Piston spark plug fire slot shape

[dhidaka]

I am having to shim my spark plugs excessively and index them precisely to get clearance with my piston dome. So, I need to cut .060-.080". This is a 2-valve hemi with 14.5CR. How would be the best to aim a fire slot? Do I direct a slot to the intake or exhaust or just cut a flat? Scribe mark indicates the location through the spark plug hole at TDC. Advice please. Also, curious on "swirl" indication on the gas ports.

[Protech Racing]

My guess on the gas ports is that youre getting some oil pull in from the top ring. Does it smoke a tiny bit?
Also , it may run better, faster, if you remove the sharp edges in side the chamber. Maybe cut a gap to open up the plug area .

[Tuner]

Perhaps a different spark plug electrode gap style or a surface gap plug?

[In-Tech]

Perhaps a different spark plug electrode gap style or a surface gap plug?

Good call.
https://www.ngk.com/ngk-3129-br9eq-14-m ... spark-plug
I used to use these on my Toyota 2t/3t stuff and I think the same link shows a surface plug too.

[Tuner]

I don't know if these are still available. Mazda rotary leading and trailing plugs, I recall heat range 7 (leading) and 9 (trailing) were available. We used the 9 range 30 years ago learning how to use 3 or 4 kits = 600+HP multi-layer nitrous. Mazda rotary plug designed to avoid melting the side electrode and they worked great with nitrous for the same reason.

https://www.ngk.com/ngk-4764-bur7eqp-su ... spark-plug

https://www.ngk.com/ngk-5255-bur9eqp-su ... spark-plug

[fordified]

Use a radius end mill and plunge cut at the same angle as the plug. Chamfer the inside and edges with a cartridge roll. Check the dome thickness first to make sure you have enough material to cut that deep. If it’s an angled cut, the amount removed will be less than .080” depending on the angle.

Don’t worry about the gas ports. If you’re rings are sealed you’re fine.