Tool and Die question

M

mcfoo

Well-known member
I need to tap a 1/8-27" NPT threaded hole into the front of a block off plate.

The "book" says to use a 'R" lettered drill bit which measures .339".

I've got quite a few tools but no lettered bits.

Does anyone know if a fractional or numbered bit would suffice to accomplish the same thing and what would it be?

Since this is a coolant block off on the front of my Miata prod motor I can't afford any leaks.

Thanks

Tom B.
 
I have always used an "R" drill at 0.3390 in. An 11/16 in. is 0.3438 in. and should work well with the tapered pipe fit. The next fractional smaller size from "R" is 21/64 in. = 0.3281 in. That size may be hard to start the tap. You could tap some scrap and check what works.
 
11/32 0.34375 inch
R 0.339 inch

11/32 is 0.005 inch larger than an R sized drill

A 1/8 27 NPT male or female is tapered

I'd use two smaller drills before the final drill size (all sharp drills) to control the hole size.

Buy a hardware store R drill and your set for future holes. Tap and drill at Home depot $4.42
 
Tom,

Suggest you use the special tapered reamer for your specific tap size. Kinda pricy, but yields uniform thread depths, minimizes seizing on major diameter, and centers the threads better. McMaster-Carr lists them under reamers, even though they're specifically for use with. pipe taps.

Very helpful to use copious amounts of the right lubricant for the material your tapping. Aluminum very grabby; brass & cast iron chippy; steel just plain tough. Each prefers a different lube. (lard actually works well on cast iron)

Ron Sorensen
 
Also when tapping a tapered pipe tap. DO NOT run the tap all the way through the threads. I suggest several passes and use a fitting as a go no go for how deep to tap. You can over tap a pipe thread very easily and leave yourself with a piece of scrap metal.

Tapered reamers work well, but the taper on a 1/8 npt is very small and not necessary.

i suggest if you are going to do this more than once...Hangsterfers Hard-cut or Super All-Tap
 
one more for ya... if the material is soft, then the cheap drill bit sets from Horror Freight have a letter, number, and fractional set. think I paid like $19 for it a decade ago and I still have most of it. looks like similar/same is $39 now. https://www.harborfreight.com/high-spee ... 61620.html

I wouldn't use them on anything tougher than mild steel, but if you're doing a block off plate I assume that's going to be alu or something somewhat easy to cut.

Anyway, those huge sets get you all of those weird sizes you use once every few years... and lots of cheap pilot bits. ;)
 
Thanks guys.
The reason I have a fair number of tools is that I once owned two(2) ACE hardware stores....but for some reason we never carried letter'd drill bits(my bad).
so I never had the chance to bring any home..(lol)
Some of the crap they sell in HF turns my stomach as they now refer to themselves as "America's Hardware store".... sure they are.
Thankfully most of my own stuff is USA made including my Irwin bits. I even own some Channellock and ViceGrips made in the good ole USA.

I happened to find an "R" bit at a fairly local mill supply store. When I called to ask if they had any he said "sure how many do you want"....now that's a real hardware supply store.

My block off plate is aluminum so preparing it or a AN to NPT fitting should be fairly easy.

Appreciate all the input and from a guy who contracted covid back in the early days, do what is necessary to protect yourself and loved ones...I was never hospitalized but was pretty damn sick for 10+ days.

Regards
Tom B.
 
well that's cause most of America now buys their hardware from China, and horror freight gives them what they ask for. they're opening new stores all over down here, so their business model is working.
I have a select set of cobalt drill bits for high volume or hard steel work, but otherwise I just use the cheapies and they do well enough.
I needed some metric stuff a while back and called a machine tool supply place called CW Rod Tool. I was amazed at the options.. want a cobalt, left twist, 12.15mm jobber length bit? got it. need a 3-flute instead of 2? got it. wait wut? all of it on the shelf in Houston. dang. then again, it was a $30 drill bit...... but I got it!
 
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