Temperature Gauge thread size

[Scott21]

I finnaly drilled out the plug that was inplace of my temperature sending unit port. I have to retap the hole. Does any one know the thread size in the temperature sending unit?

Scott

[wesk]

I believe it was either 1/2" P or 3/4" P thread. I hope you didn't drill too much out. That thread is tapered and reguires a drill size closer to the small end diameter of the hole. If you drilled the entire set of threads out you will be forced to go to the next larger Tapered Pipe thread size which may not fit in the space available in the head.

Male threads: Measure the outside diameter of the large portion of the thread at "A"; Find figure nearest this dimension in column 1 or 2 of chart. The dimension in column 3 will be your nominal pipe thread size.

Female Threads: Measure top diameter of thread at "B"; Find figure nearest this dimension in column 1 or 2 of chart. The dimension in column 3 will be your nominal pipe thread size.

Column 1 Column 2 Column 3 Column 4 Column 5
OD Fraction Inch (for quick reference only) Actual (decimal) Inch. Pipe Thread Size Normal engagement for tight joint (dimension "C") Threads per Inch
5/16 0.3125 1/16 0.2611 27
13/32 0.405 1/8 0.2639 27
35/64 0.540 1/4 0.4018 18
43/64 0.675 3/8 0.4078 18
27/32 0.840 1/2 0.5337 14
1-3/64 1.050 3/4 0.5457 14
1-5/16 1.315 1 0.6828 11-1/2
1-21/32 1.660 1-1/4 0.7068 11-1/2
1-29/32 1.900 1-1/2 0.7235 11-1/2
2-3/8 2.375 2 0.7565 11-1/2
2-7/8 2.875 2-1/2 1.1375 8
3-1/2 3.5 3 1.2000 8
4 4.0 3-1/2 1.2500 8
4-1/2 4.5 4 1.3000 8
Two common pipe thread sizes exist, the tapered National Pipe Thread (NPT) and the straight National Standard Free-Fitting Straight Mechanical Pipe Thread (NPSM). The tapered threads are for joining and sealing, the straight threads are only for joining. The Dry-seal thread (NPTF) allows for joining without sealants. Three less common threads exist, the Garden Hose Thread (GHT), Fire Hose Coupling (NST) and British Standard Taper Pipe Thread (BSPT). The NPT and NPTF threads are interchangeable with sealants such as PTFE tape. None of the other thread standards are interchangeable. Female NPT threads can be designated as "FPT" and male NPT threads can be designated as "MPT."

Pipe sizes do not refer to any physical dimensions. The outside diameter of each pipe or fitting must be measured and compared to the table for size identification. For example, a 3/4" NPT pipe thread has an outside diameter of 1.050 inches. Each thread size has a defined number of threads per inch (TPI). The 3/4" NPT pipe thread has 14 threads per inch. Both the TPI and OD of the thread are required for positive identification of thread size because several sizes have the same TPI.


Definition of pipe thread acronyms
NPT National Pipe Thread
FPT Female Pipe Thread
MPT Male Pipe Thread
PTF SAE short tape pipe thread
NPTF American National taper pipe thread for dryseal pressure-tight joints
NPSM American National straight pipe thread for mechanical joints
NPSI American National straight intermediate pipe thread

Your sender will fit like the illustration C with a rather shallow threaded hole. Therefore when you get ready to tap the threads you must not just run the tap all the way to it's end. You have to insure the diameter at the bottom of your hole is correct for the required small end diameter.
You will normaly drill the pilot hole to the small end diameter then using the correct tapered reamer you ream the hole to a taper. Then you run the tap.

You can check these links for more info:
http://en.wikipedia.org/wiki/National_pipe_thread
http://www.plumbingsupply.com/pipethreadsizing.html
http://www.colder.com/Portals/0/pdfs/NPT.pdf

[Scott21]

Wes
Thanks. There is still a small amount of the plug in the head. I was going to clean it out with a tap.

Scott

[warriorg1]

It should be 1/2" npt. if it is drilled to big for 1/2 you can go with 3/4 npt and use a reducer bushing.

[whydahdvr]

Is this the same for the oil gauge sending unit? Thanks.

[wesk]

The oil gage sender is 1/8" P. Uses a 90 deg iron elbow 1/4" P to 1/8" P and then a straight brass nipple 1/8" P.