Setting Stephenson Valves
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Tom Stamey wrote the following description for setting Stephenson valve gear on Allen Models locomotives, 20 April 2014.
Preparation
Take the heads (valve covers) off both cylinders. Take a soft lead pencil and mark the exact edge of the opening on both the front and back of the valve head (the plate the valve slides over). You will have to move the engine to do that (i.e. rotate the drive wheels).
Then, with the reverse lever in neutral move the rod crank (on the same side as the valve being set) to the position of 12 o'clock. Take some time to make sure you have the crank squarely on 12. See Dead center. The valve must be equally located between the front and rear openings in the head. If not use the adjustment screw on the valve rod to center it. Do this on both sides of the engine. This is called "centering the valve" and is absolutely necessary to properly time the engine. You will find the pencil marks very handy. Assuming your adjustment nuts never get loose you won't ever have to do this again.
Timing the Engine
1. Place the rod crank on the engineer side of the engine at 3 o'clock taking care to position them properly as close to 3 as possible and lock the drivers down. Move the reverse lever to full forward. Adjust the forward eccentric until the valve just barely covers the front opening in the head and lock it down. Then put the rod crank in the 9 o'clock position, lock down the drivers, move reverse lever to full reverse and adjust the reverse eccentric until the valve just barely covers the back opening in the head and lock it down.
2. Go to the fireman's side and put the rods at 9 o'clock, lock drivers down, reverse lever in full forward, move the forward cam to where the valve just covers the front opening, and lock it down. Put rods at 3 o'clock, lock down the drivers, reverse lever in full reverse and move reverse cam until the valve covers the back of the opening. Again the pencil marks come in handy.
3. Understand that moving the reverse cams will slightly affect the forward valve position and vice versa and that you will have to repeat the above steps a couple of more times to get the engine to time as close as possible.
It GREATLY simplifies things if you have one person watching the valve while another is under the engine moving the cams. They can tell you when to stop or to go the other way etc. Or, if you have not yet put the boiler on you can time it and see the valve from above.
If your cams are properly spaced 90 degrees apart and you have done everything else right while building it, you should have no difficulty in timing it.
Some locomotives, such as the Allen Models Mogul and 10-wheeler, have adjustment rods on the eccentrics. These can be used to make adjustments after making sure the eccentrics are properly spaced 90 degrees apart.
Good Luck
Tom