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NKP 765's chuffs seem to be perfectly "quartered" ( not sure if this is the correct term)  While PM 1225 has a bit of a hitch to its giddy up.

What is done to achieve accurate timing, and how does timing affect the locomotive performance?

PM 1225

NKP 765

Last edited by Rich Melvin
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What you are hearing is the difference between a steam locomotives with the valves slightly "out of square" (1225) and a locomotive where the valves are set perfectly square (765.) In the 1225 video, you can also hear rhythmic "squeaks" and "whistles" from the engine, which is steam blowing by the valve rings.

The 1225's valves are worn and need a little work, that's all. At one time, the 765 sounded like that, and even worse!

Gentlemen, please lets not confuse the squareness of the valve settings with the quartering of the main crankpins, at EXACTLY 90 degrees. The "off-quartering" is what produces that "hitch-in-the-gittyup"exhaust sound. Proper setting of the valves is what produces that "extra sharp" exhaust crack, with absolutely no "mushiness".  With the out of quartering it is also pretty difficult to properly set the valves.

As Jack said, proper quartering of the crankpins is what sets the "rhythm" of the exhaust. If the quartering is out, the exhausts will not be evenly spaced. The "rhythm" won't be even and steady, like this scene with WM 734:

The other aspect to setting the valves is correctly positioning the valves in the fore and aft direction within the valve cylinder. This is what Steve describes in the post above this one. In this case the exhaust may be perfectly timed and square, but one exhaust beat will be noticeably louder or softer than the others.

Last edited by Rich Melvin

On a similar note, I came across this video a while back of the 1225 from 2003. As you'll notice in the final scene pulling into Cadillac, starting about 14:40, the exhaust is out of quarter and one of the chuffs appears to be completely GONE.

I've become accustomed to 1225 having one chuff that's quieter than all the others, but I've never heard one completely disappear. Any reason for this? Is it just an extreme case of the things already mentioned?

At the risk of sounding redundant here and/or adding fuel to the discussion, a locomotive sounding square is when the exhaust occurs at an even tempo. Assuming that everything is good with the valve gear from the eccentric crank right up to the combination lever and valve rod, the locomotive can be brought square by proper positioning of the valve cages in the valve cylinder relative to the valve spool. The width of each ring set on the valve spool between the steam and exhaust edges of the ring set takes into account the width of the ports in the cages plus the amount of lap and lead, plus or minus the amount of exhaust lap or clearance between the exhaust edges in the cage ports and the exhaust edges on the spool ring set. The NKP originally had ¼” exhaust clearance on the 2-8-4’s but changed in later years to line and line exhaust edge alignment.

The locomotive can be made square by proper positioning of the valve cage exhaust edges front to back to correspond to the exhaust edges front to back on the valve spool. Of course the position of valve spool in the cylinder must be known. One way would be to measure the position of the valve in the cylinder by noting its location at both front and back dead center which are good points to measure as the valve gear imparts no motion to the valve at these points. Valve motion at these points is imparted only by the combination lever. As a nice check point, open up the peep holes front and back after the cages are set, and check if the lead is the same front and back.  

A mushy, or different sound intensity of a squared exhaust could occur when there is blowby between the rings and cages, or port exhaust edges not parallel to the rings.  

Jim Kreider

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