AJS & Matchless Owners Club

New solid copper gasket, hone the cylinder, and fit new piston rings and i reckon that will sort it all out

Place the head on the cylinder without the gasket and just nip it down, now measure the gap with feeler gauges where the gasket would go. I can't help thinking this might be a cracked head, a head that has been skimmed but not had the recess matched or otherwise warped head that is causing this, that is just too much oil from the bottom of the engine, if there was that much oil making its way upwards I would expect it to be thrown out of the exhaust but you don't mention this. That ring gap is not excessive by a long way, I have had engines run properly without oiling with 1mm ring gaps, most of the time the combustion chamber is under pressure and therefore will force oil downwards not up, unless you have a hard vacuum on induction i.e. blocked air filter or similar, so I think you may discount the rings or bore. The only other scenario that may cause a lot of oil to get into the combustion chamber is a bad inlet guide and the oil is being sucked from the rocker box through the guide and into the chamber on the over run.
One thing I might mention, have you removed the crank case drain plug to see how much oil is in there? should be minimal amount.

I capped off the oil supply to the head and don't get the chance to be in overrun.
Next thought maker is the piston is from Wellworthy and went bust late 60s. I've followed the trail to AE pistons and so far..........
Somebody must have encountered this before, any tips appreciated.
Ps Max, when I had the chance to keep the engine running long enough oil was being chucked out the exhaust but things have got steadily worse.

I did my apprenticeship at 'Wellies' in the 70's and have been in vehicle engineering most of my working life, that oil can only really be coming from the top end of the engine, I would be very surprised if that much oil has made its way from below and I doubt there is anything wrong with the piston, even if it was a size under it is unlikely to allow that much oil in there. Did you carefully check the head and clearances of the head to cylinder face as well as looking at the inlet valve guide? Do you have another head you can try? Check the head for flatness in all directions with the edge of a steel rule or similar known flat edge and a feeler gauge.
Did you see how much oil was in the crank case by removing the rear drain bolt? there should be almost none.

Thanks for all your tips Gents. I've checked all you recommended Max apart from seeing how much oil was in the crankcase, I don't have a spare head. Anyway process of elimination, I've found rings at Cox and Turner and could be on my doormat in a day or two, unfortunately the doormat is in Chester and I'm in Germany.
Then I'll hone the barrel and I know that's then good.
Progress is going to be very slow due to logistics and location but I'll get there.

Well Max your time at "Wellies" was not wasted I only wish your suggestion of wet sumping was a bit earlier.
The symptom of all this oil in the cylinder has got steadily worse this summer (hot here in Bavaria) even though I've not been able to ride. Along with oil level down in the tank and increasing large amounts of cylinder oiling. The heat particularly in my garage has obviously dropped the oil visocity and the sae 50 is travelling, almost 1/2 ltr in the sump!
I thought the G5 didn't wet sump; well it does! Thanks Max.
The prep to travel is well underway before I made the discovery and it'll be honed with new rings in UK, it's become a problem getting spares down here post Brexit.
In about 5 weeks I'll report success....hopefully.

Well Gents, time has moved on and this G5 is now back home in the UK.
I honed the barrel and fitted new rings. Existing new ring gaps from top to bottom 020,012 and I opened the oil ring to 010. Starts and runs as it should, no smoke at all.
At the moment I can't keep it running for more than about 2 mins, seems like a float bowl full. The plug when removed seems slightly wet and smoke may exit carb.
Compression reading is 92psi , a bit low but it's not easy kicking and holding a tester with a somewhat perished cone seal.
I can only think the engine wont continue to run due to fuel starvation. Could the rings be nipping up? Anyway significant advance, nearly there.
Any suggestions welcome.

Check the pilot jet is clear.
Dave.

Valve clearances too tight and holding the inlet valve open once warm?

What if you pop the head on dry no gasket, then feeler gauges to check any gap twixt head and barrel, then if we're to check the
Compressed copper ring on gasket with micrometer and the composite part you might get two different readings, just my thoughts on this . But I'm no expert.