[62eyadams]

Today I'm putting coolant into my rebuilt engine for the first time, and after about a gallon I see coolant pouring out of a hole at the back of the block. The hole is in the block, on the passenger side, below the spark plugs, roughly between the 3 and 4 plugs. I think it's 5/16 in diameter, and fine threaded.

I've attached a picture of the hole, with a bolt put in to keep more coolant from leaking out. You can clearly see the oil filter and exhaust manifold in the picture.

So - what should go here? I assume a bolt is wrong, but beyond that I'm a little stumped.

[Mark W]

I looked at my old 63 motor and it has.......A BOLT!

[Jack Gifford]

Appears to be a likely location for a coolant drain plug or petcock. If so, it would be a 1/8 NPT hole, which can be mistaken for a 3/8-24 thread. But the NPT is tapered, to provide sealing as it's tightened. Try a 1/8" pipe piece (plug, fitting, nipple, whatever) to determine if that's what the thread is. Do this before trying to tighten a 3/8-24 bolt into it, which would harm a pipe thread hole. If it's NPT, just use the appropriate plug with a little sealer on it. If you like the convenience of a coolant drain, buy a 1/8 NPT petcock.

[62eyadams]

Thanks, Jack. I tried a 1/8th, and it was too big. A 3/16ths plug fit loosely but sealed the hole. I'll take it back out and put some sealer on it just to be thorough.

[Jack Gifford]

Not to belabor the point, but I'm not sure what a 3/16" pipe plug is- no such thing as 3/16" pipe threads per ANS (American National Standards), neither straight nor tapered.

I didn't mean to infer that it is definitely a tapered pipe thread, just that it seemed highly likely. It's possible that it is, in fact, a straight thread intended to take a bolt (maybe with a sealing washer). Can you get a good enough look at it to see whether it's tapered or straight? Or do you have a set of pitch guages to actually measure the threads per inch? 1/16 & 1/8 pipe are 27 threads per inch, and 5/16 & 3/8 NF bolts are 24.

I'd be leery of using a loose-fitting plug and depending on sealant to hold it. The use of a sealant with pipe threads is merely to fill microscopic gaps in the thread fit, not for retention of the plug.

Maybe someone with that exact same block can chime in here?

[62eyadams]

I can't get a good enough look at it to see if it's tapered, and all I have to measure the size is a collection of bolts left over from reassembly. Funny how there's always bolts left over. The 3/16ths plug I bought was at Pep Boys isn't an "NPT" (what does that stand for, anyway?). But it fits. I should have been clearer when I said "loosely" - it went in more easily than the bolt. But that could be solely because the bolt already cleared out the paint that was in the hole.

Anyway, I put silicon sealer on the plug and put in coolant, and everything is nice and dry. I haven't fired up the engine yet (due to unrelated problems), but I'm not worried about this leaking.

[george kujanski]

NPT is National Pipe Thread which is a specification for a tapered thread that locks itself into the mating threads the tighter you turn it. A bolt thread is a straight thread.

I agree, that location looks like a cooing jacket drain plug and should be NPT.

BTW, a 1/8" NPT plug is nowhere near 1/8". It's larger. Typical pipe threads are 1/8", 1/4", 3/8", 1/2" NPT.

George

[Stuart]

My '61 Bonneville has drain plugs in either side of the 389 block in that location, so I assume the 4 cylinder has a similar drain plug. It's 1/8" NPT thread. I did the same thing once - forgot to put the plugs back in before pouring in the antifreeze. You can get a 1/8" NPT plug at any hardware store that will work, but I'd put in a petcock instead - it will make it a lot easier to drain the block the next time. I used petcocks like you'd find in the bottom of a radiator. You should be able to get them at any good auto parts store that has the boxes of Dorman products in the back.



https://www.dormanproducts.com/cgi-b...ist=1378:468:0