So....on this same topic, does someone have the definitive answer as to what size the balance tube holes were originally tapped to? How about the outlet ports on the thermostat housing? When I bought my '71, the PO had roughly carved a plug out of wood and wrapped a piece of bailing wire around the balance tube and over the plug to hold it in....I drove it that way until I pulled the engine for my current L28 swap project. At one point, I picked up an NPT plug and tried to plug it but realized the NPT was not the factory fit, so I put the wood back in and left it 😄 (Now that I think about it, that may have been a contributor the obnoxious gas fumes....) LOL. Now I am looking to properly plug all of the unused holes and would like to know what standard was used at the factory so I can acquire the proper plugs. I have done quite a bit of searching on this topic and have not found any definitive answers. At some point, I'd like to Identify all the sizes and compile a list for the Z community to avoid future "mashing" of the wrong size plugs into parts that are becoming harder and harder to find in good condition. I just placed an order with McMaster-Carr for 1/8 and 1/4 BSPT and BSPP plugs and will use them to check the different port sizes (that is, IF I can remove all of the rusty OEM fittings!!! LOL).

I used basic brass plugs from Lowes. 1/4" and 1/8" for the balance tube. Also used thread sealant that I had from making a fuel rail out of aluminum with brass bungs.

http://ecx.images-amazon.com/images/I/51SF7HJG4AL._AC_UL320_SR194,320_.jpg

8 minutes ago, siteunseen said:

I used basic brass plugs from Lowes. 1/4" and 1/8" for the balance tube. Also used thread sealant that I had from making a fuel rail out of aluminum with brass bungs.

http://ecx.images-amazon.com/images/I/51SF7HJG4AL._AC_UL320_SR194,320_.jpg

Yep. Been down all those paths and I STILL don't know (for sure) what size they actually are. I see what you used, but I don't think you used what was intended to go in those holes. I've seen more references to BSPT than anything else, so I suspect that is what it is. There are MANY holes that can be plugged on these cars; holes in the block, holes in the exhaust manifold, holes in the thermo housing, etc. Looks like some may be just metric threads (which are a parallel thread), some are BSPT (I'm presuming, will confirm) and some others may be BSPP. I also googled, "240z balance tube plugs" as was suggested in the thread you linked to, and many of the resources provided by Google were links to places where one could purchase said plugs, but they were not identified as to which size or threadform they were, in fact, a couple of the sellers provided two different plugs with two different threadforms and said both, "will work". Call me picky, but I prefer to use the proper fitting when a threaded hole is provided.

Edited 2 hours ago2 hr by astrohog

Sounds good! Make a plan and stick to it.