AJS & Matchless Owners Club

I own a 1957 G11 and to reduce television interference (and to improve starting) I fitted some NGK BPR7EIX iridium plus as recommended by the experts on this forum. All was well until the right pot stopped firing. When I took the plug out I noticed that the earth electrode had bent. I have measured the iridium plug and it's about 2.5mm longer, from the washer to the top of earth electrode, compared to the old B7ES.

The left plug is fine...

I did nip the spark plugs up after 50 miles of riding and I wonder whether this has caused the plug to interfere with the piston. Alternatively something very horrible has happened in the cylinder.

Any advice would be appreciated.

Regards

Chris Gee

Not the piston but one of the valves. They come very close to the ground electrode if it's the protruded tip type. To be on the safe side you have to measure how much clearance there is with the head off.

Michael

Hello chris
i had the same problem but with a couple of b7es plugs i did a shearch in tech articles and
found one it would appear that some plugs are
made just a bit to long i overcame this by buying a new plug from a different supplier
with a different batch and has been fine since
maybe it has happened with your plug?
cheers b-o-n

Not in this case. The "P" in the NGK part no. indicates a projected tip. I have never seen a B7ES with projected tip but BP7ES does. The "R" indicates a built in suppresion resistor.

Cheers, Don.

Hi Don
Have you looked at the article? under ignition
excessively long plug's, Have a look and let
me know what you think? it seems the problem has been around since 2000
cheer's b-o-n

I did read the article when first posted & I use not a few B7ES myself but have never seen the long ones here.

I have probably 30 or so used B7ES in a can & they are all the non-projected type. When I install new plugs, I clean the old ones for spares.

Cheers, Don.

I cant imagine why you would use iridium plugs in an old bike.

For cleaning plugs try soaking them in bleach over night --You will be amazed at how clean they come,---like new again.

Why is it unimagineable to use iridium plugs in and old bike?

Unecessary expense. Why would you recomend iridiums?

Given the longer life of iridium plugs, I would suggest extra expense is not an issue. Several people on the site have reported improved starting and performance using these plugs. A searchy will bring out all the pros and cons. For myself, I fitted iridum plugs to a G3 and a G11, there was no discernable difference to the single, but the twin was improved in the respect of pick up after downhill drifts, there was always a hesitation on opening the throttle, this vanished with the iridium plugs, picking up cleanly and immediately. The search will also go someway to dispelling doubts on the suitability of the plugs, due to the built in surpressor, with magneto ignition.

Regarding the valves fouling the earth electrode, I thought this was rubbish until I ran some tests on a couple of twin heads. The depth of thread in the heads varied by 50 thou, using the shorter thread head, two of six plugs tried ended up with the earth electrode fouling the inlet valve, the worse being a Champion N7Y with a projected tip. Examining the threads on the plugs showed that the start of the thread never had the same relationship with the earth electrode, plug to plug, in essence the earth electrode could end up anywhere.

Somewhere I read an article by an American gentleman of the adviseability of fitting plugs such that the point where the earth electrode joined the body should face away from the inlet valve, this apparently gave improved combustion, it would also solve the problem of bending the electrode. His method was to mark the plug body in line with the earth electrode, screw the plug in and note where the mark ended up, using difference thicknesses of 'squashable washers' (his term). Thus ending up with the electrode in the desired position.