Can anyone tell me the sizes of some coolant plugs on a small block:

1) The two at the bottom, center of the block just above the pan rails on each side. They're the ones used to completely flush the each bank of the block. Mine seem to have developed a "slight" amount of rust in the last 30 years and they had to be drilled/extracted out.
2) Also what size are the holes in the cylinder heads where the water temp. sender is suppose to go (between the exhaust ports). These are Darts incase they use a different size than stock.

Now for a stupid question, can someone explain the difference between NPT and coarse threads? The differences between coarse and fine thread is obvious but how does NPT compare to coarse threads? I thought it had something to do with the way they were measured, but what else is it? Like the angle (pitch?) of the threads or what?
Thanks for the help

The small drain plugs in the lower side of the block are 1/4"NPT. The standard plugs are steel and rust, you can use brass or stainless steel plugs if you want - but be sure to coat them with teflon or use anti-seize on the threads so you can remove easily. I often put petcocks (like in your radiator) in these holes if the block will need to be winterized or drained often, like in boats.
The sender units - the sender units - man, I think they are 1/2"NPT - but my mind is drawing a blank - they could be 3/4"NPT. :confused: Just take the sending unit to the auto parts or hardware store and match the sender to their plugs - you'll get the right size.
NPT vs SAE threads - not to hard to describe the main difference. NPT threads have a slight taper to them which results in a tight interference fit as the joint is assembled and tightened. The taper is just a couple of degrees so it is almost invisible to the eye. SAE or standard type threads are straight.

Hey Vintage, thanks for the info. I have the water temp unit in the intake and on the old engine the camel humps didn't have a hole but the new darts do so I need to plug them. If its only those two sizes I can try both and return what doesn't fit, thats no problem.

Now the problem, where can I get a 1/4" NPT tap(as in tap and die) to rethread one of the block holes. I have never seen a set for NPT threads, and I really don't want to buy a whole set just for the one size. Is there any store (preferably a store that I can go to, and not have to order over the net or phone) that carries them?
Also is a 7/16" hole the right size to drill it out to for the 1/4" NPT? I saw this on a chart and want to make sure I read it right.

Home Depot sells the taps separately.

The tap will have the drill size on it.

Like 'Mr. Drop' said - any good home improvement store such as Lowe's or Home Depot will have the Tap in the plumbing or tool dept. Vermont American Tools sells a set that has both the drill and the tap in it.

I was able to measure the head sender hole and it is 1/2"NPT. graemlins/thumbsup.gif

Thank you very much guys, ya know I just might have this engine finished up by the time spring gets here after all. :D