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 Thursday at 12:40 AM2 days by astrohog

Sounds good! Make a plan and stick to it.

You will be quite supposed on how easily the bspt plugs thread in. You will know right away.

I'm also planning on removing the water tubes that run through the intake manifold. Are these BSPT as well? I'll find out soon enough, I suppose.

I believe that all the tapered fittings are BPT. Probably dictated by world manufacturing climate at the time.

Some of the BPT and their respective NPT counterparts are different enough to easily tell the difference, and some of them are really similar. For example, with some thread sealant, it usually seals if you stuff a 1/8 NPT plug into a hole threaded for 1/8 BPT. But, I think the BPT is correct.

There also does exist some locations where a straight thread was used (temp gauge sender unit for example), but I've found them to be straight up metric thread, not BSPP.

1 hour ago, Captain Obvious said:

I believe that all the tapered fittings are BPT. Probably dictated by world manufacturing climate at the time.

Some of the BPT and their respective NPT counterparts are different enough to easily tell the difference, and some of them are really similar. For example, with some thread sealant, it usually seals if you stuff a 1/8 NPT plug into a hole threaded for 1/8 BPT. But, I think the BPT is correct.

There also does exist some locations where a straight thread was used (temp gauge sender unit for example), but I've found them to be straight up metric thread, not BSPP.

Thanks Captain! This is more the info I was looking for. I guess I will be able to verify once my assortment of plugs arrive from Mcmaster.

Yeah, hopefully it should be easy to tell.

In any event, it's got to be better than this!!! Hahaha!

On 11/19/2025 at 6:53 PM, astrohog said:

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....

OK, here we go.....

The threads on the balance tube all seem to be BSPT (1/8" and 1/4")

The threads on my intake manifold where the water lines used to go are still a mystery. I have a feeling they are just a metric thread (not pipe thread), so I will have to do more investigating when I have an assortment of metric bolts to try. Also, my thermostat housing is at my storage, so I cannot check those outlets right now, either. I guess everything is a process......stay tuned!

Here's my new dilemma; The pipe plugs I ordered from McMaster for sizing purposes are external hex head, and they do not thread deep enough into the balance tube for the head to bottom out and seat on the flat of the balance tube. Not the clean look I was going for. I think I'll try the internal hex drive (like a regular socket set screw) plugs and hope they sit flush or a little recessed when seated. If I don't get a good, clean look that I like, I may have to weld the holes up and grind them smooth.

Oh, the lengths we'll go to for these cars!!!! LOL

49 minutes ago, astrohog said:

external hex head, and they do not thread deep enough into the balance tube for the head to bottom out and seat on the flat of the balance tube.

Tapered pipe threads are supposed to tighten up on the threads, not the underside of the head. In fact, if the head were to bottom out, the threads would run the risk of not being tight enough to seal. In other words... The hex headed plugs you have there are working exactly like they should.

You would stand a better chance of achieving your "clean look" with the recessed hex head "Allen screw" type plugs.

Yes, I understand how they are designed, I just wanna use them how I wanna use them! LOL

I really wouldn't like the look even if the heads DID bottom out, still kinda klunky looking. The BSPP plugs on the other hand (they have a lower profile head), look pretty good, but they won't bottom out in the tapered holes.....so I guess I could tap the holes to a parallel thread and use thread sealant.

I think you are right though, once I get all the holes I need to plug measured, I will order the allen screw type.