Spark plug gap

[billspit]

Yesterday I was installing new Champion plugs in the 80 Spitfire engine I have rebuilt. The book says the gap should be 0.025". Its been a long time since I've installed plugs in one of these engines, but is that correct? Seems like everything now is 0.040" or so.

 

[Andrew Mace]

.025" is correct, at least for points ignitions. Larger gaps are common nowadays with electronic ignitions, and it's possible that your '80 Spitfire (presumably equipped with some sort of electronic ignition) might benefit from a gap somewhat larger than the old .025"?

 

[billspit]

Once I get the engine broken in, I will be going with a Pertronix ignition. It orginally had the electronic ignition that all late Spits had.

 

[trfourtune]

to go to the big gaps, you need a HOT coil and something like an MSD, Crane, etc.
Rob

 

[poolboy]

No question,a larger gap is going to require either a coil with greater potential voltage output, 20k vs 40k OR increase dwell time for the coil to build voltage.
The draw back is that higher voltage will tend to carbonize the rotor and contacts in the dizzy cap or worse,so carry spares.
Although this article addresses Ballasted Ignition, there's something to be learned regarding the relationship between plug gap and coil potential.
https://www.vtr.org/maintain/ballast.shtml