D valves not seating - how do I fix?

[c.p.huntington]

I think the D valves on my Little Engines C.P. Huntington are not seating. I tried running it on steam for
the first time a couple months ago. I got on board, opened up the throttle and the pressure quickly
dropped to near zero and the engine didn't move. I finally got around to trying to find the problem
last week. After filling the boiler with compressed air, I found that if I put the Johnson bar in neutral
and opened the throttle the air would come out of the exhaust nozzle. I suspect that that the pressure is
running under the D valve and up the exhaust. How can I fix this? Or could something else be causing this problem?

Thank you for any advice that you can give me.

[NP317]

Things to check:
Open the smokebox front.
When you open the throttle, does the steam (or air) exhaust up the smoke stack, via the exhaust fitting?
That could indicate valve leakage.
If steam is leaking into the smokebox from other fittings/locations between the throttle and the valve chest, solve that first.

If valve leakage is indicated:
Remove the valve covers.
Is the valve sitting above the slide surface?
If yes, solve that first. They might be stuck on the valve shaft, above the cylinder surface.

If still leaking steam past the valve, then:
Is the cylinder top slide surface flat?
Is the valve surface flat?
Resurface those until flat and the valve properly seal.

Reassemble and repeat tests.
Let us know what you find.
~RN

[Fred_V]

Also check to see that the valve is not cocked up at one end. There should be a bit of slop in the rod fit to the valve to take care of expansion and any slight cocking of the valve rod as it is supported at one end only.

[Bill Shields]

is this a new loco that has never been run or one that has just been sitting for an extended period of time?

[c.p.huntington]

This is a new locomotive. It's been run on air just a few times. This was my first attempt to run it on steam.

[Rwilliams]

Often times I have seen D valve equipped models and other D type valve equipped steam appliances that either have too much wear, corrosion of the cast iron seating surface or if new, tool marks from the teeth of the milling cutter used to cut the valve surface. In all cases even the small void like passages created by the above mentioned problems can cause significant leakage. There could even be a casting flaw that has gone unnoticed.

Lapping the valve face is a fairly easy process to remove all wear or tool marks. The port surface of the cylinder casting is a different challenge as often it is impossible to get that surface to a lapping plate depending on the shape of the casting.

If removal of the cylinder is not an easy option, the use of stones and light oil can remove the worst of the wear or cutter marks. To achieve a smooth surface free of all low spots will take some time. Final surface finish can be achieved with various grades of lapping abrasives until the desired finish is reached. A piece of thick glass of 1/2 inch will work as the plate to move the abrasive compound around on the port surface area. Be sure to fill all holes with some silicone sealer to keep the fine abrasive out of the ports and tapped holes. Makes cleanup far easier.

I have even made custom cast iron lapping bars to reach into small port surface areas. A nice piece of cast iron the desired size is cut with some 1/16 wide and deep grooves in a diamond pattern on one side and then lapped smooth on a lapping plate. The new custom lapping bar can then be used to bring the port surface into a super smooth and flat condition.

A word of caution, it is easy to make a high spot in the middle if one is not super careful. A guide bar bolted to the front and back of the cylinder casting and adjusted with a straight edge, piece of tool steel, can go a long way in insuring the surface does not end up with a crown shape. It is amazing how smooth and level a surface one can achieve using simple guide bars bolted to the ends of the cylinder casting.

[DianneB]

.... Or could something else be causing this problem?

Before you get into D valve seating you need to confirm that is the problem.

A likely problem could be timing that is off and for that you need to open the valve chests and be sure the valves are centred over the ports in neutral gear. Since you have it open anyway it is a good chance to check the timing and make sure you have equal valve openings in full forward and full reverse. With the valve chests open, might as well check the D valve on the steam to be sure it is free floating.

My LE American came to me at over 40 years old and the port faces and valves were only a couple of thou off flat.

[NP317]

Because the locomotive is new, and has run OK on compressed air,
I support Fred_V's comments about thermal expansion possible causing the valve to cock on the port surface.
~RN

[JBodenmann]

Hello My Friends
I once was brought an engine with this problem. It was caused by a poorly fitted gasket between the cylinder and valve chest. It was preventing the valve from seating. Just something to check. If it is the problem, it's very easy to fix.
Jack

[johnpenn74]

Another issue can be too much restriction in the exit porting / blast nozzle. If the passages or blast nozzle is too small, the exiting pressure will push the d slide up breaking the seal on the slide; ie balancing the pressure to the main cylinder on both side.

You might try removing the blast ring or nozzle if it is easy and then testing under air.

JP

[Fred_V]

For that to happen it looks like the exhaust pressure in that side of the cyl would have to be greater than boiler pressure.
Is that possible?

[c.p.huntington]

Fixed the problem. One of the engine's cylinders was purchased machined from an individual that
had some extra Little Engines American parts. The other cylinder I machined myself and it was
this cylinder that was the problem. I had milled the top of the casting flat and thought that this
would be good enough but found out the hard way that it wasn't.

I removed the steam chest and filed the top of the cylinder casting starting with a coarse file
and ending with a small fine file. A few weeks after getting it back together I took it down
to the club's track and was able run back and forth in the yard a few times. The timing was
a bit off but the engine gave a good strong "chug" as it pulled me down the track.

Unfortunately, this is where the story ends. I've reached a point where I would like to
remove the clutter and stress from my life and simplify things. I have decided to sell off
the engine and my house and move back to a condo. Look for this locomotive for sale on
the Discover Live Steam site soon.

In closing, I would like to thank everyone who helped me with this project. Your advice
was much appreciated.