Oil gallery plug size - Pontiac GTO Forum
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post #1 of 2 (permalink) Old 06-14-2019, 08:29 PM Thread Starter
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Oil gallery plug size

Hello again, guys. For a reason that I donít care to admit to, I had to pull and strip my motor again. Since the block is bare I figured it would be a good idea to tap the front oil gallery holes. Using a high pressure pump, those pressure plugs scare me.
Anyway, all of the reading I have done here says I should be tapping a 3/8Ē hole. My issue that those holes are already bigger than that. They mic out to just under 5/8Ē. Not only that but the hidden plug in the back of the motor as well as the one next to the distributor are 5/8Ē.
I measured the existing galley plugs at .60-.65. So those plugs are 5/8Ē. Something is askew here. I guess I should have mentioned the motor is a 389. It was my old mans car and Iím pretty sure he mentioned that the motor came out of a 65 Pontiac wagon.
Any useful knowledge would be appreciated.
Thanks guys. It never takes more than a response or two and I feel like hitting myself in the forehead for not thinking of it. You guys are a lot of help. Pops car hasnít moved since Ď04. I was almost ready to back her out of the garage when the most recent screw up happened. Anyway. Thanks again.
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post #2 of 2 (permalink) Old 06-15-2019, 09:10 AM
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Location: Gastonia, NC - Born & raised in Connecticut - 31 years
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Pipe plugs and thread pitches etc. seem to have their own way of measuring which is confusing to me. Best way I do it just to make sure, get a 3/8" plug and insert it in the known hole in the back of the block if it is already threaded.

3/8" is the right size for the plug. If you use them in front, make sure not to insert too deep and block/cut off the oil supply.
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