Bachmann 70-Tonner Rebuild: The HO Edition

I rarely dabble in HO, but Bachmann’s 70-ton switcher holds a special place in my heart. Way back when I was a boy and dinosaurs roamed the Earth the original Spectrum version with its odd dual-motor setup was my first ‘real’ (i.e. non trainset-trash) locomotive. Later in life I changed scales and took a break from the hobby, but when I came back from my hiatus Bachmann’s N scale 70-tonner was my first foray back into detailing and painting. I'd thought about fixing up my old HO one to match that, but half its parts have been lost over the decades. I dropped the idea until this weekend, when I spotted an HO 70-tonner sitting alone on a table at the local train show. It had been crudely painted in a very VERY thick custom paint scheme, but it was the newest version with a proper HO-sized motor and a factory DCC decoder, and it ran beautifully. I couldn’t not buy it.

(This is where a picture of the loco in the condition I found it would go, but I was too eager to get the scale inch of paint off to take the ‘before’ photo. If you really want to see what the custom paint looked like, take your own 70-tonner and dip it in pancake batter.)

Like the N version, the first steps mostly involve fixing Bachmann’s problems. I stripped the paint, removed the handrails and other details while cursing Bachmann’s obsession with gluing things together, and now it’s time for lots and lots and LOTS of cleanup. The mechanism may be a big step up from the old black-box version, but the shell is the exact same tooling as it was 20+ years ago, and it has not aged gracefully. Flashing, awkward mold seams at the hood and cab edges, and random blobs of plastic abound. Some of the panel gaps have all but vanished. It will take lots of sanding, scraping, and re-engraving with a tiny chisel before I can get to the fun details.





After sanding the edges and corners into the correct shape, I applied lots and lots of filler putty to various inaccuracies, unused detail holes, and mysterious dents in the shell.



While I was already busy chiseling stuff off the cab I removed the solid steps too. From what I can tell in prototype photos, the cab steps should be rectangular metal mesh. I have two sizes of brass mesh on hand, but my HO-untrained eye can't tell which would be the more likely one to be accurate. As for the corner steps on the sill, your guess is as good as mine. Something for me to mull over while the putty cures, I guess.

 

Well, I ordered a batch of details and decals that cost more than the locomotive itself. Starting over fresh in HO is expensive.

While I wait for those, I'll work on the mechanism. Running it away from the noise of the train show I could hear the various gear noises, along with the distinct tick-tick-tick of a cracked gear. After all, the three most abundant things in the universe are hydrogen, stupidity, and split Bachmann gears. Lo and behold, all four are cracked. Along with the u-joint cups on the worms. And of course Bachmann doesn't have the spares in stock. Does NWSL or anyone make a suitable replacement gear? I see NWSL makes a u-joint replacement at least.

 

A close #4 are split Proto gears...

I have a "twin-engine" Bachmann 70 tonner with the same cracked gear problem. In the state it's in, all four gears cracked, it's unrunnable.

One would think the industry would learn from these blatant materials defects and improve their merchandise.

 

I don't know if it's materials, design, or both. From my experience with various Bachmann gears throughout the years it seems like the holes are consistently undersized for the axles that get stuffed into them, with predictable results. But whatever it is Bachmann did, they do it often enough that I'm surprised there isn't a ready-made third party fix for the 70 tonners. Custom options certainly exist for several other Bachmann locos plagued with the same problem (doodlebugs, DD40s, Shays, etc.).

 

It really is appalling. I've ordered some brass tubing with an ID about the same size as the un-cracked axle. With any luck I can make some compression sleeves that force the axles back into shape. If that fails, I'm also working on an STL file to 3D print new ones.

 

While I explore options for printing new gears I’ve taken a stab at fixing the originals. The shaft portions of the gear have an OD of 3mm. Brass tubing with an OD of 3.5mm and a wall thickness of 0.225mm fits very tightly over the axle shafts, (hopefully) squeezing the cracks closed. With my Dremel I rough-cut sections of tubing slightly shorter than the gear shafts. Using various small files I cleaned these up and smoothed the cut edges. The brass sleeves press tightly onto the gears as I’d hoped.







