Has anyone got a 19/32 drill bit they don't mind selling or loaning to me? I'm doing some engineering but having looked, these seem to have either a tapered shaft suitable for a lathe tail-stock or silly expensive! Alternatively, a 14.75mm is the metric equivalent.
I used an online conversion website that told me 14.75 was 19/32. Basically i need the correct drill to tap a female 3/8 npt thread... so i looked up an online table that told me i needed a 14.75mm drill... but... being an imperial measurement i assumed that it must equate to a standard imperial measurement and 19/32 seemed to be it. Does anyone have a npt chart (or a bspt chart) that states the correct drill to tap a 3/8 internal thread?
Having checked again, 9/16 is close which is 14.28 so 14.5 might be ok. Any American guys know what size hole you need to drill to tap a 3/8 npt thread? Is it a numbered drill rather than an actual dimension?
There is likely to be both 9/16 and 19/32 in my workshop (need to check!) - but they will be morse taper which is no good to you. Presumably you need a reduced shank drill? Is it practical to post the part that needs the hole?
Drill America 9/16" Reduced Shank High Speed Steel Drill Bit with 1/2" Shank, DWDRSD Series Amazon.co.uk
There are two main distinctive types for the NPT, namely the basic NPT and the NPTF. Both are tapered, as with the BSPT. The basic NPT is for general plumbing and fittings for general service. The latter (NPTF) is an industry standard for fittings on pipes and devices meant to handle fuels as well as for certain electrical cable entries/glandings in hazardous industrial applications, i.e., petrochemical, etc. It is of the 'dryseal' type which, means it relies on metal-to-metal contact of the tread engagement to seal off fluids/gasses instead of using sealants for the purpose. Which one do you intend to make, Colin? If its the basic 3/8"'-18 tpi, then a standard 9/16" or a metric 14.3mm equivalent would do. A 14.25mm diam drill would be closer but hard to obtain and isn't really critical anyway. If your intention is for a 3/8"-18 NPTF, then a drilled-hole size of 37/64" diametre would be more appropriate. That'd be closer to 14.68 which means, 14.7mm as the closest standard size. But then, it depends on the tap type you're having at hand. And depends on the tapping depth's extent. Whether you intend to tap-through the hole or partial. Tapping too deep through would turn the female(inside thread) into a straight 3/8"-18 tpi instead of tapered, with dimensions assuming the larger-diam end of the tap. However, its still generally correct to assume that making a slightly smaller hole would result in a closer-tolerance fitting.
Having done a bit more homework I've settled on a 14.5mm drill which will be reamed out with a tapered reamer to take a 3/8th BSPT 18 tpi taper tap. This exercise will allow me to fit a brass reducer on an otherwise useless tank so that an alternative control valve assembly can be fitted. The tank I'm altering is neither safe, or in any way rare or I wouldn't be doing it. When it's finished I will most likely post the results.
You are absolutely right @MYN - I was getting confused but I've sorted it out (sorted myself out I guess!!!). I'm using and adapting a brass reducer in order to use a different control valve on an unwanted tank (a Coleman 275 that had suffered fire damage - now there's a surprise!). The outer thread of the reducer is 3/8 BSP 18 tpi but the smaller, internal thread is 1/8 NPT. I'm also working with a mate from the US where NPT is standard - and confusingly, it is 19 tpi. With all the different US and UK tap sizes I was getting myself properly befuddled but I've sorted it out now! The internal thread I'm cutting will be a 3/8 inch, tapered tap of 18 tpi. I'm using a straight 14.5mm drill to remove the existing threads and then using a reamer to add the correct taper which should give me a tight fuel resistant joint between the reducer and the tank's central bung or boss. I may also use loctite for extra security if I think it's necessary. This is something of an experiment and I will probably have to fiddle with the depth of the taper that I cut before I'm happy, but it'll work - oh, and it will use kero, not CF or Naptha as I don't have a death wish!!! Why am I doing this? Because the 275 tank has a lovely shape - I think so anyway - but the reputation of it as a fireball waiting to happen is a very good reason NOT to use it as originally intended. Sure Coleman say they fixed the problem with the updated version but let's be honest, no they didn't! It still uses a Schrader valve and once that reaches the end of it's life, it's fireball time again! Anyway, once I've added the brass reducer, there are a couple of different path's I might take - I haven't made up my mind just yet. Watch this space. Lastly and very importantly, I'm using bits and pieces that are genuinely useless - I've been collecting lanterns long enough to know what can and can't be reused and the lantern I assemble will not be sold or used by anyone but me. I also like an engineering challenge and creating new and useful items from 'dead' parts is fun.
I might not be correct but I've read over at CCF that the Coleman 275 has the same thread size-pitch as the Coleman 237 for the boss/valve block. If that's true, then it would be 1/2"-32 tpi Unified Thread(UN-32) size and tapered. Its not NPT and its also not exactly a standard UN size. I've made a plug for the 237 boss/bung before. A 3/8" reducer for NPT and BSPT would have their outer major diametres of around 17.055 and 16.662mm respectively.
