the head gasket used, a composite one will compress more than a solid copper one so err towards 0.020" if you're using composite. 0.010" should be OK for copper but could give a problem if a composite gasket was subsequently fitted.

Regards Mick

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Re: 1955 AJS 18CS - Fire Ring dimensions

Postby Andy51 » Fri May 03, 2019 8:48 pm

Not quite clear what a fire ring is. Is it the copper gasket or the raised spigot on the top barrel face that enters the recess on the head? If it's the barrel spigot I have an iron one in the garage I can measure tomorrow night. Let me know, cheers, Andy

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Re: 1955 AJS 18CS - Fire Ring dimensions

Postby Halfwhelk » Sat May 04, 2019 8:06 am

Thanks Andy,
Your measurement would be much appreciated.

The fire ring is the raised part of the liner that enters the recess on the head. I lost mine when the PO decided to raise the compression ratio with a coarse file. Without this the copper head gasket gives up in fairly spectacular fashion after about 20 minutes.
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Re: 1955 AJS 18CS - Fire Ring dimensions

Postby Dixter » Sun May 05, 2019 4:05 am

Matt, if I’m following this correctly, you are using an alloy barrel with an alloy head. If this is the case, there is no head gasket used. The workshop installations direct the joint to be “ground” with fine valve grinding compound on the major barrel surfaces, and course compound on the top edge of the iron spigot. This implies that when the surfaces are properly mated, their will be a thou or two clearance between the top of the spigot and the underside of the combustion chamber, and the barrel/cylinder head will make a complete seal.

I measured the spigot on a short stroke barrel I had on the bench, the spigot is 0.085” high. The OD on this barrel will be meaningless in your case, short stroke engines used a different bore than your long stroke engine.

Have you used grinding compound to verify your barrel and head are mating correctly. I’d suspect a sealing problem, not a flame front problem.

Ciao, DC
Dick Casey
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