Hi all,
Are there any "rules of thumb" when it comes to determining the optimum spark plug gap? Does the plug condition give you any clues or do generally go by engine performance perfomance?
I am running an msd unit on an hemi 265( Aussie engine) which the stock gap is .035. msds spec recommendations are .050 - .060 for engines up to 10.5:1 ( my engine is 9.5:1). . The plugs are currently set at .045, which seems to be fine.
I am a little cautious about opening the gap much more and whether I'll know when Ive gone too far.

Gappin it... Jeremy

.035 is normal for a point-type ignition. .045 is what's recommended for an HEI system.[.50 is half an inch]

Cheers 68 ... have put that 0 in the right place! 1/2 inch would be some gap!

Optimumly the bigger the gap the bigger the spark.
However eventually you reach a point where the ignition system can't maintain the spark over a wide gap because there is just too much resistance in the form of empty space. The stronger the ignition in regards to both voltage and amperage the more gap it can tolerate. Volts is the push to jump the gap and amps is the volume that decides just how strong that spark is.

.045" should be right or you could go with the MSD spec of .050". Wider gap will usually give a little better idle quality.

Make sure your HT wires are in excellent shape, or you could have problems with arcing in damp weather.