Low compression - Head off

[Duncan]

Hi Eddie, this file on the Archive contains some useful service ranges that can be used as a surrogate/guidance for things like piston ring gaps: http://archives.jampot.dk/book/Workshop ... ndards.pdf

[Expat]

Duncan, snap! I had totally forgotten where my printed spec pages came from, obvious now as they are the very same as in the link you posted and where the figures I posted came from.
What a great source of info the archives are, I found them very helpful when checking out wear of the G3 parts.

Eddie, hope they help you diagnose your issue though FWIW, it sounds like you may need to travel the same road as I.

[SPRIDDLER]

Just my two penn'orth.........
The valve seats look reasonably good and just need lapping in.
The conrod will move a bit side to side but if there's no up and down play it'll be fine.
The piston scuffing could do with cleaning up and looks as though it has nipped up at some time.
I'd be happy with a 19 though ring gap. Depending upon how many miles you plan to do on the new-to-you bike they could still be O.K. for many more years of use. I guess a purist shed-dweller would hone and fit new rings.
Check the valve springs as below for loss of tension. The gap reduces over time as the springs weaken. Clearly the high leg isn't right but even though a pair are quite expensive I'd replace them for peace of mind as you don't want to risk a valve dropping into the cylinder. Perhaps with the spring deformed it has lost its vitality and it's not holding the valve tightly closed.
Obviously you'll clean the carbon off the valve stem as this may have been preventing complete closure of the valve. From memory the ridge at the carbon deposit looks rather close to the valve head so is the valve guide protruding too far into the combustion chamber? The zorst valve would normally have a circlip to indicate the depth of insertion and prevent it from migrating further into the combustion chamber than it should be.

Hairpin spring b.jpg

From memory this ridge looks rather too close to the swelling on the stem at the valve head but it may be just my fuzzy memory or the perspective of the photo........

Valves b.jpg

[EddieDay]

Thanks Expat. That helps. It makes my 19 thou look not so bad.
Regarding my comment about both valves stamped INL,VL I went on the AMOC parts website to order a new exhaust valve and saw the description for valves for a 500 single is "inlet / exhaust valve" and only one part number. So now I am wondering if the valves are actually interchangeable.
Anyway I am enjoying working on a British bike again. I must read "Zen and the Art of Motorcycle Maintenance" again.

[SPRIDDLER]

Regarding my comment about both valves stamped INL,VL I went on the AMOC parts website to order a new exhaust valve and saw the description for valves for a 500 single is "inlet / exhaust valve" and only one part number. So now I am wondering if the valves are actually interchangeable.

The factory spares list shows different numbers for the valves which is possibly because back in the day the zorst valve stems were hollow and 'sodium-filled' to aid heat transfer away from the valve head. I'm guessing that with modern valve steels this isn't critical so the same valves can be used for inlet and zorst.

[Joker_Bones]

Extract from AMC classic spares:

valve.jpg

[EddieDay]

Duncan, thanks. I hope you haven't broken the Official Secrets Act by posting restricted documents. Brilliant source of info. I am half way between acceptable and condemned. I'll take that and carry on.

Spriddler, thanks for your comments. I'll clean up the stems and lap the valves in. Regarding the ex valve material I suppose it's not possible to tell whether you have a modern valve of better steel. I will re-use the existing one for now. I have already ordered a set of springs. Your comments on big end play help. I really don't want to split the cases as there is some good weather coming up and I want to ride the bike at least till November.

[Duncan]

Spriddler, thanks for your comments. I'll clean up the stems and lap the valves in. Regarding the ex valve material I suppose it's not possible to tell whether you have a modern valve of better steel. I will re-use the existing one for now. I have already ordered a set of springs. Your comments on big end play help. I really don't want to split the cases as there is some good weather coming up and I want to ride the bike at least till November.

A quick test is that exhaust valves are usually non-magnetic (Austenitic Steel I think?), you can use an exhaust valve for an inlet but not vice versa.

[Joker_Bones]

I looked into new valves for my G80 engine recently... As far as I could make out It seems only the exhaust is available from the main suppliers as a new replacement, and offered as the inlet valve as well. The AMC classic spares listing suggests the exhaust valve is the higher grade and therefore okay for the inlet.
Curiously... the old valves I took out of the engine were not the same exactly the same length, though I've no knowledge of their origin or correctness.
Curiously also... if you look in the army inspection manual referred to earlier, although for the 350 engine not the 500, the spec for the valve stem diameter varies a couple of thou between the inlet and exhaust.

[EddieDay]

Duncan and Joker, thanks for your comments. Both valves are magnetic so neither is austenitic SS. So to be on the safe side I will order a new valve. Out of interest I will ask the spares guys what stamping the valves in stock have.