AJS & Matchless Owners Club

Hi. I want to replace the Edison plug fitted to my G3L. I looked on various forums and the NGK 7ES seemed to be the plug recommended for my bike. I ordered one but when I compare it to the old Edison it is clearly much longer (3/4'' rather than the 1/2'' of the original). Is it possible that I need the NGK 7HS which is shorter or could it be that the original was too short in the first place and the longer reach is really what is required? I am afraid that the longer reach will cause damage if it protrudes into the combustion chamber.

I'd have thought you'd be wanting either an L10 or the later replacement L86S 14mm short reach Champion Spark plugs. You don't want to be using the long reach for a 49 single as the original was a Lodge 14mm 1/2" short reach plug.

A 49 single should have an iron head and a short reach plug. the 7ES would be for the later aluminium head. Is it possible yours has the later head?

clive wrote:A 49 single should have an iron head and a short reach plug. the 7ES would be for the later aluminium head. Is it possible yours has the later head?

A very good point, I must admit though as the plug that came out was short reach I naturally assumed it would have been the correct iron head, however 70 yrs later that isn't always the case. Hopefully all is standard and my info is correct.

ajscomboman wrote:

clive wrote:A 49 single should have an iron head and a short reach plug. the 7ES would be for the later aluminium head. Is it possible yours has the later head?

A very good point, I must admit though as the plug that came out was short reach I naturally assumed it would have been the correct iron head, however 70 yrs later that isn't always the case. Hopefully all is standard and my info is correct.

Judging by the state of my bikes when I purchased them, I assume nothing. Mind you as two of my bikes are bitsas, one of them entirely of my own making, I will probably be considered a guilty Previous Owner at some time in the future. I just wondered why the Internet search had turned up a longer plug for a 49 engine.

Surely a 14 mm would be too long ?as 1/2 inch = 12. 5 mm ? ?

14mm. is the plug size thread dia or what ever, 1/2" is the thread length / reach. Usually 14 mm X 1/2" reach in iron head 14 mm 3/4" reach in alloy. So if you put a long reach 14mm plug in a head that should have short reach. Danger it may hit the piston or perhaps firing closer to the piston it will burn a hole in the piston. Getting the
other way round, short reach that should have long reach, plug will spark in the un used part of the threaded hole. Damaging the thread in the head. Look at the picture at the start of the topic which shows both 14 mm Plugs 1/2" & 3/4" reach thread.

Cheers for the explanation Rob . Just misunderstood the post brain not engaged tonight .
Regards Chris

The difference between the iron head and alloy is the answer, many thanks for this explanation. Mine does indeed have the original iron head so requires the shorter reach plug. I've ordered the correct length so should have the bike back on the road asap!

Thanks again all, much appreciated.

Chris back in 1969 as an apprentice chippy, I remember the absolute panic among some of the old boys when everything was moving over to metric. And, the thought that their ancient and cherished rules and tape measures would be redundant didn't bear thinking about.
I'm assuming you are a wood butcher and not a fishn'chip merchant.