I’m new to Scotts – currently getting a ’38 500cc Flying Sqirrel ready for the road (DPZ engine).
The engine runs, but the spark plugs are spares from my car – rather than the correct ones, so I’m a bit reluctant to run the engine for any length of time – can anyone tell me the correct ‘plug to use for this engine?
Thanks in anticipation
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.
Thanks, Rod, for the information. If I understand correctly, the ‘plugs listed are all 18mm thread. My engine has (bronze?) inserts in the head to take 14 mm plugs – is this normal? has it been ‘converted’? can I remove these and use the (recommended) 18 mm plugs?
Over the last 36 years running 14mm without inserts detachable head motors with various compression ratios up to 15 to 1(Full squish head).
I have found that NGK B6HES(lohg reach) with spacers, or B6HS(short reach) with the combustion chamber modified so that the electrodes slightly enter the combustion chamber work very well. I have used these for racing,sprinting and with nitrous oxide injection. I do however run less advance on all my engines, around 25deg.
Ian Pearce used the same grade of plug(eg a NGK 6) when racing and uses an “adjusted” B5HES for the road with excellent results. I’ve never bothered to run a 5 as I’ve had no trouble on a 6(I use a mag, Ian is on points with his road bike). Ian also uses less advance than std.
There does seem to be a power penalty if a short reach plug is used .
I have only run one engine with inserts, but only for a short time. I suppose you might get a hotter running plug and perhaps in that case you’d need to run a B7HES.
I wish you luck!
Glad to be of help. My knowledge of Scotts is limited, but not engines in general. Personally I would keep the inserts. If ever you strip a thread it would give you an extra option on how to repair and could be the reason the inserts were fitted in the first place. Others have done the same conversion and it has worked allowing you easier access to easily obtainable plugs.
The next thing to look at is the plug condition, if the colour is correct and the bike is running well, what’s the point of upsetting things. I’m not suggesting that new ideas should not be tried, and if we can make an improvement then why not. But removing an insert from an alloy head that has been in any time from an unknown quality of thread could leave you with a headache that you might regret.
As I say I am not an expert on Scotts but the above is just a personal view.
Just for the record
I use NGK Iridium BR8EIX for racing with 21 deg BTDC ignition
I would use BR7EIX for a tuned iron engine. ie gas flowed and ignition back to about 27 BTDC
I would use BR6EIX for a standard engine with lower compession pressures and thus lower burning speed and ultimate temperatures. Ignition advance standard ie 34 / 35 BTDC.
Note that if you improve the breathing you must retard the ignition or you will risk detonation, overheating and seizure.
If you want to check timing set TDC, make mark on flywheel, use flexible thin steel rule bent over flywheel, mark every 2mm. This is almost exactly 1 degree on a standard flywheel, so you only need to mark as far as you need.
Obviously you choose a reference line for zero on your crankcase and lay a rule or strip accross, or make a pointer.
All the NGK Iridium plugs seem to have a built in resistor. I have heard that this can damage the magneto. It there any truth to this?
Sparking Plugs with an inbuilt resistor are bad news for a magneto. As a result of the resistor the magneto has to generate a higher voltage to overcome the resistance of the resistor plus the resistance of the air gap. This increased voltage puts a greater strain on the armature windings and their insulation. This will eventually cause them to burn out and fail.
I would also advise against using resistor plug caps for the same reason.
I run both types of plugs on engines with points but only non resistor plugs with magnetos and you run a relatively soft plug for a Scott. If you find it not running totally clean go a grade softer but always check plug colours before. There are so many variations (14mm, 18mm, long reach, short reach) of head I cannot recommend a plug without knowing that info but once that is established you are down to just a couple of choices.
I personally use Champions as I always have found them more consistent than NGK’s (manufacturing quality wise) and for serious beans a 6 to 8, general fast riding an 8 or 9, and town and slow riding a 10 or 11 grade. I’ve run as hard as a 4 but you need a spot on mixture and to ride it flat out to get it to run clean and hard plugs are generally more difficult to get the bike started. Note the numbers are for the general N range and do not directly translate to the L82/L86/D14/D16 etc
Technicalities seection 5.4 covers the subject too.
Let us know what thread and reach your head has and I’m sure you’ll get a few recommendations.
I note the very first post mentions using plugs from a car. Of course Minis used N9 plugs (now N9YC or N9YCC) for long reach 14mm which work very well in a Scott too! About the right plug for general riding if they fit your head.
Further to the discussions about moving the plug further into the chamber this of course is easliy acheived by using a projected tip plug which most soft plugs tend to be.
Hope that helps,
A suppressor is normally about 20K ohms and will make very little difference to the HT voltage, in fact it will reduce it slightly. When I was serving my apprenticeship at the BTH in Ford Street in Coventry in the early fifties they did tests with suppressors on the magneto’s and actually found that it improved the spark, it cut out spiking and gave a cleaner spark. In those days you could be prosecuted for causing interference to the television signal. I run my Scott on Champion L86 and have no problems, they last for ever, if you do need to replace them you can buy them from the Green sparkplug gentleman for about £1.50.
In my old reply on 18mm A6 NGK plugs I stated they could be machined down to increase the reach. I no longer recommend this as it leaves very little metal and it is easy to shear off plugs, I have seen a friend do it.
In my opinion the best way is to Helicoil the plug holes down to 14 mm.
I did this and found an unused pair of KLG FE 65P thats 14mm, longreach (3/4) inch and extended nose. These plugs have been in over 3200 miles and not given an ounce of trouble
When I bought my bike it came with a pair of NGK B7HS plugs but they look like they have been in the engine for a looong time so I thought a change would be good.
Some folks seem to like the Iridium plus but they are all resistor types.
The B7HS fitted means that I have 14m holes and implies short reach but I don’t know how to go about confirming that reach is indeed 1/2″
B7HS does seem a little hard for road use. Too see the plug reach you can use a small torch and look down a plug hole – you can tell the difference in length between 1/2″ (short reach) and 3/4″ (long reach) easily with your eye.
Running long reach plugs in short reach heads gives two problems. The first is the electrode can touch the piston crown and close up the plug gap making starting a tad difficult. The second is the 1/4″ of thread from the plug that sticks down into the combustion chamber gets carbon coated and this makes the plugs very difficult to remove and strips the head thead in the process.
For NGK provided you have the 1/2″ deep threads in the head you would probably want B6HS or B5HS, with the respective Champion plugs being L82C and L86C. I note the post from Ted below about using L86 too.
You can find the full Scott fitment Champion range on my website if you are interested.
OK, I have a pair of unused B6HS so I’ll stay away from the Iridium plugs and give the B6HS a try. Any suggestions on gaps?
20 thou is generally the accepted gap for mag and points ign, going up to nearer 30 thou for CDI ignition.
If you set the gap to 20 thou you should get a nice fat blue spark when just spinning the mag over by hand. If the spark is feint, pale or non existent at that speed either the leads/pickups/carbon brushes are shot, points gap is wrong, the internal timing is wrong, or it is time for a mag rewind. 1001 starting problems are fixed with a rewound mag!