Cylinder drain cock thread?

My 3 x 5 twin Victorian steam engine restoration plods on. The cylinder ends each have a threaded hole which I naturally assume was for some kind of fitting. The only things that come to mind are drain cocks

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My main query is about the thread size, it measures 1/2" OD ot the top, with a core about .48" narrowing to around .46" about 3/8" in. but it's 25 TPI. I've checked this numerous times. I just cannot seem to find any reference anywhere to 25TPI standard threads at these sizes (e.g. BSP / BSPT).
Are these just one-off specials or is there some obscure 'standard' I've just not been able to locate?
Also, I was under the impression that drain cocks weren't necessary on older slide valve engines, as any water build up can be forced out via the slide valve lifting off its seating. If so, what other purpose could these threaded holes serve?
Any thoughts appreciated.
Regards Mike

BSB is 26tpi

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Please forgive my impertinence, Mike … but I have to ask:

How did you check the tpi of the threaded holes ?

… presumably not with a thread gauge, or they would be to some standard.

MichaelG.

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This is probably a ‘Red Herring’ but:

https://racingnorton.co.uk/Douglas-2-3/4hp-Early-Square-Brake-Rod-Bolt-Alternative-25tpi-Douglas-Thread-Stainless-Steel

Edited By Michael Gilligan on 08/07/2021 19:31:41

The old-timers'way to measure an inside thread in an off-site location is to fill the threaded hole with candle wax. I have seen it done with the blunt end of the candle, just pared down with a pen-knife, then screwed in. The modern way to do it would be to spray inside the hole with WD40, then fill the hole with mastic. Stab a flat bladed screwdriver into the mastic when set, then screw the plug out and measure it. Note that 25tpi is quite close to 1mm pitch, although if the engine is old British that is quite unlikely, but not impossible. As an aside, I needed to cut a concentric 1mm thread on a drill spindle, to take a Unimat chuck. I looked at screw cutting it on an Imperial Myford and balked at the time and expense involved. I screw cut it 26tpi , but left it oversize, then finished it with a die.Job done.

Just to clarify, DiodeDick …

We all have our own preferred techniques, but

I actually wanted to know how Mike checked the threads in his cylinder ends

MichaelG.

You could try thermoplastic, it's pretty cheap these days, it's plastic pellets you put in a mug of boiling water, 40 seconds later you take the now soft pellets out squeeze them together and mold to shape, if you block one side with your thumb, you could force it into the hole and create a perfect male pattern, advisable to leave some sticking out to grip and unscrew.

It takes a minute to harden to a tough nylon like material, maybe a bit harder.

It can be reused by putting it back in boiling water.

Mike -

With great respect I think you may have mis-gauged the thread, for as you say 25tpi doesn't match any standard.

I think it is most likely 1/2 " X 26tpi BS Brass, whose tapping-drill size is very close to the core diameter you quote.

You could try the dodges others suggest, but I would make a test-gauge by threading a short piece of brass (or use a 1/2 " BSB fitting if available) and carefully try that. The reduction in diameter suggests it was not cut right through for some reason, perhaps to create a taper-thread effect.

If it actually is 25tpi, it is an oddity indeed.

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You also ask the holes' purpose.

I don't know the engine but if it's a vertical the holes may be for tallow-cup lubricators. If so, I would expect smaller holes in the cylinders, close to the ends, for drain-cocks. If a horizontal then they very likely are for drain-cocks. Those are the outer end covers: what equivalents exist in the crank-end covers

I consider it wise, indeed normal practice, to fit drains "even" with slide-valves, but as I say I don't know this particular engine's design.

Whilst it is possible the water might lift the valve off its seat sufficiently to relieve the pressure, there is no guarantee it will do so, and the engine still has the problem of disposing of the water.

Hi Mike just cut a piece of round wood which will be slightly larger than the hole core and longer than an inch. Screw it into the hole and it will impress the thread like a die but not damage the hole thread and you can measure the TIP.

David

Pete may be referring to a specific thermoplastic, polycaprolayone, otherwise polymorph, and various other names. Quite useful but it is biodegradeable, so not so useful for permanent uses.

