As I'm working on a 260z engine, I found somebody decided to plug some holes in the balance tube by using two 10mm straight thread cap screws (bolts). The original holes were tapered but they forced metric cap screws into the holes, ruining the threads. Notice they were only able to get the screws in part way. I've attach a photo, I circled the two locations. Without drilling the holes out I was able to thread the holes with a 1/8-27 tapered pipe tap, thus allowing me to use 1/8" plugs. All other plugs I've found seem to be the proper threads.

Jim

Well let's hope fixing those two plugs helps your running issue. If the two 10mm head bolts were screwed in somewhat tight, they probably sealed "mostly". If those bolts are the only vacuum leak you found, they probably weren't large leaks.

Oh, and I don't see anything else glaringly wrong in the pic, but was wondering... When the flat top to round top conversion was done, did they (you?) use a new intake/exhaust manifold gasket, or reuse the old one?

I can't remember on the gasket, swapped carbs years ago. Going down another rabbit hole now with brake problems. Can we still get the gasket?

I was considering a master vac leak, pumped on brakes, pedal went to floor, resivor for rear brakes was dry, filled up, fluid going somewhere. Rebuilt cylinder, now other issues. Should put that on hold and get back to vac leak problem.

Cap the booster line for now. The booster is probably ruined from the fluid. It'll need a rebuild. Pretty common problem...

What he said. Just cap off the brake booster and the transmission vacuum lines for now.

As for the intake/exhaust gasket, yes they are cheap and readily available. Only problem is in order to replace it, you need to pull both the intake and the exhaust off the head. And that extra trouble step is why sometimes people try to reuse the old gasket.

If yours was in good enough condition to maybe reuse, then some silicone would probably make it leak free. But if half of your old gasket peeled off the head and stayed with you original flat top intake manifolds.... Well then, you're asking for trouble.

A friend of mine has a smoke machine, good for finding air and vacuum leaks, if my intake gasket is not leaking, I'm not going to touch it.

Jim

Okay, so I've received various plugs and set screws from both McMaster and Bel-Metric. I ordered 1/8 and 1/4 NPT, 1/8 and 1/4 BSPT, M12 x 1.50 straight thread and M12 x 1.50 tapered thread, and I am still a bit confused!

I won't hold you hostage until the end, I'll give you what I THINK the holes were from factory, and then I'll provide some pictures and notes from my "testing".

I think the factory balance tube (US market 240Z) came with 2 ea. - 1/8" BSPT and 5 ea. - 1/4" BSPT holes

BALANCE TUBE 1/4" BSPT HOLES

• it was clear to me that 1/4" NPT is too big, couldn't really even get a thread to start, so it makes sense that, 1) One COULD force 1/4" NPT into the holes in the soft aluminum and create some sort of "seal", and 2) a hole that had been "buggered by a previous (or current!) owner could likely be salvaged by tapping to 1/4" NPT.

• The 1/4" BSPT plugs I have encounter resistance at 1-1/2 turns and are fairly tight at 1-3/4 - 2 turns. This leaves them sticking out farther than I would like for a clean, plugged look as I would like them to be as close to flush as possible.

• The M12 x 1.50 straight thread, socket set screw I tried fit very loose initially, and threaded quite a ways in (farthest of the three options I tried) before finally hitting resistance. I feel like it MIGHT have finally created a "seal", but not ideal in my opinion.

• The M12 x 1.50 TAPERED plug encountered resistance at about 3-1/4 turns and was fairly tight at 3-1/2 turns in. This left it pretty close to flush to the surface and provided a good seal and I think this is what I will be using to plug 1/4" BSPT holes. (see pictures below)

BALANCE TUBE 1/8" BSPT HOLES

• it was clear to me that 1/8" NPT is too big, couldn't really even get a thread to start, so it makes sense that, 1) One COULD force 1/8" NPT into the holes in the soft aluminum and create some sort of "seal", and, 2) a hole that had been "buggered" by a previous (or current!) owner could likely be salvaged by tapping to 1/8" NPT.

• The 1/8" BSPT plugs I have, encounter resistance at 1-1/2 turns and are fairly tight at 2 turns in. This leaves them sticking out more than I would like for a clean look (see pictures below)

• now I want to experiment with M10 fine thread (both straight and tapered) in the 1/8 BSPT holes

INTAKE MANIFOLD WATER HEATING HOLES

• The outside (inlet and outlet) holes come from the factory with 1/4" BSPT holes

• Again, the 1/4" BSPT plugs don't thread in very far and sit proud.

• The M12 x 1.50 TAPER thread plugs thread in farther and sit pretty flush for a clean look.

• the inner holes (two holes on the inside of each part of the two-part manifold that connects the manifold halves with "bushings" and metal tubing) come from the factory with M16 x 1.50 STRAIGHT thread.

• I have some M16 x 1.50 straight AND taper plugs coming to try in those holes. I'm now thinking that a straight-threaded plug with a low head and Loctited, may be the ideal ticket for this hole since there is a machined spot to accommodate a bolt head. Heck, any old M16 x 1.50 bolt will work here! Keep in mind that these holes, if not being used (not connected to water lines), can stay open, they do not need to be sealed/plugged, as they will not affect anything other than keeping the intakes warm on initial start-up in cooler weather/climates. I just want to plug them for aesthetic purposes.

Here's some pictures to clarify my rambling!

Edited 1 hour ago1 hr by astrohog