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Old 02-07-2007, 08:56 PM   #1
noahdallas
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Unhappy Spark plug gap for a '75 350 please...

I have asked before but could not find the thread. I have a '75 350 bored .040 over. I have a '69 pickup and thats what I "assumed" the engine was. I got the block and head numbers and it came up to be a '75. When I thought it was a '69 the guys at Autozone said the gap was 35 and for the '75 it is 60 if I remeber correctly. Any help would be appreciated.
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Old 02-07-2007, 09:07 PM   #2
Russell
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Re: Spark plug gap for a '75 350 please...

I'd run the plug gap at .045 myself, and run a newer HEI distributor Not sure what the gap should be with points though...
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Old 02-07-2007, 09:22 PM   #3
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Re: Spark plug gap for a '75 350 please...

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Originally Posted by Russell View Post
I'd run the plug gap at .045 myself, and run a newer HEI distributor Not sure what the gap should be with points though...
oops... it does have HEI... I was hoping changing the plugs would give me a faster response. I did change the plug wires, timing chain, cam, lifters and carb (Holley 650)about six months ago. After some tuning and carb adjusting it has been running pretty decent. I just need a little faster response till my big block is done and the way my kids have been eating might be a little while.

Last edited by noahdallas; 02-07-2007 at 09:22 PM.
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Old 02-07-2007, 09:24 PM   #4
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Re: Spark plug gap for a '75 350 please...

HEI should be .045.
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Old 02-07-2007, 10:12 PM   #5
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Re: Spark plug gap for a '75 350 please...

Points is .035
Hei is .045
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Old 02-07-2007, 10:17 PM   #6
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Re: Spark plug gap for a '75 350 please...

.045 it is... thanks everyone...
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Old 02-07-2007, 11:31 PM   #7
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Re: Spark plug gap for a '75 350 please...

noahdallas from what I have come across most books will tell you .045 for HEI, but this is misleading because the plug manufatuer actually determines the plug gap. Let me explain the OEM for you engine may be .045 for 75 chevy 350 with say AC R44t spark plug BTW that is the correct number for you, Then say since you want a better plug you go to a AC R44tx same thread, same reach, same taper, plug gap now .060 all because of the plug design. The main thing you have to be concerned with is getting the right thread size, reach, and taper if needed from your parts store wich can be tricky on rebuilt engines (ex. heads not off of original engine and vise versa.) SO that being said plug gap depends on what plug your parts guy gave you, and by no means don't trust all the counter guys at your local part stores.
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Old 02-08-2007, 12:59 AM   #8
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Re: Spark plug gap for a '75 350 please...

I think GM was recommending .060" back then, but changed their recommendation to .045" after a few years. Might be why early HEI plug wires used to get fried before they'd get to 50,000 miles.
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Old 02-08-2007, 06:27 AM   #9
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Re: Spark plug gap for a '75 350 please...

you will cook a stock hei if you run a gap bigger than .045. the coil and module cant handle it. wider gap means more electricity to jump the gap.
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Old 02-08-2007, 11:23 AM   #10
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Re: Spark plug gap for a '75 350 please...

Quote:
you will cook a stock hei if you run a gap bigger than .045. the coil and module cant handle it. wider gap means more electricity to jump the gap.
This is true to a point, which is the case with most stock replacement plugs. But many of todays performance specific plugs are being built with different resistance properties as is the case with many wire manufactuers also, So inturn you can run wider gaps for better combustion and gas milage at no risk to our older ignition systems. You do need to do some homework to make sure where you are at in this scenario. Technology is a wonderful thing sometimes and I think this is one of them.
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