npt taps

Has anyone used npt taps before?

I have piece of 10mm aluminium flat bar, my plan is to tap 3 x 1/8 and 1 x 3/8 NPT threads into it, cut a small section out of the top of my inlet manifold and weld it in for vacuum ports.

Did some googling and brought the right size drill bits, but read conflicting advice thereafter and confused the shit outta me lol

My question is how far how far do I tap? how threads/ turns are required? Is my aim to have the maximum number of thread contact when I screw the fitting in?

In the past I've just done a trial and error, tap abit, try the fitting tap abit more etc, worst case if you go too far then abit of ptfe can help alot....

I've attached a simplified version of a spec we use for NPT's at work below. The 0.53 size is where we normally draw the end of the thread to on drawings. The 0.267 size given in the chart is roughly how deep the fitting would screw into the hole. So, if you tap the hole such that the end of the lead-in taper on the end of the tap is over 1/2" into the metal you should be fine. You could always use an NPT fitting as a gauge as well. Tap per the above and then install the plug to make sure it goes in far enough. You're not going up to 1000's PSI internal pressure (hopefully, lol!) so you shouldn't really need to go too deep to get a good attachment and seal out of it.
I might be telling you how to suck eggs here but, you need to put a minimal wrap of PTFE tape around the fitting before screwing it in as technically, NPT's don't actually seal by themselves.
The second chart is roughly how to install/torque the fitting...
Hopefully I haven't confused the issue! 

Tbanks lads, that makes sense, ill just take my time and sounds like i cant go too wrong.

Thanks for that diagram mike, gives me something to go by. I did an ebay search on those fitting gauges and some were damn expensive. Im sure for this application if its close enough should fine and ill use tape aswell

Yeah don't use much of the tape though, especially on a small size like this. One 360° wrap of the tape should suffice 

Thanks mate, thats great, ill give it a go this weekend

Try use paste instead of tape as most are unaware that tape only goes on one direction.

On alloy use a smaller drill bit than on steel.

Drill the holes in the plate than weld it on before tapping. Drill hole into the intake after welding. Weld the plate on top don't cut out a section as it won't be as strong.

If you don't have taps let me know and I'll post down mine. Saves u buying a set for limited use. Or send it to me and I can do it all for you.

Sorry for point form but tired, getting late and have to be up at 4am for another long ass hot shift.

Thanks for the offer mate!

After re-looking at it, I could weld the on plate without cutting it, originally I was thinking the radius of plenum would be awkward to weld a piece of flat bar to but it will be fine.

I have the right spec drills bit that match mikes diagram above and the taps, the only reason I was going to drill and tap before welding was incase I fcked it up lol.

Rest up so you have energy to work on your car