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Too much steam going up the chimney

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John Hilton07/09/2015 17:06:53
113 forum posts
22 photos

I would appreciate some help with my 3 1/2" gauge loco please.

It is a GWR County Carlow design. Stephens Valve gear - outside piston valves.

The problem is that far too much steam is always going up the chimney.

I have had it running today after resealing all the gaskets etc. From a full boiler it will empty the steam in a couple of seconds of opening the regulator, although it will then plod away quite happily on the remaining 25 psi! (jacked up - not under load),

So I don't think the valve settings are a problem.

When i open the regulator at any wheel setting there is a huge blast from the chimney. Could it be the piston valves - I am not sure of the best way to make these reasonably steam tight. I tried to follow Don Youngs plan to make them a push fit and put some graphite grease with them, but they are either too tight or too loose!

My only other suggestion is that some steam is transferring via the lubricator pipes.

Any suggestions very gladly received.


John Baguley07/09/2015 17:25:15
458 forum posts
49 photos

Hi John,

There's a very good chance it's the piston valves leaking. It's very difficult to get plain metal bobbins to seal well enough. Even if you machine them to as good a finish as you can and a good fit, the high spots soon wear off and the valves soon start to leak. Don's method is to make them a very tight drive fit, coat them with Molybdenum Disulphide greae, force them into the liners and keep knocking the valve from one end of the liner to the other until they become free. The idea is to remove all the high spots and leave the liner bore and the valve bobbin with a very smooth surface. I have to say I tried that some years ago but didn't have a lot of success with it!

I've given up with solid bobbins now and use PTFE heads with complete success. The only problem is that once you run on steam the valves will leak if you try and run the loco on air cold. No problem on steam though.

There are another couple of possible causes. The piston packing may be leaking allowing steam to blow past or the valve liners may not be sealing into the cylinder block and allowing the steam to pass straight from the inlet to the exhaust.


John Hilton07/09/2015 17:45:02
113 forum posts
22 photos

Hi John. Thanks for this..Some comfort in not being the only one who found the Don Young method a tricky one! The Pistons each have 2 o rings and I don't think they are leaking. Can you enlarge on the PTFE HEADS please. Do you mean all or part of the piston valve is PTFE? Many thanks

John Baguley07/09/2015 22:07:38
458 forum posts
49 photos

Hi John,

Here are a pair I made for a chap at our club. They are for a 3½" gauge 75000 class loco which originally had plain stainless bobbins that leaked badly:

valves 1.jpg

valves 2.jpg

The valves consist of a central bobbin made of bronze, stainless, or even brass. The heads are rings of PTFE held onto the bobbin by threaded caps. You machine the PTFE heads to a push fit in the liners and the fit sorts itself out as soon as you steam the loco.

The bottom photo shows that I altered the method of valve adjustment to a much better method. The originals were adjusted by altering the nuts on the threaded valve rod which is a right pain to do. In my method the valve floats on a central 'bush' threaded onto the valve rod. To adjust the valve you just rotate the bush and secure it with a locknut. Think I got the idea from an article in an old ME

There's a bit of a description of these valves here:


Unforunately, I didn't take any photos of the bobbins before I assembled them but hopefully you get the idea. I might have done a CAD drawing before I made them, I'll have a look.

It might be an idea to see if you can check the valve liner bores to make sure they are truly round and parallel. It's possible they may have distorted when they were fitted to the cylinder block? PTFE heads will accommodate slight ovality etc. in the bores though but better if they are true.



Edited By John Baguley on 07/09/2015 22:14:16

julian atkins07/09/2015 22:52:15
1235 forum posts
353 photos

hi john,

classic case of leaking piston valves im afraid.

i doubt if john baguley's excellent PTFE piston valves will work ok on County Carlow as designed due to the shape of the ports in the liners. see ME 20th June 1969.

you really must have another go at the piston valve bobbins, and check the liners at the same time. you wont be the first person to make 2 or 3 piston valve bobbins. have you access to a 5/8" reamer in very good condition, or a 16mm reamer in very good condition if your liners are a bit oversize in bore?

ordinary grease is no good with Don's method - you need molyslip as described by john baguley, and it's a pretty time consuming and fraught process belting the bobbins in and out till they slide nicely but definitely not loose. piston valves are not easy! precision turning of the liners and fitting same, and making the bobbins is far from easy.

incidentally Don only built one piston valve loco which later spent some time in my workshop - his 5"g K1/1.




Edited By julian atkins on 07/09/2015 22:53:33

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