3rd member options?

I'm in the process of ordering a new complete 3rd member for my 30spline 8 3/4" and my question is what should I order? The vehicle is a 69 Roadrunner with a 500hp 440 backed up by a 5 speed manual transmission. This is a street car with 275 60 R15 BFG's and it will see no tracktime. In my other cars I have always used a Detroit Locker and I'm happy with that but I'm curious  about the Eaton true track and Doctor Diff's Helical Gear Sure-Grip ("Sure-trac II")
Please advise me.

Pretty happy with my Detroit Lockers. I'm not sure what the modern version of the old clutch type Sure-Grip is, but that would be an option.

Talk to Cass at Doctor Diff. I'm pretty sure the helical is the way to go these days. They are much smoother than a locker and much more reliable than the old clutch pack design.

Not sure I agree with the reliability comment, other the then clutch style by design does/ was intended to wear over time, and that time I suspect does vary, other then that I'd also lean for street to the helical style.

Yes. Call Cass.
Avoid the Eaton Detroit Truetrac. They have quality issues with the Chrysler 8-3/4 version. I got one and had to send it back. It had a tremendous amount of side play on one of the output worm gears that engage the axle splines. Many reports on Mopar sites about this problem. The play causes the axle splined end to move side to side, in two dimensions, and therefore the wheel to wobble. People have also reported clunking noises.

Robert

I really don't know what could be more "reliable" than the clutch style sure-grip unit. I have had them in Dana rears and 8 3/4 rears on the track and street with zero issues. If they do happen to wear out you can rebuild it No downside at all IMHO
Gus

I also run the clutch type sure-grip on several vehicles and they always work and never had a problem with them, for me that's the only choice since I have several of them lying around with different ratio gear sets.

Thanks a lot for the information. I will give Cass a call.

These are good info. Cass build my A-body 8 3/4 and I had the issue with the play mentioned above. Cass then built me a 489 case 3rd member with the helical Eaton. The only thing I can add is to make sure you use the gear oil without the friction modifier in it. I used the dinosaur oil initially, but now I'm going to use the Redline synthetic without the friction modifier in it. The issue is how hard the wheels lock together which changes with the various gear oils.

are you saying the dino oil locks the gears tighter than synthetic ?
or slower to lock ?

Eaton says they use conventional oil when they test/design the units.They state that using friction modifier, in any gear oil whether it be conventional or synthetic will reduce the amount of bias the helical gear set-up will have. In other words it won't lock as hard. They even say that different weights of gear oil have a subtle effect.

Friction modifier is used to reduce potential and actual clutch chatter in the clutch style units.

I am going to use a synthetic without a friction modifier added, which may or may not alter the bias. I have 3.91 gears with a .67 overdrive, so I am looking to reduce heat at modern freeway speeds with the synthetic.If it affects the bias too much, I'll drain the synthetic and go back to conventional gear oil. I just had a drain plug welded into my 8 3/4 to make gear oil changes easier.

They recommend conventional gear oil only with the helical units. No synthethic.

I had a customer who I furnish a new , from Cass , clutch type 8 3/4 posi unit that he had a local shop assemble and installed. I furnish 2 quarts of the non synthetic Valvoline 75/90 WT gear oil along with a bottle of friction additive for the clutches, the shop guy who installed it in the Lawyers car(my customer) use their synthetic gear oil with no modifier.
The owner called me very upset that he had almost wreck his 1967 Plymouth GTX on the way home foromm the shop when he started to turn right up his road to his home and the posi unit locked up and almost made him go straight forward into the ditch instead of letting his car go around the corner like normal.
I replaced the unit and he had the same experience again so I ended up providing him a helical gear type posi before I found out what that shop was doing to make those posi units act that way
My message is to make SURE you use the correct gear lube for your type unit