The title says it. The part that screws into the carburetor appears to be ¼" NPT. Anybody know what the other thread is called?
The part that screws in the carb is 1/8 NPT. The other thread is 1/4 NPS.
Hard to tell from the photo but the stock T carb inlet is 1/8 npt where it screws into the carb and 1/2 x 18 TPI (straight thread) where the cap screws on.
Pipe threads were available in straight and tapered and the taps and dies are still available today.
Yep, I got ½-18 when I measured the big end. Now that I know it's called ¼ NPS I know what to search for.
More detailed information on tapered and straight pipe threads is available in this Acrobat document from Reed Machinery.
The 1/4" NPS thread is 18 threads per inch on a .540" diameter.
It is not NPS thread, it is UNS.
Interstate - 1/2-18 UNS, 1 Inch Outside Diameter High Speed Steel Round Die
Right Hand Thread, Adjustable
MSC Part #: 63323430
Interstate - 1/2-18 UNS, H3, 4 Flute, Bright Finish, High Speed Steel, Straight Flute Standard Hand Tap
Plug Chamfer, Right Hand Thread, 3-3/8 Inch Overall Length, 3B Class of Fit
MSC Part #: 04844080
Price: $37.62 ea
I suspect that the tap and die Stan has linked to are the same thread per inch and diameter as 1/4 NPS. However the devil is in the details isn't it?
I bought some reproduction fuel line nuts from a T part vendor a couple years ago. They were the proper thread per inch and proper ID, but the nuts would not thread onto original fuel fittings like the one in Steve's picture. They would lock up within the first couple of threads.
I had to run a 1/4" NPS tap in each one to make it work.
I have no idea what UNS thread is, but suspect it differs in an important detail from the correct NPS tap in the thread angle. Perhaps the reproductions are using 1/4 UNS thread. That would explain the fact that they don't fit.
Here's a 1/4 NPS tap for sale on eBay:
IIRC, UNS stands for Unified National Special
Keep that original. Even though they are not scarce, the modern ones don't look the same.
My Snap On Tap and Die sets have taps and dies marked with "NC" "NF" and "NPT". Not one tap or die in any of the sets I own are marked UNS. Is this something that came after the Reagan administration?
The U is for a unified standard thread form which has a radius at the root. The nc or nf thread form has a flat at the root. Modern standard threads are unf, unc, unef etc. The uns threads are non standard threads per inch with a unified standard thread form. NPT is standard pipe thread, tapered. NPS is standard pipe thread, no taper.
I have probably 30 or 40 special thread taps and dies. I'm willing to bet your Snap On set doesn't have a 9/16 x 24 or 3/8 x 27 but those are very common carburetor threads.
My sets for 7/8 x 24; 7/8 x 32, 1 x 24 and 1 x 32 were well over $200 for just those four specialty taps and dies.
Should have said well over $200 each set; well over $400 for the two sets.
I mentioned my Snap On Set only because it is the newest one that I own. I bought it in 1980 for about $200 I think. It has paid for itself many times over. Good tools are worth having if you use them.
That's why I have a couple thousand dollars worth of specialty taps and dies. Use them all the time. I wish they were American made, most are from Japan, China and Taiwan. If Am made are available I try to buy them.
There is a list of specialty taps and dies listed in the MSC catalog. When restoring my Packard Twin Six most threads were 2 threads per inch from standard thread practices. This early thread was often called carriage or blacksmith thread.
The specialty taps and dies are mostly from Poland and actually are quite accurate and the quality is very good.