Before re-inserting the half-axles I reamed out the holes with a #53 drill bit, since the stock diameter was obviously a hair too small. The goal is to have the half-axles fit tightly enough to hold gauge and transmit power, but not so tightly that they stress the plastic.



Once the fit seemed right I gauged the wheels, reassembled the loco and gave it a test run. Two of the axles ran like new, the other two still had a noticeable click. I inspected the clicking ones closely and noticed that the half-axles had forced the cracks open a little bit despite the brass sleeves. I reamed these gears out again with the same #53 bit. I can’t find my #52 bit, but I suspect it would be a little too large. The goal is to find the sweet spot where the gear has enough friction to hold the half-axle securely, but not enough to stress the plastic. A couple of the half-axles ended up fitting slightly loosely; I added a drop of extra thin CA glue to these and pressed them back in. Everything seems to be staying in place now, and the loco runs smoothly.

Well, sort of.

As soon as it hits a curve or grade it makes a horrible clattering noise and grinds to a halt. With the cracked gears fixed, the cracked U-joint cups on the motor and worms are under extra stress. And unlike the gears, these are pretty irreparably messed up. I see NWSL offers replacement driveline parts, has anyone here used them before?

 

Luckily, this project isn’t all about fixing problems with the mechanism. There are also plenty of major problems on the shell to fix before I can start adding road-specific details

So far I re-engraved the shallow and uneven panel gaps around the hood doors. Filled in some panel gaps on the cab that only exist in Bachmann’s imagination. Sanded the radiator hatch flat, replaced the blobby reinforcing ribs with styrene strips, and engraved a gap around the bottom of the hatch. Replaced the engine access door on the roof with a piece of 0.010” styrene.



The one body flaw I haven’t dealt with are the hood door louvers. Aside from the fact that some of them are in the wrong spots, they’re pretty sad looking moldings. In the above picture that's not just sanding schmutz clogging up the rearmost three louvers, the mold is wearing out there and the louvers are blobby. I considered making new louvers from styrene siding, but after looking at my siding collection it’s all just a little too flat and shallow to really represent HO louvers. So I fired up Blender and modeled yet another part I need to figure out how to print:



Each louver piece is smaller than a dime, but Shapeways wants $7. Per louver. Times eleven. To paraphrase Luke Skywalker, I could almost buy my own 3D printer for that price. This is shaping up to be the project that finally pushes me into 3D printing.

 

I like your solution to the cracked gear problem. It might work on the "twin-engine" 70 tonner that has been in "the morgue" for a while.

I might try to revive that corpse... at least test the concept on one truck. At worst, I'll learn something. At best, I might have it back in service!

Nothing to lose, and everything to gain.

 

I've been using them on my shay build, they seem more than fine, and have quite a few sizes available. Made of tough engineering plastic, they are a tight fit.

 

Good to know, thanks! I've just ordered a set (plus a spare just in case). And some 1.5mm rod in case I need to build a longer dogbone than the one they provide.

Not much progress, otherwise. I priced out some 3D printer options and it made my wallet cry. Printers with high enough resolution to print the tiny detail parts I always need are on the expensive end. Still cutting out the original louvers, though. One way or another they are getting replaced with something; I probably can't do worse than Bachmann.

 

Nowadays in N Scale Land I'm struggling to find detail parts as basic grab irons. In HO you can find multiple styles of door handles. It's enough to make a multi-scale modeler weep

And while I'm ranting, I really wish Microscale would scale the individual decals down for their N versions but keep the sheet the same size as HO. Having more than a single molecule's width between each individual decal would be worth paying a couple bucks more.

Alright, back to cutting out louvers I go.

 

The NWSL U-joints arrived today and driveline repairs are probably complete (more on that in a minute.) The stock dogbone driveshafts (there's one in the top left of the photo below) do fit in the NWSL cups, but are a little loose and rattle around. Since 90 percent of this loco's well-documented performance problems seem to be related to things being loose and rattly, I wanted to put a stop to that. So I went ahead and made custom driveshafts out of brass tube and the ball ends included in the NWSL set:



While I had everything apart again I also added little dabs of super glue inside the plastic brackets that hold the worm bushings, to tighten their grip. As I received the loco the sintered bronze bushings were held so loosely in the brackets that they were spinning with the worm rather than staying still and doing their job. The extra slop in the worms did the mechanism no favors, but now that the bushings are stationary the worm assemblies turn smoothly and vibrate less.

