• When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.

Correct Spark Plug Gap?

Ronald Weishaar

Well-Known Member
Local time
11:13 PM
Oct 18, 2018
Reaction score
I been setting my spark plug at .035 but I'm second guessing that it should be more. Engine is a 540 cu.in. 10.5 to 1 compression, Indy 440-EZ-1 Heads, MSD 7AL-2 ignition box, MSD Distributor, and MSD Pro Power coil. I'm using Champion RC12YC plugs recommended by Indy. Does my plug gap need to be wider?
I was going to say I run about .040 on my 500in motor very similar to yours.
It depends on how much spark your ignition system can deliver. With my HEI, I can n do run .055 on my 496.
That MSD system should fire larger than .045, so that's plenty safe, maybe up to .060. Mopar recommended .060 for early MSD 404BC, predecessor to 7AL.
Open your gap by 0.005" at a time, and when it starts to miss, back it off 0.005". I ran my 426W with and Accel coil and MSD 6AL with the Autolites gapped to 0.055".
The higher the cylinder pressure and the wider the gap, the more energy is required to make it spark. That was the main point in using magneto's for performance and aircraft applications. The faster the engine spins, the more output by the Mag.