So I've had a driveline vibration issue in my Lemons Dart for a while now. The rear end is a Mopar 8.25" rear out an '84 Jeep Cherokee. I doublechecked the pinion angle multiple times, and I'm pretty sure that's not the problem. The vibration starts around 65 mph and lasts until you get to about 75, then goes away completely. No strong correlation with applied throttle or braking, although the speed range where it makes noise might shift a little lower when decelerating. I had the driveshaft professionally spin balanced about a year ago and that greatly improved it; it was still there but much reduced in amplitude. It also may have lowered the resonant frequency a bit; I recall it happening more in the 75-85 mph range previously. Both U-joints were also replaced at that time.

Unfortunately, it started getting worse again during the last race. I checked the rear U-joint and found that the straps holding it into the pinion yoke were a little loose on the U-joint caps. The bolts were properly torqued and the strap was tight against the pinion yoke, but the caps were still able to slide back and forth slightly. (Luckily this yoke has little tabs that keep the U-joint caps from flying off.) I wrapped both of these U-joint caps with a couple layers of aluminum tape just to get the car through the race. That reduced the vibration to a less worrisome level, although the caps could still slide a bit in the yoke if you pushed hard.

Anyway, I got a call from Charlie Schmidt the other day, who read about this in my race report post but is having trouble actually typing posts on his tablet. (apparently the cat screwed up his computer somehow ) He mentioned that there were two Mopar U-joint sizes with two different accompanying yoke straps. Unfortunately the diameters are so close that it's hard to tell the difference by eye. He suggested that maybe I have the large straps installed on a small U-joint. (I'm pretty sure I bought these current straps out of the Dorman Help! section at Autozone, so no idea on the size.)

So I started looking up part numbers. The only U-joints listed on RockAuto for a '64 Dart that have cap diameters included show them as 1.078" (Moog 246, ACDelco 45U0120 and 45U0117, for example). Looking up U-joints for an '84 Cherokee, I see U-joints with a cap diameter of 1.063".

I just went out to the garage to measure the spare U-joints I have. They are both Moog 369, with a cap diameter of 1.063; I must have bought them using the Jeep application. But one of them has a pair of 1.078ish caps swapped on because of some shenanigans at our previous race in April. And now my memory is jogged and I think I know what the problem is...

After I got the rebalanced driveshaft back from the shop earlier this year, I installed it on the car. At our next race (NOLA in April 2018), there was a squeaking noise coming from the rear. I isolated it to the rear U-joint, pulled the straps, and saw that the U-joint caps were creased. I assumed I had overtorqued the bolts, because I had indeed just tightened them by feel and have a tendency to overdo it sometimes. (But now I realize that makes no sense; Even if the bolts were overtorqued, it will only overtighten the strap to the yoke. This can't cause the cap to deform unless the strap or the yoke itself has too small a diameter.) Not wanting to press the U-joint out of the driveshaft, I pulled a spare (Moog 369) U-joint of the parts bin and swapped the caps onto the installed U-joint. They fit just fine on the U-joint itself and looked to be the same diameter as the ones I pulled off (but were actually 0.013" smaller). They also felt snug in the yoke. I bolted the straps back on (with a torque wrench) and we ran the whole race like that. But this same assembly felt loose the next time I checked, at the race this past weekend. Probably because it was always a little loose but got slightly wallowed out with time.

In summary, I think my Jeep diff yoke takes 1.063" caps but the straps I bought are for 1.078" caps. My Dart driveshaft also takes 1.078" caps. So I basically need a conversion U-joint (which I kind of already made myself) and the smaller size straps.

The RockAuto listings for '84 Jeep U-joint straps are a mess of different axle options and 2WD vs. 4WD, and none of them directly call out a size except one which says "1310 and 1330 u-joint size," but lists a bunch of axles that do not include the Chrysler 8.25". Two U-joint strap kits are listed for the '64 Dart, one for "7260 U-joint size" and one for "7290 U-joint size." Can anyone confirm which cap diameters any of these correspond to?

This may not be the root cause of the resonant driveline vibration itself, but I can definitely see a loose U-joint allowing a small vibration to get out of hand. We'll see how it feels when I get this straightened out.

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Somehow I ended up owning three 1964 slant six A-bodies. I race one of them.
Escape Velocity Racing

Do you use the inner clips on the caps? This will keep them from sliding outward. I found two different strap kits by Moog.
#31810 (7260)
#31610 (7290)
I couldn't find any dimensions on them.
All I could find was that the 7260 caps are 1.078" dia.
I have never had a problem with the straps. I try not to reuse them and I always flatten them slightly so that they grip the caps some. I actually have a set of billet aluminum caps I picked up for another drag car I was building. I think they might be made by Moroso.

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-MattMan
LEANED & MEAN

The caps that came on the U-joints installed by my driveshaft shop had inner C-clips, but the smaller caps on the Moog 369 U-joints don't have them or a slot to install them. I don't think this is an issue in my case since my pinion yoke has tabs on the outside to keep the caps from sliding out. Either way, the caps need to held tighter by the straps.

It looks like the 7260 U-joint with 1.078" caps is the "small Mopar" U-joint, which is what I need on my driveshaft side. The 7290 "large Mopar" U-joint has even bigger caps at 1.125". I need to confirm the size of my pinion yoke, but it looks like what I need on that side is actually a series 1310 (1.062" caps, outside locking, overall width ~3.22)

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Somehow I ended up owning three 1964 slant six A-bodies. I race one of them.
Escape Velocity Racing

See if this site has any of the info you need.

http://www.fme-cat.com/MOOGUjointSpecif ... x?Brand=MU

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-MattMan
LEANED & MEAN

Thanks, that site says I can use a Moog 319 as a conversion U-joint, assuming I need what I think I need. I still need to get under the car and verify some measurements.

_________________
Somehow I ended up owning three 1964 slant six A-bodies. I race one of them.
Escape Velocity Racing

couldn't you just eliminate all the sizing noise by going to u bolts

the one linked may not be the exact size or best price, just an example

https://www.ironrockoffroad.com/product ... e-kit.html

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Doo Ron Ron and the Duke of Earl are friends of mine.....
https://www.youtube.com/watch?v=uX8Nj8ABEI8