thread tapping question

IrishBeatdown · Nov 29, 2012

Bleh, so I was making some modifications to my intake manifold, specifically where the vacuum line from the bov connects to the im. I wanted to run a stainless line in its place, so i decided to drill and tap the manifold to accept a 1/8 NPT to -4AN fitting. Drilled the hole to 21/64". Went to go thread in the fitting, and the hole is too large, to the point where the fitting barely goes in but isnt threading. I used a 1/8 - 27 NPT tap. What in the living hell? Did i need to use a different tap for this fitting? I'm damn confused, maybe someone could shed some light on this. Going to have to weld in the fitting now. :banghead:

gsxtasy · Nov 29, 2012

That is the correct size bit and tap for that fitting, it should work. As long as you're sure that is the bit size you used and you have the right fitting, I don't see how it couldn't fit right. Did you hold the drill bit up to the fitting and make certain it wasn't too big?

NHerron · Nov 29, 2012

So is the hole too big and you obviously can't go back or is it too small and the threads aren't catching exactly. Sorry this bit got me mixed up.

Cool idea though I bet it'll look great afterwards

IrishBeatdown said:
Went to go thread in the fitting, and the hole is too large, to the point where the fitting barely goes in but isnt threading.

IrishBeatdown · Nov 29, 2012

NHerron said:
So is the hole too big and you obviously can't go back or is it too small and the threads aren't catching exactly. Sorry this bit got me mixed up.

Cool idea though I bet it'll look great afterwards

The hole is tapped cleanly, the fitting is just too small for it. Hole is too large.

So I re-checked the sizes, the drill bit is dead on accurate according to my calipers. I checked size of my fittings, I have a bunch of 1/8 NPT fittings lying around, they are all .38" OD, the tap is .42" OD though. I'm starting to think this tap is labeled wrong, it clearly says 1/8 -27 on the side, but my calipers say it is too big. It was from a cheapy tap - die set. So at this point im going to go ahead and say this tap is a piece of crap mislabled whoremother until someone else can refute it.

NHerron · Nov 29, 2012

Heres a link maybe?
NPT- National Pipe Thread Taper- ANSI B1.20.1

WES_393 · Nov 29, 2012

Did you use a drill/tap set specifically for heli-coil? They will often label the parts with the next size down because thats the bolt size you will use, not how large the actual heli-coil (and hole) will be.

Shot in the dark, but possible.

IrishBeatdown · Nov 29, 2012

WES_393 said:
Did you use a drill/tap set specifically for heli-coil? They will often label the parts with the next size down because thats the bolt size you will use, not how large the actual heli-coil (and hole) will be.

Shot in the dark, but possible.

No, this is off just a regular tap/die set. I'm about to throw the whole set in the trash, serves me right for buying a 20$ set off of amazon and being a cheapass.

pauleyman · Nov 29, 2012

NPT taps are tapered. How far did you thread in the tap?

IrishBeatdown · Nov 29, 2012

pauleyman said:
NPT taps are tapered. How far did you thread in the tap?

id say 10 threads. too far?

BogusSVO · Nov 29, 2012

Thats the thing with NPT taps, go too far and it becomes to large.

Tap a little, then test fit the fitting, untill you get it where you want.

Sonds like you may want to take the hole to a 1/4 NPT, then install a reducer bushing for the 1/8 NPT

dsmcurse · Nov 29, 2012

^^beat me to it^^

Ya, what he said

IrishBeatdown · Nov 29, 2012

going to have to do something else, i dont think i have enough space to go to 1/4" as this is on the raised port of the stock 2g intake manifold. most likely welding. i need to chill out with the tap next time i guess.

Diamond Star Werks · Nov 29, 2012

IrishBeatdown said:
going to have to do something else, i dont think i have enough space to go to 1/4" as this is on the raised port of the stock 2g intake manifold. most likely welding. i need to chill out with the tap next time i guess.

Yeah slow down there happy feet and don't tap so much next time

my why don't you post a pic so we can see how much room you have? But i think i know why your saying weld it

NHerron · Nov 29, 2012

A couple options, you could get a new IM off the classifieds they seem pretty cheap. Or buy a 1/4 npt aluminum bung to weld onto the manifold, like an exhaust bung for a wideband. Or just weld like your saying

funcar · Nov 29, 2012

In my machine shop the rule of thumb for npt's is tap 6 threads from the top. So from the big end of tap count down 6 threads and tap to that point. In 15 yrs I have never been screwed with this method. Even with some of my customers who check our parts with thread gauges ,never a problem.

IrishBeatdown · Nov 29, 2012

NHerron said:
A couple options, you could get a new IM off the classifieds they seem pretty cheap. Or buy a 1/4 npt aluminum bung to weld onto the manifold, like an exhaust bung for a wideband. Or just weld like your saying

I'm going to weld a bung on. For some reason im opposed to buying stock parts for my car as a replacement if I don't have to, I feel like the ones I have belong to my ride. I'd rather fix than abandon. My head, for instance, the previous owner stripped the ever loving shit out of the exhaust mani stud holes, even tapped one to m10 and stripped that. Would have been cheaper to buy a new head, but i coiled a few of the holes myself and paid to have the m10 hole fixed at the machine shop so i could keep it.

The Top DSM Community on the Web

thread tapping question

IrishBeatdown

gsxtasy

NHerron

IrishBeatdown

NHerron

WES_393

IrishBeatdown

pauleyman

IrishBeatdown

BogusSVO

dsmcurse

IrishBeatdown

Diamond Star Werks

NHerron

funcar

IrishBeatdown

Similar threads

Latest posts

Build Thread Updates

Vendor Updates

Latest Classifieds