When superheating became practical, it provided as much power as the Vulcan Compound with far fewer complications, so most of them were converted about 1906. SP 1727, the mogul on display in Dunsmuir, CA was one converted at that time. When she was donated for display, apparently the Stephenson straps and large bearing ends were removed, and the valves locked in neutral by welding some angle iron on their valve rods. This would permit easy movement of the pistons in the cylinders. We have been unable to find any sign of them anywhere. It seems the big ends would be the most complicated part to have made, the straps themselves were flat steel in cross section
Of course, at this point in time, it is sadly unlikely that 1727 will ever be restored to operation, so the replication of the pieces is moot--however, 1744 is still intact, and one could start with it for a parts pattern--1785 may have her straps too.

David, I'm afraid that I'm not quite following you on the missing motion from 1727. Are you describing the eccentric straps which wrap around the eccentric sheaves as "big ends"? The big end is normally the 'thick' end of the main rod (aka connecting rod) whose bearing runs on the crank pin of the driving wheel. The eccentric rods, mainly "flat bar" with a fork at one end transfer the motion from the eccentrics to the expansion link, and via the die blocks to the valve rods.
So are you really missing the 4 eccentric straps and 4 eccentric rods? I'm also puzzled why anyone would lock the valves "in neutral" (aka mid-gear) as it would actually cause the pistons to compress air and thus move less freely. If you're going to drag a loco any distance the main rods would normally be removed so the pistons aren't moving at all but leave the Stephenson's motion alone apart from giving it and the valves plenty of oil.
Ray.

"David, I'm afraid that I'm not quite following you on the missing motion from 1727. Are you describing the eccentric straps which wrap around the eccentric sheaves as "big ends"?"
Yes, sorry for being confusing, yes, the four eccentric rods are what's missing from 1727. The Links are there, as are the die blocks and valve rods (ie: all the parts visible from outside the locomotive).

"So are you really missing the 4 eccentric straps and 4 eccentric rods?"
Yep, just have the now-rusty cams on the driver axle. We were hoping the parts had been tossed into the tender water tank, but no such luck.

"I'm also puzzled why anyone would lock the valves "in neutral" (aka mid-gear)"
Well, not having any diagrams, I assumed (since this was done) that there was enough "lap" in the design that at that location, both cylinder ends would be partially open to exhaust up the stack (Although in this instance they would alternately suck air in and then expel it out).
1727 was towed from the Dunsmuir roundhouse to Mott (probably about 12 miles or so by rail) where it was shoved on a siding. The end of the siding was dug down, and the flatbed truck trailer shoved up against it, and the tender rolled off to the trailer and then trucked back down the hill via highway 99 (now I-5) to the display site. The tender sat there by itself for a few days (as I recall, being all of about 5 years old at the time), and then the locomotive itself was rolled onto the trailer and driven down to the display site (about 4 miles via highway.
Trivia--I have found no record of the California Highway Department granting permission for the locomotive move, which would have required permits. I think it was done "under the radar" which was much easier to do back then. This round-about move was required to avoid taking the locomotive over the tall concrete arch highway bridge between downtown and the display site at the city park entrance.

.

The solution to that problem, I suppose, would be the tandem compound, Which had both cylinders in line on a common piston rod... But those took a lot of space, and were only tried on a few large engines.

My old steamboat had a "tandem" compound engine (Steeple compound we called it) and it was a TALL engine (vertical cylinders), I can't imagine how cumbersome that would be horizontally. There are other problems though, due to the transfer chamber connecting the low pressure to the high pressure; it didn't always want to reverse!
The Delta Queen steamboat is a cross-compound engine, one cylinder is high pressure, the other cylinder (on the other side of the boat) is the low cylinder. I don't think I've seen anything like that tried on a locomotive. Did anyone do it?

Just about everybody but Baldwin, and likely them too, initially, since it was the original compounding scheme. The Vauclain compound was a Baldwin patent, IIRC.

Here is an interesting read on the different systems of compounding used on locomotives:

https://books.google.com/books?id=1D9KA ... nd&f=false

Scroll up to see other builders cross compound schemes; scroll down for discussion of the tandem compound. The section on compounds actually starts on page 326, covering the Vauclain compound first.

