Sunset 3rd Rail belt drive

I am currently getting more involved into the O-scale world and might have some newcomer questions. I am starting to build up a little roster of brass locomotives and just found out that the Sunset models all have a belt drive. These are not too expensive and after all I read about it one would better replace the mechanism with NWSL parts for better running qualities. 

Do you guys use the original belt drive when running these or is the replacement of the mechanism common practice? I also wonder where to get a spare belt in case of breakage. My first project is a Samhongsa USRA 2-10-2 with the saied mechanism that I am going to convert into a more or less convincing C&S 900 class. I just hope that NWSL will stay in business for a long time :-)

Sarah Griessenboeck

Original Post

the drive Sunset uses is well designed and a good quiet runner. one of the better drives in O scale.

there should be no need to replace a belt, except perhaps after many years if it dries out.

then a company like Stock Drive Components-Sterling instruments will have belts

I am John Galt !


I have a sunset 2-8-0 that was built back in the 80s with the original belt drive and it still runs fine.   As someone said, "if ain't broke, don't fix it".

These drives are smooth and quiet if properly adjusted.   I was told  years ago, the belts were "the same as the ones used in printers" and will last forever.   

The smooth running is one big avantage, probably in part due to having teh motor shaft and the gearbox shaft parallel to each other.

In addition, with this type of drive, the Belt tower can be entirely hidden in the fire box on a steamer, thus leaving the space under the boiler above the drivers open/see-thru.    This is very protypical and generally not possible with a conventional gearbox drive.     If you look at some backlit photos of steamers, you can see right through between the boiler bottom and the tops of the drivers.


Based on innumerable web posts and (and my personal experience) modern toothed belt drives have proven to be reliable and smooth running.  As others have posted - don't change out the drive if it is running well.    You should know that not all Sunset and  Sunset 3rd Rail locomotives were built with belt drives.  Several 1990's era Sunset/Sunset 3rd rail locomotives such as their PRR S2, J1, and I1 models used enclosed worm die cast gearboxes.    


Ed Rappe           PRRT&HS 421


The belt drives are fine and they eliminate the visible worm shaft below the boiler shell.  The worm shaft runs horizontally from the front of the firebox low in the frame to the Sunset gearbox.  I too was suspicious of this drive when I first saw it but they have proved to be very reliable.  I have several Sunset steam models and they are all good performers.

Joe Foehrkolb

One other nice feature of the belt drive is the ability to easily change drive ratios.  Some of my best runners are Sunset/3rd Rail PRR decapods.  These have a "traditional" gearbox with a 31.5 to 1 drive ratio.  I have a couple of Sunset/3rd Rail PRR 2-10-2's with belt drive that had an 18 to 1 drive ratio as delivered.  The stock belt drive used a 12-tooth pulley on the motor and a 14-tooth pulley on the lower shaft to the worm.  I got some replacement pulleys from 3rd Rail (they have several sizes, or at least they did when I ordered) and installed a 10-tooth pulley on the motor and an 18-tooth pulley on the lower shaft.  This changed the drive ratio on the 2-10-2's to 28 to 1 and they now run great when double headed with a decapod.

Thanks a lot for the input! I still have to experiment with speeds but adjusting the drive ratio sounds like a great opportunity. The Samhongsa 2-10-2's drive belt is still on but might come off every second. The engine runs way too fast and I'll try to get in touch with 3rd Rail regarding different pulley dimensions. 

The idea of a frame without drive shaft and gear box visible from the side is really nice. First I thought of replacing the belt with a custom built gearbox but that would be a lot of work. But now I got your Sterling Instrument input, Chris, thanks for that!

I'll keep you updated, wish me luck :-)


I agree with all the posters above - with one caveat:  Some early Sunset (the unpainted era) came with brass axle gears.  Those gears must ultimately be replaced with bronze, and last I heard, Sunset had the bronze gears.

I am a great fan of the NWSL non-tower mod 0.6, and use it in place of even the vaunted USH, as well as Lobaugh.  However, these Sunset gearboxes are just as good - maybe better!  Maybe lots better!

I'll let Bob respond, but doing the math it sounds like the original axle gearbox is about 15.5-to-1.  Then, 12/14 pulleys give you 18:1 and 10/18 pulleys give you 27.5:1.  From what I recall, the chassis has to be "hogged" a little to make room for the large 18-tooth lower pulley.  Also, the motor might have to be slid lower on its mount to make the belt reach without putting tension on the bearings.

When I heard Bob's idea I did this mod to my Greenbrier, that's how I know ;-)

"It's the gear ratio Brian, it always has been the gear ratio."

Ted is correct, the final drive ratio is a combination of the pulley ratio and the worm gearbox on the driver axle.

I made a poor man's lathe by pressing the 18-tooth pulley on the shaft of a spare Pittman motor and used a sharp chisel to remove most of the outer "belt keeping" flange from the pulley.  It then fit into the frame without needing to modify the frame itself.  I don't recall needing to adjust the motor position for this project.  There were others that needed it, but I don't think this one did.  To be honest, most of these mods were done 10 or 12 years ago so memory is a little foggy.

Over the years I've used belts, timing belts (what Sunset 3rd Rail uses), chain, gears, shoot you name it, I've tried it.  Timing belts are a great resource for some applications.  I use a lot of them on my diesel drives.  I also use NWSL when I need a new gear box on a steam engine model.  Always ball bearings

Stock Drive Products (SDP) is one of the primary resources I use for timing belts and pulleys (I also use McMaster-Carr).  They have a great tool for computing ratios, length of belts, spacing, etc.  It is a tool I use all the time.  I will attempt to post a link.

I use mxl and I thought Sunset did also but I was wrong.  They use 40 D.P.  They look close but are not the same.  I know this from experience.

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