I am trying to locate a bolt to replace a leaky sending unit installed by the previous owner of my '71 240z.  I got the sending unit out but it was 1/4 brass pipe and not the correct thread or size. The picture below is from my lime green '72 240z but it is pointing at the bolt I need.  To my absolute shock I do not have one of these in my collection (hoard) of bolts.  I know it is larger than M12x1.5 as my bolt gauge only goes up to that size and it was still loose.  It may be a M13 or an M14 but no bigger than a M14.  Any input is greatly appreciated.  Thank you!

If memory is correct the thread is BPT. Try McMaster-Carr.

So here is what I am working with.  I am actually glad I took these pictures as I can see I need to get my pick and pull out whatever that piece of debris is in the hole.  It makes sense British Pipe Thread but I still need to know the size.  

On my L28 that hole is the same size as the one that supplies the heater core, in the back, passenger-side, of the head.  They can be swapped.  Might help if somebody has a measurement for that part, the elbow.  It looks smaller on your L24 though, but if you have the time you could remove the elbow and try it in the hole.  Then you'd have a male part that you could more easily measure.

I have an L28 plug that I could remove but I don't have a thread gauge so would have to guesstimate.

This is the bolt from the first picture. Zcardepot sells it for $3.99



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Thank you Nick!  Order placed, $11.60 shipped.  Should be here in a few days.

Unfortunately the plug I ordered is not the right size.  It is about 1/8 inch too big and course threaded.  Anyone have any idea what size that hole is?

Definitely looks smaller than my L28 plug, in the same position, so the head elbow's not going to fit.  I just went out and looked at mine and it takes up much more of the boss than yours.  Interesting.

I'd bet good money that it's the same size as one of the coolant passage fittings in the thermostat housing.  Or maybe a carburetor/manifold fitting.  Were it mine, I'd remove a few coolant fittings and/or senders or sensors, and try them.

Mystery solved! After a lot of trial and error I figured out the hole size and thread.  It is 1/2 inch -20 (fine thread).  I had tried several 12mm bolts and they were close but still left a little wiggle room. I thought it might be 13mm but I quickly found that 13mm is almost obsolete and 14mm would be too big. Using an online calculator, 12.7mm = 0.5 inches.  So I ran to Lowe's and I could tell 1/2 course thread was too aggressive so I bought a 1/2-20 bolt and a 1/2-20 allen key cap screw.  I tested with the bolt first and eureka, IT FIT!  I then tested the cap screw and it fit too.  I wrapped the cap screw with yellow sealant tape and installed it.  I topped off the coolant, let the car warm up to temperature, and took it for a spirited run.  All was dry and looking good so I call it job done!  For anyone that needs to buy one of the cap screws, it is a Hillman brand fastener, model 880938, Lowes item 137329.