I have an SM420 that is seized. It had some water in it. The gears and synchros look to be in good shape but the transmission won't spin. The reverse lever mechanism moves well There is a lot of info on here and on the web about freeing up seized engines and stuck bolts but not transmissions. I did find a YouTube video about a guy who bought an SM 420 and he sprayed kerosene on the gears to make sure they were moving ok. I don't think his transmission was stuck - he just wanted to ensure movement. Any one have any suggestions as to what I can do to free up my transmission? Would filling it up with kerosene and letting it soak work? I want to get it moving them rebuild it. Thanks in adavance.
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Grease Monkey, Moderator General Truck Talk & Greasy Spoon
Fill it with Evaporust and let it set overnight. Pour out the Evaporust and spray with PB Blaster or Kroil and let set overnight again. Then fill with Kerosene and start working it. A little gentle persuasion with a wooden mallet might help. Good luck.🛠
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There is a countershaft in the bottom below the main shaft,if the trans had water in it ,there is a strong possiblity that the bearings could be rusted up and siezed. If you were to get it freed up and turning,i wouldnt use it. Id take it apart and replace bearings and whet ever else it neede. If you cant get it to turn at all, tear it apart and see what whats going on in there.
Fill the case to the top with Diesel fuel and let it sit for about a week. Bolt a piece of 1/4" X 2" steel flat bar about 3 feet long onto the 4-bolt output flange with an equal amount of the bar sticking out on both sides. Use a big dead blow mallet to thump the bar back and forth in both directions until the output shaft frees up, then put the transmission in 4th. gear so the countershaft will turn and free up the bearings that support it. Once the transmission turns freely, dismantle it completely and replace all the bearings.
I'll be surprised if the gears don't whine so loudly when they run that you'll need earplugs to drive the truck. Jerry
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1940 Pontiac 3sp trans from car that was always garaged BUT clothes dryer was vented into the garage it sat in for 20 years. Second pic is after it sat overnight with muriatc acid solution. Was treated not to run but to be able to disassemble. Turned and shifted fine after acid treatment but bearings sounded like it was full of gravel.
If the part isn't that rare, I would junk it and find a better one. As stated previously, rust pitting on the gears will make the transmission louder than normal. To bad you are on the other side of North America, I have a couple nice SM420's sitting around waiting for someone in need. If you want to free it up just for fun, filling it up with old drain oil and letting it sit for a few weeks or months will usually do the job...no heating or beating necessary.