'63 340hp aluminum intake 3794129 threaded holes

[Irby G.]

NC - National Course (straight), NF - National Fine (straight), NPT - National Pipe Thread (tapered) are the types of thread pipe holes I have just found out about.
I had no idea that there were different types of taps for threaded holes until I have now purchased 2 taps and both are wrong. I was just wanting the correct tap to chase the threads on my intake. Does anyone know the proper tap for my '63 340hp aluminum intake 3794129?
I'm not sure whether to use NF or NPT. Since it is the water jacket, I would assume NPT but assumptions have awarded me with 2 paper weights now. I measured the plug that was in there and it appears to me to be 3/4-14. So I bought a 3/4-14 NPT tap...WAY too big.
Thanks,
Irby

[Joe L.]

Re: '63 340hp aluminum intake 3794129 threaded holes

NC - National Course (straight), NF - National Fine (straight), NPT - National Pipe Thread (tapered) are the types of thread pipe holes I have just found out about.
I had no idea that there were different types of taps for threaded holes until I have now purchased 2 taps and both are wrong. I was just wanting the correct tap to chase the threads on my intake. Does anyone know the proper tap for my '63 340hp aluminum intake 3794129?
I'm not sure whether to use NF or NPT. Since it is the water jacket, I would assume NPT but assumptions have awarded me with 2 paper weights now. I measured the plug that was in there and it appears to me to be 3/4-14. So I bought a 3/4-14 NPT tap...WAY too big.
Thanks,
Irby

Irby-------


Which holes are you referring to? If you are referring to the heater hose fitting or the temp gauge fitting, it's 1/2-14 NPT. This is a very common thread size used on older Corvettes for pipe thread size fittings.

[Irby G.]

Re: '63 340hp aluminum intake 3794129 threaded holes

Hi Joe,
Yep. That's the ones. So it's 1/2"-14 NPT.
Where I went wrong was...at first I got a 7/8-14 NF plug tap. It was ~1/8" too large. So, had to be a 3/4"-14. Only this time I got a NPT tap. The 3/4-14 NPT was MUCH larger than even the 7/8" NF tap. Still can't wrap my head around that one. Seems to me, if it is a tapered thread, you would measure ~ 1/2 way up the threads to get the O.D. of the tap. Anyway, for whatever reason NPT taps are much larger than NF or NC taps.
Thanks for the info Joe.

[Joe L.]

Re: '63 340hp aluminum intake 3794129 threaded holes

Hi Joe,
Yep. That's the ones. So it's 1/2"-14 NPT.
Where I went wrong was...at first I got a 7/8-14 NF plug tap. It was ~1/8" too large. So, had to be a 3/4"-14. Only this time I got a NPT tap. The 3/4-14 NPT was MUCH larger than even the 7/8" tap. Still can't wrap my head around that one. Seems to me, if it is a tapered thread, you would measure ~ 1/2 way up the threads to get the O.D. of the tap. Anyway, for whatever reason NPT taps are much larger than NF or NC taps.
Thanks for the info Joe.

Irby------


NC and NF are machine threads. These are based on the "actual" OD (or ID) of the fastener but with different thread counts. The vast majority of machine thread sizes used on C1 through C3 Corvettes use either of these two thread types. There are a few exceptions (e.g. front and rear spindles) but not too many.

NPT is a tapered thread. A completely different convention is used for describing these threads. As a rule-of-thumb, the orifice size will be about 1/4" greater than the "inch" size of the tap (e.g. a 1/2" NPT tap will be about 3/4" OD). Virtually all the pipe thread fittings on a Corvette will be NPT. The only exception I can think of is the fittings for AIR exhaust manifolds.