With those driveline changes made everything seems to run beautifully. I say 'seems to,' because I've realized I don't actually have a functioning HO test track at home. Tonight I tested the 70-tonner by by clipping the Zephyr's track wires directly to the decoder's pickup tabs and driving the mechanism around my workbench like a hamster on a leash. Crude, yes, but the problems caused by the cracked driveshafts and loose worm brackets were not exactly subtle. Tomorrow I'll bring it to the train club and make sure it runs as well on track as it does on my cutting mat. On the way I'll stop by the LHS and pick up some HO track of my own

 

On my shay, the dog bone balls were a bit tight in the cups. It was making the trucks wobble. I solved it by using an exacto knife and slowly scrapping the ball until it just fit without being loose.

 

Good to know, thanks! Your Shay thread has been a great inspiration for a rookie HO modeler like me. The NWSL joints' fit is definitely on the tight side, enough so that I was concerned when I put the first one together. But the stock Bachmann parts had exactly the opposite problem (i.e. spectacular amounts of looseness in every joint) badly enough that if I have to scrape the ball ends down it will be a nice change of pace from all the shimming and gluing.

 

Heh, I laughed when I read you didn't have a test track. I've got the same issue, the only bits I have are few lengths of peco flex track I picked up at the hobby store. Soon I think ill take two or three lengths and mount them to a board. Make a straight section and two tight curves for properly testing the mechanism.

Anyway, your build is looking real good! Always in awe of modellers that modify the shell look so simple. Feels beyond my skills

 

I'm glad it's not just me who's test track challenged!

I'd like to think I have skill in customization, but more often it feels like skill in assessing the availability of replacement parts. Probably why I have a soft spot for locos that are floating around bargain bins by the thousands; I can really try new things with minimal risk of destroying something valuable. I'm pretty sure I'd treat a NIB MDC Shay like a museum piece and still be agonizing over the first piece that needs sanding.

 

Well, I'm HO test track and layout challenged too

Surpringly, I'm not too worried with the shay, plenty of replacement parts available, heh you e seen how I've already stolen parts from the second kit to feed mistakes from the first.

You need a 3d printer really, you'll see one show up in the Shay build eventually, and it would have made your louvers a snap to build (though your way makes you a god of modeling to everyone watching). You don't need to use it all the time, mine sometimes sits unused for months. It's a tool like any other, you use it when you need it.

 

Took the 70-tonner to work today, so that I could swing by the train club on the way home and test run it. There's still a slight click and rattle, but considering it originally sounded like two skeletons making out this is a HUGE improvement.



As for the shell, I'm still busily poking holes in it. Half of the ten door vents Bachmann put in the right spots are cut out; cleanly cutting a matching eleventh vent in the door Bachmann left blank is a bridge I'll jump off when I get to it. I've put out feelers to friends with 3D printers to help with the louver inserts while I scrape the couch cushions of life for the startup cost of my own printer.

But it's not all louvers. The cab side windows have also gone under the knife. Bachmann opted for an open window on the right side, but made the opening too small compared to the prototype. That was an easy fix with a little scraping and filing. The closed windows on the left side are turning out to be a little harder. The window panes are closer to the correct size, but their sliding frames need to be recessed in a rectangular opening the same size as the modified right one. After trying various ways of engraving the existing window frames down to make them recessed I've decided it'll be easier to just cut out a hole to match the right side and scratchbuild new frames in it. I've outlined the correct cutout size with some embossing labeler tape, which will provide a straight and sturdy guide for cutting the new opening.

And once I've tackled that fundamental proportion issue there's still something off about the nose. Nothing adds up. I'm wondering if Bachmann made the hood too wide like they did on the 44-tonner. At one point I had a copy of Model Railroader magazine's October 1969 article on the 70-tonner complete with HO scale diagrams, but I'll be darned if I know where it went. Right when I could really use it again, too.

 

Nice work so far. Good to hear that it's running better too!

That mental image is going to take a while to clear out of my head...