The Home Machinist!

Yes, his name appears all over the boiler drawing.
-Kevin S.

What I hoped to find was someone who has applied a smokebox throttle to this engine.

Harry,
A smoke box throttle on this locomotive would not be easy. The smoke box is only 5 1/2" in diameter and 4 1/2" long. Not a lot of space in there with the petticoat pipe, blast nozzle and steam delivery pipes. I suppose you could cram all of that into the smoke box on a propane burner but as a coal burning locomotive, it would be next to impossible to clean the tubes from the front end let alone getting inside the box to repair something.

What I hoped to find was someone who has applied a smokebox throttle to this engine.

You mean like the one Dick Morris shows installed on his CP173 on the 1st page of this thread?

Ken-

Yep. Tight fit and tough access to the throttle valve for repair.

Dick, LL, Bruce, and Cary,
After quite a bit of thought it appears that all things considered an interior throttle will present the least problems. Attached is my variation on the RRSC throttle body (subject to further revision) and what I don't know is what was intended to happen at "X". Obviously it is the connecting point for the throttle rod, but what's the reason for the enlarged bore?

It wouldn't need to be a gland. I could see it as a trunk guide for the valve plug . . . perhaps with an O-ring or two, but why would it need O-rings? I have no drawings to show me what is going on there.

GW,

My throttle rod is just threaded to the "plug". The far end of the plug (X in your drawing) is surrounded by the stay tube as it enters the throttle body. There is a small hole about halfway between the backhead and the throttle body to prevent hydraulic lock, therefore there is boiler pressure within this section which would necessitate the use of an o-ring or gland on the plug. Did a little edit on your drawing:
At least that's the way it is on my unit. Forgive me if I'm not understanding the question.

Ken-

Thanks Ken, this what I suspected but couldn't guess based on the drawings I had. I'm away from my main computer today so I can't revise my drawing, but one thing that isn't shown is that there will be a 1/4" rod "tailpiece" silver soldered the the throttle body which is coaxial with the drypipe. This will make the staying action a straight pull, and avoid the possibility of creating a moment between the upper and lower axes of the assembly, even though that might be negligible. That being the case, and all else being the same, would there be any reason to include a trunk tube between the throttle body and the throttle rod gland?

Harry,

On the original design, the stuffing box for the throttle rod tube is on the back head. This eliminates the need for the o ring at the throttle body. At point X, a 3/16" rod is threaded into the slug with a lock nut for added security. This rod extends thru the stuffing box and on to the throttle lever. The is a small drain tube on the bottom of the throttle rod tube to prevent water from sitting in the tube.
There is a lubricator tube that is in line with the dry pipe to provide the axial support line through the throttle body from the back head to the front tube sheet. The intent of this lubricator line is to feed cylinder oil into the throttle and into the steam flow and on to the cylinders. I still like to have my lubricators feed directly into the steam chests above the D valve for easy "first lube in the morning" shot of oil to the valves. The lube tube is still in place on my engine but it is capped off on the back head. It is more of a structural item now.

I didn't build mine so I'm not sure what the design criteria was. There is an o-ring on the slug itself and 2 o-rings at the stuffing box on the back head. The stuffing box is a 2 piece brass affair. The thottle rod support screws into another piece both sealed with o-rings. I'll see if I can dig up a picture.

Ken-

Thanks Ken, I have the stuffing box covered, the important bit to learn about was the O-ring on the slug. This throttle assembly won't use a trunk tube.

I've made up a revised throttle valve for the CP173, built from bar stock, tweaked here and there from my sketch above. Thanks for everyone's helpful input. The photo is of a trial assembly of the parts, awaiting silver soldering. A dome collection standpipe will be screwed in from the top. The tube exiting to the right is the drypipe, the rod to left is a backhead stay. The thing that still makes me uncomfortable about this valve is that once installed it will be forever entombed within the finished boiler. I need to devise a way to do a steam test, or at at worst an air test. I hope it works.