Barrel length

Information relating to the Matchless G80 or AJS Model 18 500cc Heavyweight.
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Chris Berry
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Barrel length

Post by Chris Berry »

Hi all I am in the process of replacing my existing barrel which is bored to 0.0060” OS to a barrel that I have had resleeved and taken back to standard. Initially upon fitting the barrel and head I noticed the push rods were extremely sloppy. I adjusted the slack and then noticed that the clearance between the valve cover and the frame was to small to fit the top support bracket without modification. I have had the head off and on three times to check that there is nothing untoward happening (binding push rod tube washers etc) but found nothing. It was only when I measured the barrel length that I discovered that the new barrel was approximately 5mm longer than the old one. When measured from the top cooling fin to the bottom of the seating flange (remembering that the engine is assembled) the new barrel measures 146mm against the old barrel measuring 141mm. The old barrel has the number 013792 cast into the side whereas the new barrel has no cast numbers. I have the spacer plate fitted to the base of the base of the barrel (around 3mm thick) and am thinking that I should remove this but am seeking advice. Can anyone help me please
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clive
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Re: Barrel length

Post by clive »

As I was reading my immediate thought was to ask if you had a spacer plate at the bottom of the barrel which you do. The next question is where was the top of the piston sitting at TDC. The edge of it should be flush with the top of the barrel whether the piston is the flat top or domed type. So removing or not the spacer plate should be based on this in my opinion, others may differ!
clive
if it ain't broke don't fix
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Duncan
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Re: Barrel length

Post by Duncan »

Do both barrels have the same number of fins?
Chris Berry
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Re: Barrel length

Post by Chris Berry »

Duncan wrote: Sun Sep 25, 2022 6:18 pm Do both barrels have the same number of fins?
Both barrels have eleven fins
Chris Berry
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Re: Barrel length

Post by Chris Berry »

clive wrote: Sun Sep 25, 2022 4:00 pm As I was reading my immediate thought was to ask if you had a spacer plate at the bottom of the barrel which you do. The next question is where was the top of the piston sitting at TDC. The edge of it should be flush with the top of the barrel whether the piston is the flat top or domed type. So removing or not the spacer plate should be based on this in my opinion, others may differ!
Hi Clive thanks for your response so quickly. As the engine is together I can't check the position of the piston in relation to the top of the bore but having checked the old one (5mm shorter) the piston crown did come up level with the top of the barrel. I would assume form this that the new piston would be 5mm below the surface. Both pistons are domed and are the same brand. I'm thinking at this stage I have two choices
a. Strip the engine down again and remove the spacer plate: or
b. As there appears to be nothing sinister leave it as it is and accept that there may be some minimal loss in performance remembering that both options are reversible.

What do you suggest?
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clive
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Re: Barrel length

Post by clive »

If you run it for long with the plate in place you will need to hone the barrel as the rings will have started making a ridge. Not a big problem if you have access to a hone or if you only run it for a bit. But I am inclined to think you are going to have very little compression with a 5mm gap and whilst it might run you would be disappointed with the performance. If it was me I would bite the bullet and remove the plate. This would allow you to check what is happening with the piston at TDC and would also probably sort out fitting the headsteady without modification. Some of the early barrels had a less deep seating flange and hence the use of the plate. I am unconvinced measuring the bottom of the seating flange to the top fin is going to give you an accurate measure of the functioning barrel. With the engine in the frame its fairly easy to whip the barrel off, remembering that you will need to loosen the crankcase bolts at the top to allow the barrel to be withdrawn (hopefully easily). It would be worth measuring the height of the seating flange to see if there is a difference if the new one is substantially higher I think your decision is made. You will need new base gasket probably and the head gasket if it is the thin round type. If it is the later plain flat type it would be worth re-annealing it.
clive
if it ain't broke don't fix
Chris Berry
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Re: Barrel length

Post by Chris Berry »

clive wrote: Mon Sep 26, 2022 12:29 pm If you run it for long with the plate in place you will need to hone the barrel as the rings will have started making a ridge. Not a big problem if you have access to a hone or if you only run it for a bit. But I am inclined to think you are going to have very little compression with a 5mm gap and whilst it might run you would be disappointed with the performance. If it was me I would bite the bullet and remove the plate. This would allow you to check what is happening with the piston at TDC and would also probably sort out fitting the headsteady without modification. Some of the early barrels had a less deep seating flange and hence the use of the plate. I am unconvinced measuring the bottom of the seating flange to the top fin is going to give you an accurate measure of the functioning barrel. With the engine in the frame its fairly easy to whip the barrel off, remembering that you will need to loosen the crankcase bolts at the top to allow the barrel to be withdrawn (hopefully easily). It would be worth measuring the height of the seating flange to see if there is a difference if the new one is substantially higher I think your decision is made. You will need new base gasket probably and the head gasket if it is the thin round type. If it is the later plain flat type it would be worth re-annealing it.
Thanks once again Clive. I bit the bullet today and removed the barrel to check it and there is a difference in the length of the barrel skirts (the part that sits in the crank case). The barrel with the longer length between the flange and the fins had the shorter skirt. This would suggest to me that the shorter skirt was designed to be fitted without the spacer plate as the skirt would still sit in the crankcase to the same depth while maintaining the same overall height. Hence I have now removed the spacer

Cheers and thanks for your help
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clive
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Re: Barrel length

Post by clive »

sounds good, just need to get it on the road now!
clive
if it ain't broke don't fix
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