Piston or slide

[tbsteam]

I am getting to the point where i will be needing cylinders for my hall( ;D) but i don't know whether to go for piston valve or slide valve.
Any suggestions appreciated.

Cheers

[spamcanman]

Wow this will open a can of worms
I own a 'Maid of Kent' it's the only one I know of that has piston valves, there are a few in my club all slide valve and my loco seems to have more power and pulls more passengers than than theres but I have no idea if that's the reason

What has to be spot on is your oil lubricating system it's very important on Piston Valves on mine I tend to have it over oil slightly (you can't be too carefull)

[baggo]

It all depends on how accurate you can machine the valves and the bores. Slide valves only need a good finish on the port face and the valve face, piston valves need to be virtually a perfect fit with a very, very good surface finish on the valve bobbins and the liners (assuming plain bobbins are used which is usual in 3½" gauge). The situation is a little less critical if the valve will have rings fitted. A well fitted piston valve should remain steam tight for many years and needs a lot less power to drive it than a slide valve (unless of the balanced variety). There is, therefore a good chance that a similar loco fitted with piston valves will be more powerful than a slide valve version - less power is wasted driving the valve.

The problem with piston valves with plain bobbins is that if the surface finish is poor, they will rapidly wear and soon start to leak. Don Young's method of fitting them (and I've used this method myself) is to make the valves slightly oversize, coat them with molybdenum disulphide grease and literally drive them into the bores. They are then driven backwards and forwards until they become free. By this time both the bores and the valves will have gained a mirror finish and further wear should be negligible.

John

[Shawki Shlemon]

I have used both types and I agree with the comments above . for piston valves I used plastic valve (Teflon) as described in the ME 4/12/1987 page 664 on NSW C 38 ,NSW C 36 ,and GWR King and they run very well . It is a hard choice to make , there is more work in the piston valves but once done they are better in my opinion .

[ilvaporista]

Generally I tend to agree. Do not be put off by the people who say you will never make good fitting valves. You have already done the operation once for the pistons and now you've got experience..

Now if you want to be really flash you can set up a toolpost ginder to get a fine surface finish on the valves once turned, remebering to make allowance for . If you are poor like me you can use an air powered spindle or a quality pistol drill as a substitute. Run it at top speed, dress the wheel so that it is parallel to the axis and take very fine passes. I used this trick a lot on spool valves for pneumatics and then to refurb a Torquay Manor.

[tbsteam]

If i do decide to go for piston valve cylinders, is there anybody that would machine them for me as i don't have enough experience to do it myself and thats the reason i am swaying towards the slide valve cylinders, i would be able to do them.

I would prefer piston valves, but like i say, i wouldn't want to do them myself.

Cheers
Tom

[daveburrage]

Another issue worth discussing is the power required to drive slide valves. Large slide valves (5g & 71/4g) need a lot of effort when loaded at boiler pressure and the driving linkage needs to be substantial if it is not going to wear rapidly.

The designer of a scale model of a loco intended for piston valves must have a choice between scale appearance or enlarging the linkage to take increased loads.


regards

Dave Burrage