Printed Part Gets Classic Truck Rolling

When working on classic vehicles, and especially when modifying them outside of their stock configurations, things can get expensive. It’s a basic principle in economics: the rarer something is the more money somebody can charge you for it. But if you’ve got the skills and the necessary equipment, you can occasionally save yourself money by custom-fabricating some parts yourself.

After changing the gear ratio in his 1971 Ford F100, [smpstech] needed to adjust his speedometer to compensate. Unfortunately, a commercial speedometer reducer and the new cables to get it hooked up to his dash would have run into the hundreds of dollars, so he decided to try designing and 3D printing his own gearbox. The resulting development process and final product are a perfect example of how even a cheap desktop 3D printer, in the hands of a capable operator, can do a lot more than print out little toy boats.

The gearbox contains a large ring gear driven by a smaller, offset, spur gear. This compact inline package drops the speed of the input shaft by 25.5%, which [smpstech]  mentions is actually a bit slower than necessary, but it does give him some wiggle room if he decides to change his tire size.

Even if you’re not looking for a speedometer reducer for a nearly 50 year old truck, there are some lessons to be learned here in regards to 3D printed car parts. The first version of his gearbox, while functional initially, ended up looking like a deflated balloon after being exposed to the temperatures inside the F100’s engine bay. His cheapo PLA filament, which is probably fine for the aforementioned toy boats, simply wasn’t the right material for the job.

[smpstech] then reprinted the gadget in HTPLA, which needs to be annealed after printing to reach full strength. Usually this would involve a low-temperature bake in the oven, but he found that simmering the parts in a pot of water on the stove gave him better control over the temperature. Not only did the HTPLA version handle the under-hood conditions better, it was also strong enough that he was able to use a standard die on the connections for the speedometer cables to create the threads instead of having to model and print them. Definitely a material to keep an eye on if regular PLA isn’t cutting it for you.

This isn’t the first time we’ve seen 3D printed parts used to get a vintage vehicle back on the road. Building these custom parts would have been possible without a 3D printer, of course, but it’s a good example of how the technology can make these types of repairs faster and easier.

[via /r/functionalprint]

19 thoughts on “Printed Part Gets Classic Truck Rolling

  1. Classic example of a terrible use case for 3d printing.

    That gearbox wont last more than 1000 miles if he gets lucky, I wouldn’t be surprised it is blew after 100 miles considering that is is being run inside the engine compartment of a car at 1000 rpm (speedo cable is 1000 turns per mile).

    In the mean time, he could have paid $30 for a brand new 1:0.5 reducer https://www.ebay.com/itm/263838552676 that would have let him use a much more reasonable 14 tooth speedo gear ($5-10 at any auto parts store). Total time about 15 minutes to install.

    Probably works out cheaper than the filament and thermal insulation needed to print the first gearbox, destroy it, buy a $40 spool of premium filament, print a second gearbox…

      1. 1000 turns per mile makes sense to me. If you’re at a cruising speed of 60 mph, that’s 1000 turns per minute. so 1000 rpm seems like a good typical use case. 2000 rpm would be a bit fast unless you’re a race car. 500 rpm would be running around a city, but might as well be a bicycle speedometer if you’re only going 30 mph.

    1. And if it fails he can make another…what’s your point?

      I had a similar issue with an old F150 back in the dark ages, but installed a tachometer and knew roughly how many RPM in a given gear for common speeds…made it feel like I was driving a race car

        1. Well like I said it was the dark ages (circa 2005)…i had to work it out with the gear ratios and tire size, but never got any speeding tickets if I followed my gear/RPM rules.

          Now I’d just stick my phone to the dash and call it a day.

      1. Back in 2000ish I had a 1990 Nissan where the speedometer didn’t work. In college and completely broke, a buddy of mine taught me how to use the Tachometer to judge my speed in any given year ratio.

        Then that piece of shit broke so I just kind of matched my speed with whoever was in front of me. I kind of figured if they were doing no more than 85 in a 65 zone, I would be OK.

        I drove it that way until 2004 or so when I got a new vehicle. The dealer didn’t want it so I traded that Nissan in for a couch at the Goodwill. By that time, I think the couch had more recoverable metal than the Nissan.

      2. This was standard practice back in the Dark Ages (when Detroit ruled). When your speedo failed, you’d get calibrate the tach. Only needed a few points (25/35/60 etc.) for most speed limits and could fake it most of the time by following traffic.

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