18mm Spark Plugs

[Paul W]

For many years i successfully used 18mm NGK a6 plugs in my blind head, short stroke flyer. I then began to have problems with them in more recent times (premature failure after just a few miles). I read somewhere that manufacturing processes changed but whether that really affected how the plugs worked in the Scott combustion environment i’m not sure… anyway, i’m about to fire up the Scott after a complete magdyno rebuild and re-wire for solid state voltage regulation and am looking to get some new 18mm plugs (i don’t want to use 14mm with a reducer at this stage): does anyone have any recommendations for modern manufactured 18mm plugs in a Scott (like D14, AB-6 etc) that actually last ok ?

Cheers,

Paul

[BRIAN MARSHALL]

I’ve heard that the problem is caused by the porcelain insulator no longer being glazed, but can’t really confirm that. If I were you I would try the repro KLG ML50 plugs from the Green Spark Plug Company. They are a longer reach than the NGK 18mm plugs so you MUST put an 18mm tap down the plug holes to clear any carbon build up from the innermost threads, before trying to install them. The full story is that these plugs are being made at the behest of the Bugatti Owners’ Club, who were desperate for long-reach 18mm plugs. When Kenelm Lee Guinness died his KLG plugs were made by Smiths Industries, but in recent years the long-reach plugs went out of production. Tim Green was able to acquire the tooling and commission a batch to be made. Other benefits of these plugs are that they give an increase in compression, and no Japanese trade marks !

[Roadster]

Many years ago I had a similar problem: I would lose my spark after about 100miles, I would fit a new pair of plugs, manage another 100miles then same thing. After about half a dozen pairs of plugs I pulled the mag. off and there was an almost invisible crack in the slip ring. New slip ring fitted and I then had six pairs of good useable plugs!
As your mag. has now been rebuilt is it not worth trying your old NGK A6 plugs again?
Rod

[Stan Thomas]

Firstly, don’t use those ‘orrible reducers, as they alter the intended heat-value of the plug and can also lead to pre-ignition and/or “running-on”.

Instead, use Champion D21 18mm plugs (or equivalent) which can easily be obtained off the internet without paying fancy prices.

Champion D21 plugs (or equivalent) have a heat-range which will accommodate the sort of “oiling” associated with Sotts.

[Tman]

I bought a pair of 18mm’s from the GSP co some years back for a French single I was renovating at the time, and they both failed to fire the engine as they weren’t sparking. I contacted the GSP people for a refund and he wouldn’t issue one as apparently I hadn’t “followed the procedure”.
This (he said) meant starting the bike on the old plugs then removing it/them and putting in the new ones while the engine is still warm.
“But this is a rebuild and I never had any old plugs” to which the answer was “tough, you’ve sooted/wetted the plugs under the insulator nose now and they’re useless”.
Tried all the usual methods of cleaning them but to no avail.
They have a good range of stuff but I avoid them now. Another customer lost…

[Paul W]

Many thanks for the replies… I think my previous experiences echo some of the comments where new plugs have failed immediately or after a few 10’s of miles. Also, i believe there are plentiful supplies of fake plugs out there which, unfortunately, raises doubts over buying on-line unless from a reputable supplier.

I still have a couple of unused NGK AB6’s which i will try as well some of the KLG and Champion ones recommended below. I’ll post an update depending on how i get on, and once the winter salt subsides…

Cheers,

Paul

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