pre '49 G80 valve guides - circlips?

Information relating to the Matchless G80 or AJS Model 18 500cc Heavyweight.
hatrack
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Re: pre '49 G80 valve guides - circlips?

Post by hatrack »

Thanks. And thanks to all who have helped.
hatrack
Posts: 69
Joined: Thu Sep 07, 2017 3:36 pm
Location: Somerset

Re: pre '49 G80 valve guides - circlips?

Post by hatrack »

I got the had and rocker box on today.
There was a delay because of a puzzle over push rod tubes. The ones fitted measured 7.5/16" a length not listed anywhere.
Careful search of parts lists, numbers and suppliers lists fetched a pair of correct new tubes from Classic AMC Spares. Next day delivery! (I must add that the club spares scheme has also been very prompt as well).
The new tubes revealed that the old ones had been shortened at the top, possibly seeking easier assembly.
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Rob Harknett
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Re: pre '49 G80 valve guides - circlips?

Post by Rob Harknett »

The ones fitted were not listed anywhere ?? was that because they were shortened? If so were they shortened because the compression plate under the barrel was left off? Now you have the compression plate on you need new tubes, like you already had, that had been shortened.
hatrack
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Re: pre '49 G80 valve guides - circlips?

Post by hatrack »

No compression plate on it when I got it. Should it have one?
Yes, tubes not listed as they were the original ones shortened by about 1/4"
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Rob Harknett
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Re: pre '49 G80 valve guides - circlips?

Post by Rob Harknett »

hatrack wrote:No compression plate on it when I got it. Should it have one?
Yes, tubes not listed as they were the original ones shortened by about 1/4"
I am sure it states compression plate under barrel in the parts list. If there was not plate and there should have been one, that may be why the tubes were shortened.
hatrack
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Re: pre '49 G80 valve guides - circlips?

Post by hatrack »

Yes, I now see the compression plate in the parts list, but thanks to Matchless Parts List just showing little pictures of each bit, rather than a proper exploded diagam, from which you can see what goes where, and the fact that my engine wasn't fitted with such a plate, meant that I wasn't looking for one. Looking at that page, that little drawing looks just like a cylinder base gasket (which they don't bother to show!)
Rant over!
Looking through the Bernal Osbourne/Temple Press book, on page 13, under "Compression Ratios" I find:
"On pre1952 models the compression plates can be dispensed with after the ridge formed in the top of the barrel is honed out; it is absolutely essential to do this."
This is in the context of whether the compression can be raised by fitting later pistons or not, and in the case of my machine seems to raise the CR to about 7:1 Not a particularly hairy set up, but perhaps enabling slightly better use of modern fuels.
As my bike was running, without a compression plate when I got it and had a recent rebore, I think I can ignore the dire warning about honing out the ridge.
Regarding the push rod tubes, cut down or not, removal of the plate may have caused a misguided decision to shorten them.
Again, in the Osbourne book it is carefully explained that, with the compression plate fitted there should be a steel washer above and below the top rubber seal. If no compession plate is fitted there should only be one steel washer above the seal.
Looking at the full length tubes in situation, the top of them does not come up against the cylider head.
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Rob Harknett
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Re: pre '49 G80 valve guides - circlips?

Post by Rob Harknett »

hatrack wrote:Yes, I now see the compression plate in the parts list, but thanks to Matchless Parts List just showing little pictures of each bit, rather than a proper exploded diagam, from which you can see what goes where, and the fact that my engine wasn't fitted with such a plate, meant that I wasn't looking for one. Looking at that page, that little drawing looks just like a cylinder base gasket (which they don't bother to show!)
Rant over!
Looking through the Bernal Osbourne/Temple Press book, on page 13, under "Compression Ratios" I find:
"On pre1952 models the compression plates can be dispensed with after the ridge formed in the top of the barrel is honed out; it is absolutely essential to do this."
This is in the context of whether the compression can be raised by fitting later pistons or not, and in the case of my machine seems to raise the CR to about 7:1 Not a particularly hairy set up, but perhaps enabling slightly better use of modern fuels.
As my bike was running, without a compression plate when I got it and had a recent rebore, I think I can ignore the dire warning about honing out the ridge.
Regarding the push rod tubes, cut down or not, removal of the plate may have caused a misguided decision to shorten them.
Again, in the Osbourne book it is carefully explained that, with the compression plate fitted there should be a steel washer above and below the top rubber seal. If no compession plate is fitted there should only be one steel washer above the seal.
Looking at the full length tubes in situation, the top of them does not come up against the cylider head.
You do really need to check parts lists. Bikes now can be + or - parts, wrong parts etc. There should be no doubt where , compression plate barrel fits, seeing the shape of the plate and barrel base. The little drawing of the plate can be seen as thicker than the head gasket.
Ashampoo_Snap_2017.10.07_12h08m15s_003_.png
You always need to check for a lip at the top of the bore, in particular when fitting new piston or rings. The lip could soon break the top ring. If it appeared the tube measurement would not reach the head, did you consider. The correct O rubber between the crank case and tube, will raise the tube upwards by about 2 mm. When fitting the new tubes please take care when tightening the head. Observe the bottom of the tube and O rubber.
If the tube etc is not fitted correctly, you could flatten the tube seat for the rubber. So be careful you do not have to tighten the head so the tube touches the crank case. Stop and investigate or you will have to buy another set of tubes if they start to flatten out on the crank case. No good using them if flattened, they will leak oil.
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hatrack
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Re: pre '49 G80 valve guides - circlips?

Post by hatrack »

I agree on checking parts lists, I have done lots of that, rebuilding the forks, but as I said, with no base gasket shown, and I've got gasket sets, I wasn't looking there.
As I also said. There is no lip. Its had a recent rebore and was running satisfactorily, in that respect.
When I said it dosen't reach the head, I meant that it does not come up against the head. I have the right metal washers and rubber sleeve, and "O" rings at the bottom.
It is all boted up, torqued down and everything is sitting pretty. There is no problem.
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Rob Harknett
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Re: pre '49 G80 valve guides - circlips?

Post by Rob Harknett »

hatrack wrote:I agree on checking parts lists, I have done lots of that, rebuilding the forks, but as I said, with no base gasket shown, and I've got gasket sets, I wasn't looking there.
As I also said. There is no lip. Its had a recent rebore and was running satisfactorily, in that respect.
When I said it dosen't reach the head, I meant that it does not come up against the head. I have the right metal washers and rubber sleeve, and "O" rings at the bottom.
It is all boted up, torqued down and everything is sitting pretty. There is no problem.
Should be OK then. The plate was removed and the other parts not effected by the height lost by not using the plate. Higher compression gained. WD bikes also used a compression plate. I believe on the head. This was to decrease compression to help save valuably fuel during wartime also to help cope with a poorer quality fuel. Probably why the early 40's post war machines continued using a compression plate.
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