By continuing to use this site, you agree to our use of cookies. Find out more
Forum sponsored by:
Forum sponsored by Allendale Jan 24th

Piston Valves - much ado about nothing?

All Topics | Latest Posts

Search for:  in Thread Title in  
La Cathedrale23/02/2021 14:56:53
7 forum posts

I want to build a loco with piston valves instead of slide valves, but I see them much more rarely than slide valves.

Other than the challenges of turning the bobbin and the associated faff of another cylinder to bore, more piston rings to make, etc. - is it intrinsicly more difficult? It just seems like a bit more long winded, rather than actually problematic?

If not, and I am being overly sure of myself - is there a suggested resource for me to get my teeth into?

Many thanks,

Andrew Tinsley23/02/2021 15:18:30
1335 forum posts

Simple answer is that slide valves do not need very high precision to stay steam tight, Piston valves on the other hand do need high precision to prevent steam leakage. Piston valves are forced onto their seats by steam pressure. Piston valves rely on excellent fits to prevent leakage.


Tomfilery23/02/2021 15:37:44
128 forum posts
4 photos


I suggest you look up John "Bogstandard"'s Paddleduck engine for a place to start.

You didn't say how big a loco you are thinking of building and John's is a small marine engine for driving a small paddle steamer. It was aimed at beginners, so provides a lot of good constructional detail and advice.

Regards Tom

La Cathedrale23/02/2021 15:59:25
7 forum posts

Hi Andrew,

Isn't this the 'question' with cylnders also (i.e. requiring precision to prevent steam leakage) ? i.e. we need to do it (i.e. precise machining) twice rather than once if we use a piston valve?

Hi Tom,

I was looking at either an A1 or a B1 - much like the question of piston valves, I'm also very interested to know how much of a factor a tapered vs parallel boiler is. Basically, is it something that just takes longer with more prep, or is it a cliff edge brick wall of difficulty that will require a bought-in boiler/etc.

La Cathedrale23/02/2021 15:59:57
7 forum posts

ps. I wil review the paddle steamer engine right now

noel shelley23/02/2021 16:47:35
483 forum posts
14 photos

If you have wet steam, a slide valve will lift and prevent a hydraulic lock and blown cylinder covers with luck, a piston valve will not. Cylinder drains are vital with piston valves. Noel

Nigel Bennett23/02/2021 16:53:32
395 forum posts
11 photos

Many piston valves have been made as solid bobbins, with or without "oil grooves". (The theory is that leaking steam passes into the first groove, expands, and then leaks into the next groove at a much lower pressure. By the time it reaches the fourth groove it's got fed up, and the valve has now reversed direction and steam is on the other side now anyway...)

To maintain steam-tightness with such things requires that the bobbins are turned to such a dimension that at working temperature steam does not leak through to any significant extent. To that end, people have used O rings and (more usually) cast iron rings to provide a seal. It is not easy to provide the necessary porting in a piston valve bore so that the O ring isn't chewed up the first time it moves.

Material choice for the bobbins, liners and/or cylinder block is also a minefield due to dissimilar expansion rates. Non-metallic piston valve bobbins have been made from solid Fluorosint, or cunningly-designed composite arrangements whereby the sealing element can be expanded into tight contact by opposing tapers. I tried solid PEEK in my last loco but I ended up making solid stainless ones in the end. They still leak a bit more than I would like.

It only needs a small amount of detritus (ash, swarf, grit or whatever) to chew lumps out of a piston valve bobbin and/or the liner and start it leaking. Restoring such damage inevitably means renewing the bobbins, whereas with a slide valve you may get away with a quick rub of the damaged faces on an oilstone or whatever to restore the sealing surfaces.

LBSC always maintained that piston valves were easy - read up about it in for example the Speedy book...

La Cathedrale23/02/2021 20:00:06
7 forum posts

@nigel - Mr. Crispin on YT (rolls royce fitter?) uses cast iron pistons/spindle valves with stainless steel shafts, inside cast iron lined cylinder blocks - materials-wise, that seems fairly straight forward? That said, as far as I know despite being an excellent engineer this is his first loco build.

Paul Lousick23/02/2021 21:42:54
1707 forum posts
627 photos

If you have wet steam, a slide valve will lift and prevent a hydraulic lock and blown cylinder covers with luck, a piston valve will not. Cylinder drains are vital with piston valves.

I used to drive an Aveling and Porter steam roller which was fitted with piston valves. The action of the throttle lever was much easier and smoother to operate than those with slide valves. It was also fitted with spring loaded, automatic drain cocks on the cylinder to prevent hydraulic lockup.


Andrew Tinsley23/02/2021 21:45:31
1335 forum posts

Piston valves are smaller diameter than the pistons themselves, thus are more difficult to make and remain steam tight. I have made both piston valves and slide valves and I know which are the easiest to make! By all means use piston valves if that is your fancy, but be prepared for more servicing to keep them tight.


All Topics | Latest Posts

Please login to post a reply.

Magazine Locator

Want the latest issue of Model Engineer or Model Engineers' Workshop? Use our magazine locator links to find your nearest stockist!

Find Model Engineer & Model Engineers' Workshop

Support Our Partners
Eccentric July 5 2018
JD Metals
Eccentric Engineering
Subscription Offer

Latest "For Sale" Ads
Latest "Wanted" Ads
Get In Touch!

Do you want to contact the Model Engineer and Model Engineers' Workshop team?

You can contact us by phone, mail or email about the magazines including becoming a contributor, submitting reader's letters or making queries about articles. You can also get in touch about this website, advertising or other general issues.

Click THIS LINK for full contact details.

For subscription issues please see THIS LINK.

Digital Back Issues

Social Media online

'Like' us on Facebook
Follow us on Facebook

Follow us on Twitter
 Twitter Logo

Pin us on Pinterest