Advice on thread required

[Peter Sanders]

Hi

I have this comercial sprinkler/irrigation system part. (I'm fixing it for someone else). It is a cast brass part and has a gear at one end and two other holes into which sprinkler arms are fastened. (The gear is used AFAIK to drive the unit as the sprinkler arms rotate)

One sprinkler arm has broken, leaving the broken end "corrosion welded" into the central part.

I have since managed to extract the broken piece, the thread suffered some minor damage in the process. I would like to clean up the internal thread and put a new thread on the broken sprinkler arm (as well as on the none broken arm, making both arms the same length).

As the device (we believe) was made in the USA, would I be correct in thinking this would be an NPT thread?

The sprinkler arm has an ID of about .5" so I am guessing this could be a 1/2" NPT thread. What would you think?

FWIW, a 5/8" UNF bolt screws (loosely) into the internal thread.

Thanks for your help.

[smiller6912]

First, you should be able to see a taper (~1.75 deg) on the thread if it is NPT. Second NPT threads are way bigger that the thread name size. A 3/8-18 NPT has an OD of .675, a 1/4-18 NPT has an OD of .540

[Peter Sanders]

Hi

I didn't mention that I am in Western Australia. So I am not completely familiar with NPT as most of our pipe threads are BSP.

Observation of the thread shows what appears to be a taper at the beginning of the thread and potentially parallel for the remainder. I am using an outside thread on the same cast brass part to assume the internal thread profile is the same only samller in diameter.

It does not appear to be as tapered as a BSP thread.

I measured the internal diameter of the thread and this is ~.597" perhaps then this is a 3/8" NPT"

Do you know how close a 5/8" UNF thread is to 3/8" NPT thread?

[SGW]

See http://en.wikipedia.org/wiki/National_pipe_thread for info on NPT.

5/8" NF is 18 tpi, so it may well be 3/8" NPT.