That is my understanding about DY too. Even if you got them to seal, they would start wearing out from day one. This might not matter depending on the intended use for the engine. For regular heavy passenger hauling, you want piston rings. With gunmetal cylinders, Teflon rings can be used and there is a detailed thread on MECH Forum about this. The general belief at my club is that plain bobbins wear and start to blow very quickly. However there was one engine known that sealed perfectly long term, so perhaps the DY method can work if executed perfectly.
Hopefully someone with Don’s magazines might come along.

Totally agree with Nick Clarke 3, Don Young wrote and drew a lot but built very little.

An armchair view for what it's worth!

Industrial experience has shown fitting sliding surfaces by forcing one over the other gives poor results.

The idea is that driving the two parts removes the exact amount of metal in all the right places needed to get a good match. This happens, but it doesn't leave the metal in good condition. As neither of the two parts is a cutter, metal is removed by gouging and galling whilst smoothness is achieved by smearing peaks into the valleys and infilling them with debris. The resulting surface is full of microcracks and weaknesses, and galling causes deep damage.

At first a valve or bearing made this way will perform well, lasting long enough to fool the workman into thinking he's done a good job! Unfortunately, the damaged surfaces wear rapidly; bearings slop, and valves leak.

Depending on the application it may not matter: the bearings and slide valves in model locomotives aren't subjected the thousands of hours of hard running, and it might take years until a slide valve made by driving has to be replaced. The inferior method doesn't matter at all if the engine is tested a few times on compressed air and then displayed in a glass cabinet! But it wouldn't do for a hard working engine intended to give reliable service.

In contrast, lapping takes a multitude of tiny cuts with a fine abrasive, removing metal cleanly without much smearing, and the process leaves a high-polish on a mostly undamaged surface. Although lapping increases the working life of bearings and engines dramatically, it too causes enough damage for industry to routinely apply even finer finishing methods.

Remember 'Running In Please Pass' signs on new cars? Necessary because the bores and bearings were imperfectly lapped and gently knocking off the tiny remaining scars and changing the oil after the first 500 miles greatly increased the life of the engine. Still pays to run a new car gently at first, but it's no longer essential to keep under 20mph for a few months!

Dave

Edited By SillyOldDuffer on 26/06/2022 08:38:56

I have started to build a 5" loco that uses piston valves.

Fits and limits would suggest that the fit should be sliding, that is a clearance of between 0.2 to 1.0 thou. on an inch diameter. Obviously this is when hot and since the materials are likely to be different thermal expansion may have to be taken into account. If metal rings are used they will travel across steam ports so some form of pegging may be needed.

Now the question: Martin Evans, in his book "The Model Steam Locomotive", suggests turning a number of grooves (about 1/32" wide and deep) on the piston valve as an alternative. These, he reasons, will help hold the required oil for sealing. I also recognise this as a very crude, ineffecient, labyrinth seal (better than nothing). Has anyone had any experience using such grooves?

JA

hi, I'm building a 31/2 gauge Schools to more or less the Jackson design and want to fit it with piston valves. Partly this is because I've never made an engine with piston valves but also because it only looks right with walschaerts valve gear. I'm fabricating the cylinders because I want to minimize the weight at the front end. I have the castings for the slide valve cylinders but they are SO heavy. I've been using Don Young's articles about fabricating cylinders and have read the references to fitting the valves. I had already decided to try Fred Cottam's PTFE valve heads and have bought some for the purpose. He says the heads should be loose on the spindles because they will expand and also made about 0.005 small for the same reason. Has anyone any experience with PTFE bobbins please? On a design note I am pushed for space for the valve diameters because the Schools has a flat on the side of the steam chest so what would be the smallest diameter for the valves? I had been trying to get 5/8th but don't think I can manage it.

Hi again, I'm still hoping to get some information on how to use PTFE for piston valve heads. There seems to be a scarcity of information out there. I've heard of filled PTFE, the carbon filled material isn't much more expensive and may be better. Also flouroscint in mentioned but I don't know of a source where I can buy some small diameter. Anybody help please?

Thanks Andrew, I found a place in the uk that sells 25% carbon filled PTFE for a reasonable price [about £21 per metre] but I don't know what they charge for shipping. Does that machine better? A thread about making piston rings from it say it does. The difficulty with all these is getting the small quantities we need.

Edited By John Rutzen on 22/09/2022 11:02:36

Just scanning through EDP's website (they have comprehensive materials properties data online) FLUOROSINT could be a contender, it's PTFE filled with Mica. It's worth giving them a ring they do make stuff and not just supply material so they should be able to give you some info machinability etc.

regards Martin

Try Noggin End for flouroscint, I’ve had a piece from him in the past.

Sorry if this is a dumb question but with all this talk about wear on valves and them leaking etc. Why aren't they built in a similar way to a model aero engine cylinder and piston? I've had engines doing 10,000 rpm plus for hours on end without any noticeable drop in performance so they cant have worn that much. They also get a lot hotter than a steam engine cylinder. Or why aren't they made like a car engine with iron bores and piston rings, they seem to last pretty well? Thanks.

Hi John, sorry for the late reply. I've not been on the forum for a bit.

I make the PTFE valves a push fit in the bores. They will sort themselves out once they see steam.

It helps if you can make the PTFE heads as thin as possible as that helps negate the problems with the PTFE expanding. Those on my Helen Long are only 0.0625" thick and they still seal perfectly after all these years.

I've never used anything but the plain PTFE for valves or piston rings , indeed, never found the need to. The plain PTFE works fine for me.

John