Bill

I did not spend long in the tool room during my apprenticeship in the 60's but I never saw anything like a micrometer that could used to measure the diameter and pitch of an internal thread. If there is one now, I would very much like to read about it, although I doubt it would be affordable for the home workshop. Back then, the only ways were ready made test gauges, which were go-no go limit gauges or to section the part, then measure with a Shadowgraph, or similar. A plug cast from whatever is to hand remains the only credible non-destructive route for hobby use.

Lots of lovely, useful feedback!

Michael : I greased the thread then squeezed a pellet of epoxy putty on it. Result was reasonably sharp and I compared it under high magnification against my thread gauge, certainly didn't appear to closely match 24 or 26, but was near as at 25.

Nigel - it's a horizontal engine. Originally there was ONE tapered 'plug' in situ. Rather stupidly (on reflection) I assumed it was the correct one and spent about 2 days on and off duplicating a second one exactly the same. Only to happily discover that didn't fit. After the usual round of bad language, discovered the thread I was duplicating was 20TPI and the hole was - as per the object of this thread - much finer. Oh joy. Someone in the past had simpy jammed in what was to hand - fortunately doesn't appear to have cross threaded either hole!

I'll look into the possibilty of the BSB (not something I was familiar with ) even with 26 TPI may be able to squeeze something in at a pinch. If they were for drain cocks, I would assume they would be of brass so logical to use a complimentary thread? BTW however, there's no provision for similar at the other end of the cylinders.

David: Never thought to use the 'wood' trick, you learn something new every day. Thanks

Really appreciate the time and effort all have taken to respond. I'll try and update you on how I get on.

Mike

Working in an inspection department in the early 70s, we used a plastic to copy internal threads. This came as a powder mixed with water, and set very quickly. The form and pich were then measured using an optical projector. We were discouraged from using it due to its expense. I haven't been able to identify what the material was. Along with other suggestions, I offer plaster.

Bill

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That sounds suspiciously like it could be 25.4 tpi [i.e. 1mm pitch]
… especially as it’s on a taper

Have you tried just laying an M6 screw into it ?

MichaelG.

On an engine from the 1850's all the threads are Whitform so far (I've had to repair numerous to date on this machine) some being heavily corroded. The ones in question don't look to bad in comparison but are obviously not in pristine condition so not easy to get a precise 'image' of them. Funnily enough as per your suggestion Michael, I've just gone out and checked and a Metric thread does match pretty well 100%! Could be someone has re-cut it many, many years later and they only had metric kit to hand?

With the limits of equipment / techniques available I don't think I shall be able to precisely differentiate between 25, 25.4 & 26TPI, so I think it will be a case of cutting test pieces of each in brass or alloy and see what is the best fit!

Regards, Mike

I don’t think this will help much … but it’s a good overview of the various ‘systems’

**LINK**

https://cdn.shopify.com/s/files/1/0068/3235/7429/files/Thread_Identification_Guide_Trimantec.pdf

MichaelG.

Bill, that sounds like a Rubert material but I couldn't see this powder on their website. Maybe someone else does it it but I have not found anything yet

Maybe I have! There is a company called RepliSet but I cannot find any prices.

Brian

Edited By Brian H on 09/07/2021 11:13:19

"Could be someone has re-cut it many, many years later and they only had metric kit to hand?"

Think you're correct with that one Mike. Bosses look to be too small in diameter for thread. Wonder if originally was 3/16" thread.

Regards Ian.

Brian H, Replicet looks very useful, but definitely an improvement from what I used all those years ago. It seems to be a silicone based material, capable of reproducing fine features such as surface finish or, in this NASA document, crack detection:

Replica based crack detection

I thought, today, that my unknown material might be a dental cement (e.g., ToofyPegs - over the counter filling repair ! ) which contains zinc sulphate plus zinc oxide and sets very fast. However, it generates some heat, which I don't recall from my experience, however the metal components would have conducted heat away very readily.

Mike now has many options for reproducing the thread; does anyone have a design for a shadowgraph type projector?

Bill

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A flatbed scanner would be adequate, if Mike can section the replica

MichaelG.

That is an excellent idea Michael, I'd not thought of that! I must give it a try.

Presumably if it's a thread then a suitable profile can be downloaded from the Internet?

Brian