I recently adapted a Dana 20 (from a late-1960s FSJ) to the 4L60E in my 2010 V-8 Chevy Colorado. I’m running Tera-Low 3.15:1 gears and an Advance Adapters HD Output shaft. I’m having a couple issues after the full rebuild and gear swap.
First, while in 2WD High, the transfer case slips into Neutral coasting at highway speeds. Everything pulls great under load and only slips out when coasting. I installed stiffer detent springs and shimmed them as well. I pulled the case to shim the detent springs and checked the output shafts endplay at the same time. Everything checked out on the tight side of tolerances (0.002-0.003). I am running full synthetic fluid from Amsoil and using a JB Fab twin-stick shifter setup.
Second, with the transfer case in Low range, the transmission has a grind when shifted into Park. It sounds like it would if shifted into Park while rolling. There is no grinding when shifting into Park in High range.
The staffer with the most recent firsthand experience with a Dana 20 is Editor-in-Chief Christian Hazel, so we forwarded your questions to him. Here’s what he had to say.
My Spicer 18s would do that in my flattie when they got worn after a few years of hardcore off-roading. Most of the time it was the bearings inside the intermediate gear. They’d cock and allow the sliding gear to move just enough to slip past the detents, and then the pressure on the gear teeth would do the rest to pull it out of gear. What intermediate shaft/bearing package are you using? With cheap import bearings or a soft intermediate shaft, the main gear can cock in the case and sometimes cause stuff like you describe. Be aware there are several terrible import shafts out there seemingly made out of stale cheese. I wouldn’t even bait a rat trap with one. I’ve had the best luck with intermediate shafts from Novak Conversions (novak-adapt.com). For bearings, use CE-grade Nakagima since they have the correct Rockwell hardness.
I’ve only run a couple Dana 20s behind an auto transmission, and none of them had any grinding going into Park. Obviously something in there is still spinning or under load, so try hitting Neutral in the transmission for a couple of seconds to allow the rotation to slow before jabbing it right in Park.
Grinding in Park is a common complaint with stock GM applications. We’re not sure why it happens, but we’ve experienced several GM trucks doing what you describe, and it’s probably nothing to worry about. Try going slower when shifting into Park; giving the transmission a second to engage before going into Park seems to alleviate the issue.