47 G80 Motor

Information relating to the Matchless G80 or AJS Model 18 500cc Heavyweight.
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Yak
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47 G80 Motor

Post by Yak »

Hi.
I am trying to ascertain if I have the correct barrel for the above. With compression plate in place the piston rises level to the cylinder, but only leaves 1/4 inch of thread on the studs to secure it. This is without using washers, though I see there is none listed in spares list so maybe I don't need them. The two top fins are scalloped out to the front of the barrel on both sides, prompting one visitor to remark it was for a twin port. Part No.
45 G8L E1 is cast into the barrel, but I can find no reference in the archives, although when I googled it one entry came up as being from an Ariel! Hopefully a couple of pictures are attached.
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century
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Re: 47 G80 Motor

Post by century »

gday mate looks like twin port barrel.earlier for sure as far as i know than 47.maybe the prob,cheers
bodgie dave
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Duncan
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Re: 47 G80 Motor

Post by Duncan »

Hi, it's certainly a twin port barrel but should still be useable, do you need to use the compression plate?
Yak
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Re: 47 G80 Motor

Post by Yak »

Thanks Century and Duncan for clarity on the barrel. With the plate removed the piston at TDC stops that much above the level of the cylinder. Without looking I think the plat is 1/8 thick. This actually makes it very difficult to put nuts on the studs as they almost foul bottom of the barrel. A compression plate is listed in the spares. Also I have a NOS wire wound piston which is flat on top. I never thought to see if the piston fouls the valves with plate removed, the piston sitting so proud doesn't look right. Might be a job for today.
Thanks again.
Yak
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Re: 47 G80 Motor

Post by Yak »

Ok, made some measurements. Push rods 9 9/16 excluding adjusters, tubes 7 9/16, barrel 5 5/8 from bottom of base to top spigot. Interestingly (to me) my 350 barrel is about 1/2" taller. I put the barrel and head on loosely. Pushrods, which I know come from my 350 are roughly 1/2" too long. The tubes seemed to fit and tighten down ok (sans pushrods). I had 2 nitril "o" rings on each tube and room for another. I think with correct pushrods it should go together ok. Start saving up now.
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Duncan
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Re: 47 G80 Motor

Post by Duncan »

Hi Yak, I have just has to strip down the top end of my late 1940’s 500 due to a stuck exhaust valve. I am using a twinport barrel too with no compression plate, the overall height of the barrel is approximately 5 5/8”, if your piston is coming out by about half an inch more you may have a pre 1947 7 3/8” connecting rod mixed with a post 1947 piston with a lower gudgeon pin height.
Iron Head 500 Piston at TDC.jpeg
I am using the early pushrod tubes with the short end spigot so have to use the rubber sleeves into the head as the o-rings would pop out, on my alloy head bikes I use 4 o-rings your 2 sounds odd.
Iron Head 500 Pushrod Tube.jpeg
I have used iron pushrods in the engine, the pictures are a little distorted but they start at the same point as the ruler.
Iron Head 500 Steel Pushrod.jpeg
Iron Head 500 Steel Pushrod Adjuster.jpeg
I hope this helps.
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Yak
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Re: 47 G80 Motor

Post by Yak »

Hi and thanks Duncan. The piston only protrudes 1/8" with the compression plate out and level when in place. I have had the bottom end replaced entirely
some years ago and am almost certain (memory😯) I have the shorter con rod in there, and I definately have the piston with lower gudgeon pin. From your pictures it looks as though your pushrods are 3/8" shorter and the tubes 3/16". The rockerbox and head seem to tighten down ok without pushrods in place. I am able to get 3 "o" rings on the tube spigot, the same as my 350 which is oil tight. Shorter pushrods and tubes, I think, may be the answer but seem to be hard to find. I have a tatty spare pair from my G3L and I came upon a forum thread on shortening them, so may be the way to go. The spares list shows different part numbers for 350 and 500 models but no length is mentioned.
Thanks again
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