Spark plug confusion
March 23rd, 2015, 12:21 PM
#1
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Location: Kettering, OH
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Spark plug confusion
the car has been gone for about 15 years so I can't remember much about some of these things. 68 cutlass s 350. it now has a olds HEI distributor. I tried to find an answer on this site and the internet but I am confused with what I found. one person says one thing and one says another. when we got the car back it has AC-R45TS plugs in it. ir ran fine on the way back with it except a small miss at idle. in removing the plugs 3 of them were only finger tight. R45TS is a tapered plug, the head looks to be made for a gasket plug. I think I used a gasket plug on it before we sold it but I can't remember. what is the right type plug for the engine.
March 23rd, 2015, 09:38 PM
#7
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No, you do not need a six tenths of an inch gap. 
As said above, .045" will work fine.
There is no requirement to increase the plug gap with the HEI. The gap will depend a lot upon the compression ratio of the engine: the "smog engines" of the late 70s/early 80s had low compression and diluted intake charges so a wider gap was used to help ignite the lean mixture; on the other hand, an engine with higher CR and denser air:fuel mixture will typically use a smaller gap so that the spark can jump across the denser mixture.
As said above, .045" will work fine.
There is no requirement to increase the plug gap with the HEI. The gap will depend a lot upon the compression ratio of the engine: the "smog engines" of the late 70s/early 80s had low compression and diluted intake charges so a wider gap was used to help ignite the lean mixture; on the other hand, an engine with higher CR and denser air:fuel mixture will typically use a smaller gap so that the spark can jump across the denser mixture.
March 24th, 2015, 06:16 AM
#8
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You've already received much information. Here's a bit more. I hope that it doesn't create more confusion. If the heads were not modified you will need a spark plug with a flat (washered) type seat and a 3/8" reach (not counting the "S" extension). In 1968 there were high and low compression 350s. The originally recommended plugs were: for high compression, AC 44S, Autolite A42, and Champion UJ-10Y; and for low compression, AC 45S, Autolite A52, and Champion UJ-12Y. All spark plug gaps were recommended at 0.030". The use of resistor spark plugs started in 1969, but could easily be used for your engine also. Their designations will include an "R". The HEI will usually allow for a larger gap. Larger gaps will generally produce better combustion but will also increase the chance of misfire. Some early specifications for HEI spark plug gaps were as high as 0.080". After everyone sobered up and discovered the error of their ways, it was usually specified at 0.060". These were generally lower compression engines. When the HEI is used in a high compression engine, the gap should be less. Some spark plugs come pre-gapped at the larger gaps, but you can reduce them to whatever suits your situation. (You can also widen the ones with the smaller gaps.) I have a high compression engine with an HEI. I try to keep the gaps in the 0.045-0.050" range. All seems to work well. The tightening torque for the plugs is 35 ft-pounds. If you put a small amount of oil or anti-seize on the threads they will like you better. (And be sure that the gasket seats are free of crud.) 
March 24th, 2015, 06:30 AM
#9
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Location: Marble Falls TX
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Dug these out of dark storage. I bought them yeas ago for the low-c HEI
455's if memory serves. They are factory gapped at .060. I don't remember
what the asterisk ment. Maybe that was the replacement # for the plug they goofed on?
455's if memory serves. They are factory gapped at .060. I don't remember
what the asterisk ment. Maybe that was the replacement # for the plug they goofed on?
Last edited by tru-blue 442; March 24th, 2015 at 07:08 AM.
March 24th, 2015, 06:59 AM
#10
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Tru-blue those have an unusually short porcelain. Maybe the asterisk refers to that? I don't ever remember seeing either in my travels. Either the asterisk or the short porcelain (at least on AC plugs). I've seen them in other brands for header clearance but not on a stock engine.
March 24th, 2015, 08:38 AM
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March 24th, 2015, 06:31 PM
#13
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Location: Kettering, OH
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thanks for all the help and info guy's. I went to the parts store and we came up with the R45S plug. it looks to be the right plug. I installed them with a .045 gap with new plug wires It will be a couple of days before I try to start it. the carb will be back and I will let you know what happens. thanks
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