I’ve copied the debate that took place on just this topic on the old forum. I hope this is of help. The A6 NGK plugs work perfectly well on my 1949 600 Flyer with detachable head and it starts first time every time without any mods to the plug. But I have no comparison with other plugs on performance. Hope the comments and replies are of help.
Posted on 03/10/04 at 12:47:18 by Shaun Matthews
On my 1930 Rep (Blind Head)the Spark plug holes are 15/16″ long, Head to Piston clearance a further 3/16″. – Modern 18mm plugs (NGK A6’s) are 9/16″ reach. – This means that the Spark plug is over 1/2″ clear of the piston crown and the spark gap is recesed 15/32″ up the Spark plug hole.
Every other engine I’ve ever seen (excepting Side valve 4 strokes) has the spark plug projecting into the top of the combustion chamber.
The Scott book recomends plugs up to 3/4″ reach.
Is there any benefit in recesing the plugs by 3/8″ so that they are in the ‘ideal’ position (Can easily be done using 10mm fine wire plugs and an adapter) Will also increase compression a bit.
Has any one done this and if so did the timing need retarding?
Posted on 04/18/04 at 05:01:55 by Bob Mather
Further to my report,I have just returned from a 120 mile ride. The engine has more urge, hill climbing is much improved and the ignition does not now require as much advance. 3/4inch reach is definitely the way to go.
Posted on 04/08/04 at 22:10:19 by Bob Mather
I measured my 1932 FY engine. Length of thread in head 1 inch. Head to piston 3/16 inch. The waist in the NGK A6 plugs is no problem.I machined the body above the thread right up to the hexagon which was lightly faced.Dia above the thread 0.628 except the last 0.150 below the hex. which was tapered out 8 deg. giving a better seat for the washer and a plug reach of 13/16 inch. Starting seems the same but up my winding test hill, which normally req. a change down to 2nd the bike climbed quite easily in top.The ignition advance does not seem to req. altering.
Posted on 03/11/04 at 15:36:46 by David J Waring (North West Secretary)
Martin Heckscher is correct – I bought TK5209 (1930 2 speed super) from Glyn Chambers around 1995 .It was running successfully on machined down NGK plugs , which I have replicated and used.
However, Jon Hodges is correct that about 5 years ago or so NGK plugs acquired a “waist” in the body so that machining is not feasible. I have consequently approached The Green Spark Plug man in Timperley for old stock 18mm plugs with full bodies.He is most helpfull on 0161 973 6755 , and has 1944 stock of Champion 7 plugs and other suggestions.
Get stocked up – regards , David.
Posted on 03/12/04 at 02:49:52 by David J Waring (North West Secretary)
Glyn Chambers spark plug theories & practical experiences appeared in “Yowl” in Oct 1993 ; a table of recommended plugs for Scotts was in “Yowl” Dec 1960 – both articles were reprinted in “Technicalities”.
Champion 7 was a “normal touring” grade for a Scott. Those that The Green Spark Plug man has are £5 each.He sold out of the likes of KLG M50 some time ago.
My 1929 Flyer runs really well on Champion UK 10 , which were all I had when I rebuilt it some years ago – maybe it shouldn’t , as I think they are a commercial grade , but fortunately nobody told the bike !
Posted on 03/11/04 at 13:26:34 by Jon Hodges
With ref to machining NGK A6 plugs down check that there is sufficient material to do this if you are buying new ones . The last A6’s I bought had the body relieved in the middle which I would think would preclude machining them down though I haven’t checked this out.
Posted on 03/11/04 at 05:45:36 by Martin Heckscher
A few years ago Glyn Chambers wrote up his experiences with many spark plugs in relation to his Scotts. It included describing how (for some but not all of his bikes) he machines down NGK A6’s to bring the electrodes further down towards the combustion chamber. As far as I know it was a successful exercise. Funnily enough I’m currently experimenting along similar lines to see what difference if any it makes.
Posted on 03/10/04 at 14:39:41 by Brian Marshallby
I have tried 3/4″ reach KLG 18mm plugs in the past and found starting and running to be inferior to NGK A6’s. However they are all different and if you want to experiment why not?? best wishes, Brian.