Hi guys,

I recently purchased a 390 long block. I have assembled the engine and getting ready for first run to break in the cam. after making all connections for running the engine and filling the radiator with water I found that water was leaking from the block on the driver side from a small hole that on some fe blocks it's supposed to be for either a drain petcock or a drain plug. Mine has neither one of these two.
It looks like the metal in the hole rusted through. I cleaned up the hole and measured the size. The hole measures between .430 to .450 which I think is roughly 7/16"?
My question is how do I go about tapping this hole for a plug or petcock? Does it have to be NPT? Do I need to go one size up to 1/2"? Never done this before.

Hole before cleaning:







After cleaning:

 

I could be wrong , but I think that takes a Galley plug , (small freeze plug )

 

The SECOND picture appears to show threads. Are there any usable threads left?

If there are usable threads, or if you have to drill it and tap it, use NPT. Otherwise you will have a tough time making it seal methinks.

 

Somebody may have messed up the threads, and put a small freeze plug in it. But it takes the same size drain **** as is on the bottom of a radiator, tapered threads.

 

Thanks to everyone who answered! I figured it out. I went out and bought a 1/4" NPT tap and cleaned up the threads. It was so dirty you could not tell that there were any threads left but the tap went right in and did it's job. I plugged the hole temporarily with a 1/4" NPT drain plug. After breaking the cam in i am going to install a brass petcock.

 

There are two of those drains holes - one on each side of the block. Some have pet***** and some have threaded pipe plugs. Removal of them is the only way to fully drain the block...


If using headers, use pipe plugs either with a protruding, square head or recessed for an Allen wrench because they offer the most clearance.