[Rick Parkington]

Hi Glen, glad you got it sorted. Out of interest, the original washers on valve caps were generally copper/asbestos 'crush rings'. Sounds like yours was solid copper which may mean it was made up from bar specially to fit the undersize cap. Usually the crush washer should fit a standard cap easily.
Cheers Rick 
 

[Glen Dickens]

Hi  Glenn, I don't have a SV Sunbeam to compare - and therefore don't know what the club caps are like - but that certainly doesn't sound right! I have seen valve caps that recessed but they have been ferrous not brass and full thickness on the base. My guess would be that that one is not a club one but something modified from another type. Maybe someone else can advise there.
I imagine the idea of the big recess is to keep as much air as possible circulating around the plug to prevent overheating. But with the ability of modern plugs to withstand heat I doubt it's all that necessary. I  have experienced severe pre-ignition on vintage side valves that required racing grade 18mm plugs to overcome though, so if you do make a cap, I'd suggest tapping it 14mm for a wider choice of grades - you can always open it out to 18 later if you wish.
If you mean the cap came loose in the head (not the plug in the cap) I'd think that the thread in the cast iron may be worn - these can corrode away over time - or (hopefully) the cap was maybe undersize. Again if you are planning to make one you will have the chance to oversize it if necessary.
I have managed to recut a rusted away cap thread on a Blackburne (detachable) head by bolting it to the lathe saddle and screwcutting it using a boring head in the spindle. It was a palaver and would be worse with a complete cylinder - but it worked, just needing an oversize cap made up.
Just a thought.
All the best Rick
 

Hi Rick
Thanks for your reply. Yes I do mean the cap came loose in the head. I even tried to locktite it in but after a couple of miles it was loose again. I have just managed to find an original old Sunbeam cap and guess what?.... there's no undercut around the plug hole. I'm convinced now that this is a manufacturing fault of whoever made the one with the deep undercut (of course I could be wrong) I will try to post a picture. If the threads go on the old one, I will assume the thread is badly worn in the head (which is part of the barrel)
Machining a new thread in there will be difficult but I will find a way (it was my profession)

Regards
Glen

I thought I should update this thread as I'm pretty sure I've got to the bottom of this problem and I would like to share my findings.
The old original valve cap that I managed to find measured 0.020" bigger on the thread diameter than the one that gave me trouble, infact, to get the copper sealing washer onto it I had to heat the washer up red hot, even then it was very tight and resulted in me misforming the washer (The copper washer was a throw on fit on old/new brass cap)
 Obviously heating it up annealed it so it was very soft and malleable. I had to use the spanner to get it in the last 4 or 5 threads on the head but it wasn't too tight to concern me. I rode the bike approximately 12 miles without issue.
The old/new one would come loose about every 2 miles.

My conclusion!
The old/new one had the threads cut undersize making it a baggy fit and the "pointless" undercut around the plug hole made the section at the bottom (3mm) to thin and to weak to stand up to the pressure in the combustion chamber.

Thinking more about what I experienced, I believe it didn't unscrew, it actually "jumpt" a thread when it was hot and under pressure, because it wasn't possible to turn a full turn because the spark plug HT lead is held on the plug with a nut that was always very tight, also the movement of the cap unscrewing would have tightened the HT lead nut even more.

Lessons learned.
If you get e new valve cap and the copper sealing washer is easy to fit.... Send it back because it will fail. They need to be a good tight thread.

I'm over the moon with the Lion now and looking forward to this year's Banbury run.

Cheers
Glen

[Glen Dickens]

Hi  Glenn, I don't have a SV Sunbeam to compare - and therefore don't know what the club caps are like - but that certainly doesn't sound right! I have seen valve caps that recessed but they have been ferrous not brass and full thickness on the base. My guess would be that that one is not a club one but something modified from another type. Maybe someone else can advise there.
I imagine the idea of the big recess is to keep as much air as possible circulating around the plug to prevent overheating. But with the ability of modern plugs to withstand heat I doubt it's all that necessary. I  have experienced severe pre-ignition on vintage side valves that required racing grade 18mm plugs to overcome though, so if you do make a cap, I'd suggest tapping it 14mm for a wider choice of grades - you can always open it out to 18 later if you wish.
If you mean the cap came loose in the head (not the plug in the cap) I'd think that the thread in the cast iron may be worn - these can corrode away over time - or (hopefully) the cap was maybe undersize. Again if you are planning to make one you will have the chance to oversize it if necessary.
I have managed to recut a rusted away cap thread on a Blackburne (detachable) head by bolting it to the lathe saddle and screwcutting it using a boring head in the spindle. It was a palaver and would be worse with a complete cylinder - but it worked, just needing an oversize cap made up.
Just a thought.
All the best Rick
 

Hi Rick
Thanks for your reply. Yes I do mean the cap came loose in the head. I even tried to locktite it in but after a couple of miles it was loose again. I have just managed to find an original old Sunbeam cap and guess what?.... there's no undercut around the plug hole. I'm convinced now that this is a manufacturing fault of whoever made the one with the deep undercut (of course I could be wrong) I will try to post a picture. If the threads go on the old one, I will assume the thread is badly worn in the head (which is part of the barrel)
Machining a new thread in there will be difficult but I will find a way (it was my profession)

Regards
Glen

[Rick Parkington]

Hi  Glenn, I don't have a SV Sunbeam to compare - and therefore don't know what the club caps are like - but that certainly doesn't sound right! I have seen valve caps that recessed but they have been ferrous not brass and full thickness on the base. My guess would be that that one is not a club one but something modified from another type. Maybe someone else can advise there.
I imagine the idea of the big recess is to keep as much air as possible circulating around the plug to prevent overheating. But with the ability of modern plugs to withstand heat I doubt it's all that necessary. I  have experienced severe pre-ignition on vintage side valves that required racing grade 18mm plugs to overcome though, so if you do make a cap, I'd suggest tapping it 14mm for a wider choice of grades - you can always open it out to 18 later if you wish.
If you mean the cap came loose in the head (not the plug in the cap) I'd think that the thread in the cast iron may be worn - these can corrode away over time - or (hopefully) the cap was maybe undersize. Again if you are planning to make one you will have the chance to oversize it if necessary.
I have managed to recut a rusted away cap thread on a Blackburne (detachable) head by bolting it to the lathe saddle and screwcutting it using a boring head in the spindle. It was a palaver and would be worse with a complete cylinder - but it worked, just needing an oversize cap made up.
Just a thought.
All the best Rick
   

[Glen Dickens]

Hi All
I'm Glen, I'm new to Sunbeams, and this club. I recently brought my first Beam, it's a 1931 Lion. I've been having a problem with the brass valve cap that the spark plug screws into. It looked like a new part and had the copper washer on, but it kept coming loose after only a few miles. today it blew out completely stripping the brass thread and cracking at the bottom of the recess around the plug hole.
First question, does anybody know why it kept coming loose ?
Second question, why does it need to be recessed around the plug hole at all ??
I've measured it and there's only 3.5mm between the bottom of the recess and the compression chamber 🤷 that's not very strong. I fear if I buy a new one from the club I'll have the same problem so I'm planning on making my own (I'm a retired machinist) that would be solid around the spark plug.

Any advice would be very much appreciated.
Thanks
Glen
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