Rob Roy drain cocks

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

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

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

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

As the tapping size for 3/16x40 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.

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

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

Mike I've sent you a PM