Thomas 7-1 head sparkplug gap

[glenn in camino]

I've been using .035 but I've read on the barn that It should be less with high compression heads. What should they be set at for 7-1 compression?

 

[Pete]

I've been using .035 but I've read on the barn that It should be less with high compression heads. What should they be set at for 7-1 compression?

The plug gap depends somewhat on the type of ignition.
For most standard battery type ignitions .035 should work fine.
For capacitor discharge types around .050.
Magnetos usually will be around .018 to .025.
Plug types will affect the gap also. (projected or standard nose, side ground electrode, multi ground, resistor etc)

 

[steve s]

The larger the gap and the spark, the better ignition of the fuel/air mixture,

BUT, the higher the pressure of the various gaseous components in the gap, the harder it is for the spark to jump. Pressure in the gap is higher with high compression heads, and, with any head when you open the throttle, which is why you would decrease the gap with HC heads (all else being equal), and why marginal ignition systems produce misfiring and backfiring "under load." ALSO, longer sparks are more easily blown out by turbulence. Normally, lots of turbulence in the combustion chamber is a good thing (for preventing hot spots), but then a stronger ignition system is needed to sustain the spark.

 

[colin1928]

On my Thomas head engine I gap 0.035 no problems
the tricky bit is the heat range

 

[OHV DeLuxe]

Original Winfield book says "Head No 6: Champion No 7 gapped at .025 and for head No 7 use Champion No 13 gapped at .022"
Sounds like Winfield took the higher compression killing spark into consideration. Winfield Head No 7 is 7:1.