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Jackson-Standard Models


A division of Jackson-Standard Model & Mold



Omnicon/Samhongsa Replacement Gearboxes



In the late 1980s to mid 1990s, Omnicon was an S Scale only brass importer. Most of their models were built by Samhongsa, the premier brass builder at that time. Many of their steam locomotives including the PRR K4, L1, I1 and NYC L1 has a “free rolling” gearbox. On the surface it seemed like a good decision. They ran very well on flat layouts, but could become disengaged and truly a free rolling runaway on a downgrade. The “rocker” that has the 2 gears that engages the axle gear could center and disengage the axle gear. Other than putting the locomotive into reverse and grinding the gears there was no way to stop the runaway train.


In 2012 Bill Lane finished his PRR K4 5495 it became quickly apparent this gearbox was not capable of withstanding the stresses needed for pulling S Scale model trains. In 2 months of casual use the brass worm gear stripped out in the center where is contacted the plastic gear below, rendering the gearbox useless.


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Since this was then a 27 year old locomotive with both Omnicon and Samhongsa long out of business, getting spare parts was impossible. Further it would not have FIXED the problem of an inferior gearbox. Something brand new had to be produced.


It is not a question of IF the Samhongsa gearbox will fail but WHEN.

Locomotives living in boxes on a shelf will never break – until you RUN them. You will not get around the issue of this awful gearbox. Your models will become permanent Shelf Queens.



Jackson-Standard Models To The Rescue



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Gearbox Production

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Some time ago, I announced that I was working on a replacement gearbox for those of you have Omnicon’s PRR locos. As many of you know directly, and from Bill Lane’s experience as he has related on the Yahoo list, the original gearboxes will fail over time when subjected to even moderate loads. Unfortunately, there has been no easy way to replace them without taking the main driver apart. No one wants to pry a main driver off an axle, pull the axle gear, remount a new gear, remount the driver, and then quarter everything back to factory specs.

The gearbox I have designed makes all that unnecessary. It is as simple a drop-in replacement as possible. Installation requires drilling out the two rivets holding the torque arm to the original gearbox and shortening the worm shaft to length to fit your original universals. The gearbox reuses the Omnicon cover plate.

These are CNC machined in brass to very tight tolerances. One of my design goals was to get rid of all the slop between the worm and thrust bearings so common in brass imports. The result is smoother starts and stops. Bill and I have been testing the gearboxes with heavy trains on our respective layouts since this summer. In fact, I installed a speedometer on the layout to test the various gear ratios in my prototypes against the stock gearbox to ensure they matched.

One thing I should mention is you may need to file the cast boiler weight for clearance. I milled mine out for an earlier and taller version of the gearbox, but the final version is much lower. The I1s Decapod may need to have a small notch cut in the bottom of the boiler.  

Cost for the bare boxes is $150.00 (includes deburring) and $195.00 assembled with NWSL internals postpaid. Each side of the gearbox requires an hour of CNC machining, then they have to be mated to each other and reamed. Due to Omnicon's engineering decision to use a reverse slant axle gear, the idler gear has to be reversed from normal. Fortunately, NWSL has custom-made gears for this new replacement box. 

(Note: I do not intend to make these gearboxes a stock item for the simple reason that I have my CNC mill configured with over 24 tool holders to machine these boxes. Setting up the program, tool holders, clearances, tool offsets, and specialized fixtures takes a lot of time and is not something I would want to do again as I am moving on to my original goal of making injection mold. Accordingly, these gearboxes will be done as a single run based on the response from members of this list). 

If you want to assemble them yourself, you will need some 2.4 mm and 3mm shaft stock from NWSL, plus a 24T spur (2946-6, custom made), bearing (358-6), worm (50601-6), and a pair of 103-4 thrust washers. Hardware required is two 2mm x 8mm screws, two 2mm  x 5mm screws, two 1.4 x 3mm screws, and two 1.4 x 6mm screws. I would add knurling on the 2.4 mm shaft and use blue Loctite

Please contact me at or call 217.971.9817. Payment may be sent to Jackson-Standard, 2925 Glenmere Ct., Springfield, IL 62704.