All the two cylinder systems used some sort of starting valve to admit high pressure steam into the low pressure cylinder, usually through a reducing valve, to keep the engine from becoming stuck on dead center of the high pressure cylinder. It seems to have added a LOT of complexity to the plumbing.

Guys, the correct spelling is Vauclain, named for its inventor Samuel Vauclain (1856-1940), the general superintendent and later president of Baldwin:

https://en.wikipedia.org/wiki/Samuel_M._Vauclain

He began his career as an apprentice machinist for the PRR. "In those days, machining was a manual job with the machining done by hammers, files and chisels with the men's own hands. Vauclain's hands were left in a permanent clutching position from endless hours of chipping and filing metal."

-Philip Marshall

That is several years before the date range I understood as the advent of practical (Schmidt firetube) superheat. Likelier that the engines were being simpled because the maintenance problems far outweighed effective gains from the compounding - and that actual contribution from the LP pistons was meaningful. I think it was established that the actual MEP in LP cylinders was dramatically lower than expected, under a wide range of conditions. This would exacerbate the effect Kelly described for the different pressure peaks in HP and LP, and furthermore I'd expect one of the first results of the asymmetrical force on the crosshead would be to open up the cylinder glands and perhaps wreck the tribology of part of the piston rods in their travel. For a road like ATSF in particular, which would have adopted compounding largely for water-supply-related reasons, the increased "blow" itself might rapidly rule out big operating savings...

Plenty of cross-compound engines were designed and built - I, too, didn't think much of the idea when I first read about them, but a number of sources say they could work well and reach surprisingly high speeds if designed correctly. I think we had a discussion of them a few years ago, raising the idea that they could be poorer for passenger service as the 'twice per revolution' HP admission would lead to more surge if the LP effort were inadequate (compared to a 2-cylinder DA simple using short cutoff for long expansion, and superheat to cut down the effect of nucleate condensation on the later stages of expansion).

An interesting point about Vauclain compounds is that they only use one valve spool (and set of valve gear) per side, with a very ingenious and intricate system of passages within the cylinder assembly to distribute steam and exhaust and provide intercepting valves for the right amount of steam to the LP at starting. Perhaps needless to say, if any particular mass flow was expected, this arrangement would rapidly throttle things above a certain rate -- probably not a very high rate. I've seen discussions that advocate higher-than-usual drivers to get high rpm out of these engines (Atlantic City Railroad 1027 coming to mind as an interesting example) - these could get very fast indeed but would have remarkably quick falloff of power limiting their top speed above whatever would get through the passages properly...

I presume that no trace whatsoever of the original Vauclain valves or passages exist in the rebuilt locomotive. Mr. Dewey noted that the valve that is present was blocked to permit adequate bypass to move the locomotive (albeit, perhaps, slowly) - might be interesting to go in and see what might be there.

Don't forget that there's an intact and operable Vauclain compound at Pikes Peak:

I seem to recall seeing one at the Colorado Railroad museum?
But getting back to this thread's title question:
What might be the cost for new eccentric rods?

That depends on many variables. Certainly it's not hard to cut flat stock and machine the bits to make the forks to weld to the flat sections, join them, drill and ream for the pin holes and adjusting slots to bolt to the eccentric straps. If you have a drawing to start with, you save a lot of reverse engineering costs. Probably the two biggest factors are having good reasonably complete starting information and a vendor that's set up to efficiently either program it into his machines or an interested craftsman with more drive to take part in working on a steam locomotive than pay his bills cut them by hand. I'd start by gathering and drawing my specifications, than shopping them to different vendors to see who has the best and least costly idea about production.

Getting eccentric straps (the part that wraps around the eccentrics on the axle) would be a much more difficult part to create. One would almost need a existing one to use as model to create a ne one.

To address the topic of the thread hijackers: there is an intact Vauclain Compound NG 2-8-0 up in Yukon Territory in a museum. One has to wonder how M&PP maganged to keep their Vauclains functioning after the rest of the world gave up on them. I hav always wondered if the jackshaft-type drive had something to so with it.

He didn't ask for the straps, Earl. That's another thread.

.