Jim’s Blog: When Projects Attack

A couple of weeks back, I installed a new set of tires on my 1966 Caprice to get it to sit higher in the back, and also to eliminate a vibration. The car still shook on the freeway, so I pulled apart the only thing left that could cause the wiggle: the driveshaft and U-joints. 


At first, the fix was going to be simple. New U-joints front and rear and the car would be back on the road the same day. Already had the tools, just needed $20 in parts. No problem.


Not so fast. 


When I got the shaft out of the car and up on my bench, I noticed a weird 1310 to 3R conversion U-joint up front (on the left in the image below). That’s pretty small on the transmission side for a heavy car making gobs or torque, and probably on the verge of chucking the driveline up through the floorboard at the dragstrip. 1310 to 3R joint (left), compared to 3R solid joint (right)


The simple solution? Upgrades. So I ordered an $89 GM 3R-style yoke for the transmission so I could ditch the too-small 1310-style yoke and that dinky conversion joint. I also ordered a few $16 Spicer solid U-joints (on the left). No problem. Install everything on the old shaft and I’d be on the road again.


Not so fast.


The next day I got to looking at the shaft itself, which was kinda rusty and pitted under the paint. Would it really be wise to keep running it that way? There’s a place here in Portland that retubes drivelines for $200 or so. OK, I figured. I’ll do that too. Better safe than sorry. So I took the shaft and my new parts to the shop for the job. Fresh from the driveline shop, I’d just toss the new unit in the car and go.


Not exactly.


That bigger 3R slip yoke that slides into the back of my TH400 transmission had a larger outer diameter than the 1310-style slip yoke originally installed. The new one wouldn’t slide past the original rear transmission seal. I assumed I just needed a bigger seal. 


I went to NAPA, thoroughly confused a parts guy with all this, and finally got an output seal for a 1970 Chevelle 454 that I figured was, all things considered, the right application. $7.99. I’d just go pop it in and that would be it.IMG 4539




That’s when I learned that 1966 TH400 transmissions are unique — one of two or three years that used that thin (and weak) yoke and small seal. It was replaced with a thicker yoke for strength sometime in the late 1960s. When people talk about TH400s, and parts houses list parts for TH400s, they’re all referencing the later, stouter transmissions. 


When GM went with that bigger yoke (presumably because the smaller ones broke a lot), they redesigned the tailshaft housing with a larger bushing and that larger seal — none of which fit in my early housing.


So what to do? $75.80 later, I found another transmission tailshaft, seal, and bushing to fit my new, bigger parts. 


Now I’m more than $400 into my $20 U-joint project, it’s not done, and I still don’t know if the vibration will be gone. But any car guy will tell you that’s just how it goes when you start mixing and matching 50-year-old parts to fit your needs.


Sometimes the simplest projects are the ones that end up getting out of hand. Ever have something little with your car take on a life of its own? Let’s talk about it in the comments below.


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