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Old 11-15-2005, 10:44 PM
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recomended spark plug..

This seems to be a pretty simple, basic question so im gonna post it here..
I need a recomended plug Ive got a 68 327 with 71 heads 3986336. Its about time for a tune up But im a little confused on which plugs to buy I found a site that said my heads used 5/8 plugs, I didnt think 5/8 came till some time after 71 but theres 13/16 plugs in there.. I want to make sure I put the correct plug in there so I dont cause any damage (from a plug thats too long maybe)
Im fully aware it all depends how the engine is built wether I need a hot plug..cold plug...etc Id like a recomendation on the correct size I believe I have RY45's in there now but dont know if these are right
Does anyone have any documentation on a stock plug for 3986336 head
71 350 76cc chambers...thats all the info I have

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Old 11-17-2005, 12:37 PM
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Chevy switched to the 5/8" or "peanut" style plugs in 71. If you have 13/16", it may be a carry over early model or some one may have actually put older heads on during a rebuild.

The threads are actually the same. The difference is how the plugs seat against the head. 13/16" have a compression gasket/lockwasher to hold it tight to the head. The 5/8" have a tapered fitting and a smaller wrench size.

They are not interchangeable. Both sizes have the same heat range selections available.

I suggest you use the delco R44 or equivalent. Get the extended tip ones. I am not sure weather it is the T or S that designates extended tip.
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Old 11-18-2005, 07:17 AM
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Thanks Larry
I ended up getting some AC Delco Rapid Fire #1's. There were 3 types of plugs in there I guess from my father-in-law... But anyway wanted to make sure i was getting the correct plug..
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