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Spark Plug type for G31 600 OHV

Started by alexd912, 20 Jul 2009 at 16:18

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Hi there,

I have been trawling through the old post trying to find the model number for a 600 OHV engine. Is there anyone out there that know the correct model number and if possible for NGK as they are common round these parts:)


Best regards
1930 Douglas G31



I do not think there was quite as much of a selection of heat ranges in those days, so the engine is not that fickle. The only published data I have seen (Pitman Manuals) recomends the Lodge H1 for the sidevalve and standard OHV machines (the F/G31 would be the latter) with the recessed electrode Lodge H45 special sparkplug recommended for super sports and Dirt Track engines.

I was running my 1934 OW1 600cc OHV on Autolite BT6 spark plugs, and it seemed to be happy with them.



Hi Alex
If your machine has 18mm plugs, try either NGK A6 or A7 - they seem to work well in veteran/vintage machines generally.
Cheers, Graeme


Hi Graeme,

Thanks for this and as it happens my messerschmitt 3 wheeler has 18 mm plugs and they are AB6 if I remember correctly:)


Best regards
1930 Douglas G31


I use NGK plugs on my OHV 600 - but have sleeved the heads to 14mm. I run a 7 on the front and a 6 on the rear cylinder - probably no need for the difference now - had oil fouling problems on the rear one while it was running in and being sorted. In Australia the plain A series 18mm plugs are no longer available - AB is the only one (only difference is the size of the hex). AB6 as Graeme said seem suitable for most old bikes - I use them on a number.


Hi All
      I was recently given a copy of an original KLG Sparking Plug chart illustrating only KLG Mica plugs. I run most of my early machines on these plugs which are attractive with their polished brown mica insulation with a brass band on top. They are also detachable for easy cleaning. Interestingly in recent postings there was discussion on the "reach" of plugs and whether performance would be better with plugs that actually reach the combustion chamber. Well, according to the KLG Chart they only recommend one plug to suit all road going Douglas machines and that is Type K1 and this happens to be their shortest reach plug. (For sports models they recommend type KS5 which is also very short reach)
    Over the years collecting these KLG plugs I have picked up a few other grades, some with longer bodies and/or with longer reach but I cannot honestly say that I have ever noticed any difference in performance. My view is that in slow revving, low compression side-valve engines with the usual Douglas variations between cylinder barrels of combustion chamber volume and casting thickness it does not matter a darn as long as you get sparks, even from 80 year old antique sparking plugs. No doubt this will get some people excited.
Regards   Chris.