plug gaps

[Tony Betts]

i know this will sound a silly one to most of you.

but can someone confirm the plug gaps?

ive always set to 25 thou. but thought id better check the woodrow book.

woodrow says 18 thou?

thanks tony

[Terrytuned]

The recommended gap for most 6v coil and magneto systems was 18thou when 12v was introduced most went to 25thou, now with electronic control systems the gaps are 40thou or even 1.2mm. Terry,

[Tony Betts]

thanks terry.

this is for the super sports and an ulster rep im selling.

there both 6v.

but i feel they were better on 25thou.

so ill adjust them back to 25.

i have the van to go onto next, ill see first how it runs on 18 thou.

thanks again, tony

[Derek Sheldon]

Tony, I’ve always used 18 thou Chummy to Opal and Rosengarts

[JonE]

wasn't there another thread recently with someone saying same, that much better on 25 thou, and something to do with dwell?

[John Cornforth]

The thread JonE refers to was about contact breaker gap.

For plug gap, I have taken a consensus from 1930's Austin publications and other contemporary 6 volt cars with similar ignition coils. This suggests 18 to 22 Thou.

There used to be a combined plug gap / tappet gap gauge in the Austin toolkit, and I suspect the plug blade was 20 thou - maybe someone with such a tool could measure it ?

[Bob Culver]

Large gaps, with a system to support, are beneficial with weak mixtures but in search of power and to avoid seat recession most Sevens are not run weak.
As before the plug gap for giant Double Wasp 18 cyl aero engines is .012".
When I used to drive my car every day in regular traffic I was convinced the engine was more smooth with small gaps. I suspect a large spark gets the burning underway more rapidly and gives a slight effective advance.
Large gaps raise voltage and compund any insulation weaknesses. Must not be used with delicate magnetos.
Decades ago there was an oft repeated advert where an engine was run with normal igniton and elctronic ignition and wide gaps and certifiably tested. There was a power gain. But the ign specs were unusually retarded. and not optimised for each.