Correct Drill Bit For 3/8” UNC Tap

Hi,

I need to make a plate to hold a cylinder head on my lathe. The head will be held on using cylinder head studs. The thread on the end of the stud is 3/8 UNC. I think I have a 3/8 UNC tap but what drill bit should I use?
According to a search on the web says I need a 14.75 drill bit if the TPI is 19?

The standard tap drill for 3/8 UNC is 8mm or 5/16.

3/8 19 tpi is pipe thread 3/8 unc is 16 tpi and drill 8.25mm

14.75 doesn't sound right.

This tap chart suggests 5/16" which is 7.94mm. I'd round that up to 8mm, or even a bit bigger. (Tap drill sizes given in tables are for a tight fit, which puts a lot of strain on the tap. In home workshops it's common to drill a little bigger than recommended because it makes life easy for the tap in exchange for a moderate reduction in strength. Don't do it if the fastening has to be full-strength.)

Dave

14.75 mm is the tapping size for 3/8” British Standard Pipe Thread which is 19 tpi. As previously mentioned the tapping drill for 3/8” UNC is much smaller at around 5/16”.

Ah yes! Watch out for pipe.

BSP is weird because 3/8" refers to the inside diameter of the pipe, not the outside diameter.

In contrast, fasteners always refer to the outside diameter as in 3/8" UNC, Whitworth, BSF etc, but note that the exact tapping drill size depends on the thread form and pitch too. Although there are formula, the easiest way to find tap drill diameters is to look them up in a table.

I recommend buying a copy of Tubal Cain's Model Engineer's Handbook. In addition to the usual tables, the discussion section on %thread engagement is well worth reading. Although I also have heavy references like Machinery's, I usually start with Tubal Cain because he targets small workshop needs so well, often with pertinent advice.

Dave

The Workshop Practice book says 8.3 to 8.5 depending on the engagement - 73% to 60%.

Zeus Charts give the tapping drill for 3/8 x 16 UNC as:8 mm (BSI Recommended ) or 5/16 as an alternative.

If you have not got a set; well worth investing in. Still using mine bought in 1958! .

Always a source of reference for threads and many other things, (BA, BSW, BSF, BSP, ANC, ANF, UNF, UNC, Metric Coarse, Metric Fine, Fits, Metal Bending Allowances, Hardness Comparisons, Co ordinates for equally spaced holes, Morse taper dimensions, Mathematical tables. )

Howard

from the Machinery's Screw Thread Book:

3/8 UNC - 99% = 7.6mm, 94% = 7.7mm, 89% = 7.8%, 5/16" = 82%, 8mm = 78%,

8.1mm = 73%, 8.2mm = 68%, 8.3mm = 63%, 21/64" = 61%

Cheers, Tim

Thanks everyone for your help.

So i drilled an 8mm hole and this 3/8" Tap just seems way too big???

It's labelled STI3/8 - 16 HSS

Edited By William Harvey 1 on 11/12/2021 13:45:29

Ah just sussed it, I looked in my small box of taps and found another 3/8 NC 16 tap (left in the pic). This one is much smaller, the other one that starts STI Helicoil Set?

What type of tap is it? taper, second, plug, bottoming, spiral point, spiral flute, form tap

Someone on another forum with the same problem and it looks like he has a helicoil tap, that would need a 9.9mm drill. Can you post a link to the exact tap and a photo in case they are using a generic picture

You're right - the larger tap cuts a thread to take a wire insert core, and the insert takes a 3/8"-16 screw. STI = Screw Thread Insert. It's "special".

Unless you're in North America best not to get entangled with US threads unless you have to, especially inserts. The mixture of Imperial and Metric Threads available in the UK is quite confusing enough.

Have you picked up a bargain box of unknown tools? They often contain surprises - stuff bought to do special repairs etc. They may not match your needs, for example I associate inserts with repair work rather than Model Engineering.

To keep my non-modelling experimental workshop simple, I majored on metric threads and only buy in BA, Whitworth and UN as required - not often! A modeller might tool up with BA, BS and ME threads for the same reason, and only use metric on special occasions.

Do you have a particular hobby direction in mind? (I didn't at first.) If so, might be better build something to a local plan, and buy whatever is needed to do it.

Dave

Edited By SillyOldDuffer on 11/12/2021 15:18:20

Dave he has got a classic Mini and they use UNC threads in the A series engine