The Stovebolt Site Forums The Shop Area Engine Shop Spark plug gap.

Ok I am looking at my trusty haynes book and for spark plug gap for a 250 its got 3 different gaps. Since 1968 is not listed I am going down the years. 1970-1974 0.0035
1975 0.0060
1976 0.045
1977 and on 0.0045

Which one should I use?

Thanks

You won't go wrong with 0.035 - 0.040. Canadian dimes are 0.035! Always preferred plain old delco plugs too, had too many problems with the fancy stuff, splitfire, etc. (anybody else have similar experience?) My memory serve that they're R45's?

The differences are due to ignition. With a points distributor, gap at .035. With a HEI (75 & on), gap at .045. The .060 was a good idea, but caused the coil to burn out too early.

1978 250ci HEI plug gap on underhood sticker says set at .035

I'm sure it does, but results from guys on the 67-72 board have shown that a gap of .045 works best. The .060 gap worked real good, but overheated the coil. I remember the early HEI Lincolns were gapped at .080.

Fred, yeah, I've heard lots about 0.060 too, I think it's even reccomended for the R44XLS, but I've never got anything to run well with them that wide. Maybe my choice of coils?? Who knows, but I gave up years ago trying anything above 0.045 - found it caused stumbles on WOT acceleration. Brad is still trying to get his truck going, 0.035 is a gap that is at least will give a good start and idle to mid range. A thought though... Geography; I'm 700 feet above sea level here in Toronto with about 29 inHg. Would altitude make any difference to gap reccomendations?

Yes, probably. Whether a given gap will fire or not is a function of the pressure and richness of a mixture. A coil fires its plug by trying to force all the energy in it through the plug. The voltage at the coil builds until the spark plug fires, and then the current is limited by the resistance of the spark plug wire and the spark plug. Too much gap or rich mixture causes the voltage to get too high and it breaks down the coil insulation and overheats the wires. The optimum gap is wide enough to get a good hot spark but not wide enough to kill the coil. The wires play a big part in it too, and they die quickly in a high energy system. My Miata has a high compression (10:1) high revving (7000 rpm) engine with individual coils for each cylinder and plug gap of 1mm. (about 0.040"). It chews up ignition wires every 50k miles, even the expensive lifetime guaranteed ones. Running the gap a bit on the narrow side seems to help things a bit, and it still fires at 7k rpm. It just wears the plugs out faster, but they are cheaper than wires.