Rob Roy drain cocks

[Mike BrettParticipant @mikebrett89695]

Hi all

My 3.5 inch gauge Rob Roy came with four threaded holes in the ends of the cylinders, I assume to enable the fitting of drain cocks. I am trying to find what size the thread is but there is no mention of this either on the plans or in the book. I tried various screws in the holes and the only one to come close to fitting was a 3.4 mm outside diameter screw that went in part way then jammed. As its for a pipe thread I wondered if its possible a taper thread. Any ideas as I would like to fit drain cocks rather than just bung the holes up.

Cheers Mike

[Mike BrettParticipant @mikebrett89695]

What size thread

[julian atkinsParticipant @julianatkins58923]

Hi Mike,

They will almost certainly be ME 3/16" x 40 TPI, and this is a standard size for commercial cylinder drain cocks on miniature locos.

Cheers,

Julian

[Chris GunnParticipant @chrisgunn36534]

Mike if the original builder planned to use commercial drain cocks these would normally be threaded with an ME thread, such as 5/32" x 40 or 32 tpi.

Chris Gunn

[JasonBModerator @jasonb]

As Chris says likely to be an ME thread which is not often found on screws.

As the tapping size for 3/16×40 is 3.8mm I can't see Julians size being right as your 3.4mm OD screw would have just slid in, more like chris's. Try poking the shank of a drill into the hole as this will give an idea of the tapping drill used.

[Chris GunnParticipant @chrisgunn36534]

Mike, I did not think about trying a tapping drill in the hole as Jason suggests, you could try a drill in one of the holes you did not put a screw in, tapping drill for 5/32 x 40 tpi is 1/8", and for 3/16" x 40 tpi it is 5/52". I would not imagine it would be any bigger from what you have told us so far. I can make you a screw to test if it is an ME thread once the New Years festivities are over.

Chris Gunn

[Mike BrettParticipant @mikebrett89695]

Well I tried several drill shanks in the holes and a 1/8 drill seems a nice snug fit. So it looks like the thread is M E 5/32 x 40 tpi. What I will do now is purchase a die for this size and make a small union fitting, if that fits I am home and dry.

Many thanks for help. Mike

[duncan webster 1Participant @duncanwebster1]

Mike I've sent you a PM

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