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Drain cocks

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London_lighterman:
A'noon all.

I have a loco to repair, the drain cocks on the underside of the cylinder are of the taper plug type working together from a lever in the cab.
Please tell me is there a specific reason that these type are used and if a valve with a parallel plug machined with a shoulder to the rear.

I have been thinking this through and I maybe snow blind with it now so hence the appeal for some wise words.

regards M

Roger B:
I believe that a taper was used to allow the plug to be ground/lapped into the valve body. It did not require precise machining to obtain a good seal.

A parallel plug will require an accurate bore to be machined along with a matching plug. A taper plug will just require a tapered D bit reamer to be machined at the same setting as the plugs.

Jasonb:
As Roger says the tapered cocks tended to be used where they were sealing against pressure. On something like a water supply from a tender to pump a parallel sides valve may have been used.

Tapered are easy to make, lap to a good seal and can be held in close contact by either a nut pulling the tapered cock into the valve dody or a stuffing gland and nut pushing it in.

Some model makers use a parallel design but have a PTFE insert which is a firm fit and takes care of sealing and any slight deviation in machining.

Charles Lamont:
One view - I can't remember where from, and will have to paraphrase: "The taper plug cock was invented by the ancient Romans, and has been used ever since. It has been a piece of crap for 2000 years."

springcrocus:

--- Quote from: Charles Lamont on July 16, 2025, 08:27:43 PM ---..... It has been a piece of crap for 2000 years."

--- End quote ---
I quite agree! I built two sets of tapered three-cock gauge-glass valves for my 5" gauge Britannia and just couldn't get them to seal. Eventually, I remade them as parallel bore with two miniature "O" rings and they worked perfectly. There is a write-up somewhere on my website if anyone is interested.
Regards, Steve

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