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Hello again, guys. For a reason that I don’t 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 3/8” hole. My issue that those holes are already bigger than that. They mic out to just under 5/8”. Not only that but the hidden plug in the back of the motor as well as the one next to the distributor are 5/8”.
I measured the existing galley plugs at .60-.65. So those plugs are 5/8”. Something is askew here. I guess I should have mentioned the motor is a 389. It was my old mans car and I’m 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 hasn’t moved since ‘04. 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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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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