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Building a Rapido HOn3 GMD-1

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  • Member since
    September 2003
  • 10,578 posts
Posted by mlehman on Monday, July 18, 2022 5:24 PM


Thanks to you, Wayne, and Simon for the kind comments.

I didn't know any GMD-1s were working this far south, I know they went getting scarcer in Canada. Thanks for passing along the grteat pic!

Here's a couple of videos. The first here I haven't previously posted and discusses some of my thinking about narrowgauging this largish loco as I test run it around the Durango loop. The second I'd posted to WPF when I first got it detailed.




Mike Lehman

Urbana, IL

  • Member since
    November 2013
  • 2,362 posts
Posted by snjroy on Saturday, July 16, 2022 2:42 PM

Well done Mike! I'll have try that some day.



  • Member since
    January 2004
  • From: Canada, eh?
  • 13,029 posts
Posted by doctorwayne on Saturday, July 16, 2022 11:28 AM

That's a very well-done tutorial, Mike.  BowBow

CN is still running some GMD-1s in my nearby hometown of Hamilton, Ontario.


  • Member since
    November 2007
  • From: California
  • 2,058 posts
Posted by HO-Velo on Saturday, July 16, 2022 10:52 AM

Mike,  Great tutorial, nice work, and as always fun pics of your grand layout. 

My grandson captured a nice shot of this GMD-1 a couple weeks ago near Portland, Oregon.

Thanks and regards, Peter 

  • Member since
    September 2003
  • 10,578 posts
Building a Rapido HOn3 GMD-1
Posted by mlehman on Friday, July 15, 2022 11:41 PM

The title says it and not sure if it's been done before or not. This is how I did it for the B-B trucked versions. I plan to do an A-1-A trucked version next.

First, test your loco on DC, standard gauge, if you're not already familiar with it. It should run smoothly and quietly, it's a Rapido after all, but just to be sure that everything is right with it before your, ahem, modifications might possibly set them wrong.

I forgot to get pics of how it looked to start with. This is a good idea, especially if you've never done anything like this before. Once the loco is safely cradled upside down, you'll see the trucks, with a smooth cover over the bottom side that normally faces the track. At either end on both sides, this gear cover has D-shaped projections that engage a bump on the gear case itself, which is what retains the cover once it's snapped into place. To get them off, you pry outward, towards the rail on both sides, both ends, there are 4 total on each cover. You can do this because the standard gauge wheelsets have lots of clearance between the wheel and the sides of the gearbox housing.

There will also be wire, red or black that connects to the pickup that is part of the side frame. It's held out of the way by a big plastic nub on the side of the gearbox. The wire needs detached from the retainer nub and the slack pulled gently upward so it doesn't cause problems. Then that nub needs to be cut  off as it will interfere with  the process of narrowing to HOn3.

Each axle is composed of two metal half-axles and a plastic gear that insulates them from each other electrically. Here's a pic of the gearbox stripped down with an HOn3 wheelset already in place..

Notice a couple of things. First, the wheels are practically against the gearbox. Second, the plastic side frame of the truck has a metal electrical pickup that is also a bearing for the axle. So we need to preserve that relationship even as we narrow the gauge to HOn3 (10.5 mm). Here's another pic.

Gotta feed the cats, back in a moment,,,

How do your accomplish this? I used an old, but reliable tool, a North West Short Line The Puller. Here it is with both a standard gauge axle and a HOn3 axle after conversion.

In addition to pulling stuff, you can also use it as a small arbor press in conjunction with a V plate. You set things up so the end of the drive pin pushes on the axle end and the wheel pushes against the V plate.

Line everything so you get a good push and crank the force down firmly but gently. You will hear a POP! and the wheel will suddenly free itself. Then the turning will be easy as you push the wheel IN toward the end of the axle that seats into the plastic gear. CAREFUL to not go too far.

I would give you measurements but they would be  approximate. That's because when you push the wheels into HOn3 gauge, it's narrow than the plastic gear. Thus a small bit of the end of the gear center will need to be cut on each side. Cutting plastic is tricky with handtools because it's so slippery, at least the klind gears are made of. I believe those small black plastic nubs in the pic above are the pieces of the plastic axle end on the center gear. It's hard to get them cut off evenly but do you best, what matters is you produce an axle with 10.5 mm gauge.

Again, take it slow and fit things carefully by eye and you'll be OK, but you really do want to preserve as much of the plastic as possible, because this cutting down also lessens axle end contact, which is what helps preserve the gauge of your new narrow gauge axles, once they are assembled.

OK, now that you've narrowed the axles, you need to get the bottom cover snapped back on as well as to get the axles back into the gearbox. Here's where there's a small problem, easily solved. The original scheme to fasten things involved the D-shaped projections on the cover to snap OVER small projections on the side of the gearbox. Now with the narrowed axles, there's no way space for that to happen that doesn't push the axle out of gauge. In fact, without narrowing the nubs a little that are on the gear b0x, you can't get the axles to properly seat into the gear train, either.

The solution is to turn the D-shaped thingees into C-shaped thingees that snap on to gear box by moving in a direction, front to back, that is 90 degree from the previous arrangement.

PLUS you must narrow these down from their fatter and stronger original. They have to be able to slide between the back of the wheel and the side of the gearbox and not rub when in motion. And there's not much room, so they get thin, again careful work and constant fitting are your aids to a good outcome.

Once everything is thin enough and you have C-shaped thingees, then hook the end that is longer down first, align the center pins, and finally snap the short end into place. If you got it right, it will almost seem like factory, well almost. It does work.

Staying into gauge can be tricky with the shorthened axle ends on the slippery plastic gears. Factory either uses something that's not real aggressive or may simply be a press fit, but that can no longer be relied on due to our messing with Rapido's great engineering. I use CA and it may take a couple of tries , but I can usually get this reliably locked up with some test running. Just keep that NMRA gauge handy and suspect the axles as the problem if you're running on relabkle track. You'll find one axle or another has lost it's gauge, and the fix is easy, just pop the cover off and pull the offending axle for correction. Or you might get lucky and find that it's a great running loco, like a Rapido, once things are sorted.

Listen for rubbing noises or observe for a lope in the running of the loco from the issues caused by the narrowed  axles and revisit everything along the side of the gearboxes you've modified. Shine a flashlight in there som you b be sure everything is shaved down enough to let the narrowed axles turn freely. There is enough room, but just barely, be patient and don't overdo it as you take material off. Keep fitting until it fits then just a little more.

So my loco operates on my Four Corners Division of the D&RGW. It is kind of largish, but still plausible in terms of it's bulk versus other big narrowgauge motive power. Being painted mostly black helps with that. Here it is with a couple of my DL-535E locos. I was an early adopter, guess the Durango & Silverton kinda followed my lead on them? Wink

At Silverton. My GMD-1 is equipped with both a steam generator and dynamic brakes.

And Jason was just kidding about not taking 18" curves seriously in the GMD-1 owners manual. Here it is traversing a 17" R curve at the nirth end of Silverton.

Anyway, the GMD-1 is big power on the narrowgauge and I love the smooth running capabilities of it's Rapdio heritage. If you want it in any gauge between standard gauge and HOn3, it is possible.

Mike Lehman

Urbana, IL

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