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3rd member options? #2836369
10/23/20 06:13 AM
10/23/20 06:13 AM
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Norway
cudaerik Online content OP
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cudaerik  Online Content OP
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Norway
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.

Re: 3rd member options? [Re: cudaerik] #2836453
10/23/20 11:01 AM
10/23/20 11:01 AM
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Arlington, Texas
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bobby66 Online content
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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. work


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Re: 3rd member options? [Re: cudaerik] #2836457
10/23/20 11:08 AM
10/23/20 11:08 AM
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Oregon
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AndyF Offline
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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.

Re: 3rd member options? [Re: AndyF] #2836831
10/24/20 07:46 AM
10/24/20 07:46 AM
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Trumussia
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Originally Posted by AndyF
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.

Last edited by jcc; 10/24/20 07:47 AM.

Discovering "good trouble" everyday
Re: 3rd member options? [Re: AndyF] #2837318
10/25/20 02:23 PM
10/25/20 02:23 PM
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USA
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rftroy Offline
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Originally Posted by AndyF
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.


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


AAR 4-speed 3.91, Tor-Red;
70 440 6 pack Roadrunner 4-speed 3.54, Plum Crazy;
68 Formula S conv 383 4-speed 3.23, Electric Blue;
69 Barracuda conv Slant 6 OD4 2.94, 71 B5 Blue;
78 Lil' Red Truck, Red;
70 Challenger S/E. 505 6 pack, Passon 5-speed, 3.55, B7 Blue
Re: 3rd member options? [Re: rftroy] #2837385
10/25/20 06:12 PM
10/25/20 06:12 PM
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Rittman Ohio
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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 shruggy No downside at all IMHO
Gus beer


64 Plymouth Savoy
493 Indy EZ's by Nick at Compu-Flow
5-Speed Richmond faceplate Liberty box
Dana 60
Re: 3rd member options? [Re: fourgearsavoy] #2837428
10/25/20 10:44 PM
10/25/20 10:44 PM
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Las Vegas, NV
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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.

Re: 3rd member options? [Re: chargerbr549] #2837472
10/26/20 04:03 AM
10/26/20 04:03 AM
Joined: Jul 2013
Posts: 68
Norway
cudaerik Online content OP
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Thanks a lot for the information. I will give Cass a call.

Re: 3rd member options? [Re: rftroy] #2837515
10/26/20 10:09 AM
10/26/20 10:09 AM
Joined: Mar 2007
Posts: 3,030
West Coast, USA
jbc426 Offline
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Originally Posted by rftroy
Originally Posted by AndyF
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.


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


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.


1970 Plymouth 'Cuda #'s 440-6(block in storage)currently 493" 6 pack, Shaker, 5 speed Passon, 4.10's
1968 Plymouth Barracuda Convertible 408 Magnum EFI with 4 speed automatic overdrive, 3800 stall lock-up converter and 3.91's (closest thing to an automatic 5 speed going)
Re: 3rd member options? [Re: jbc426] #2837549
10/26/20 12:11 PM
10/26/20 12:11 PM
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north of coder
moparx Offline
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are you saying the dino oil locks the gears tighter than synthetic ?
or slower to lock ?
beer

Re: 3rd member options? [Re: moparx] #2837608
10/26/20 02:20 PM
10/26/20 02:20 PM
Joined: Mar 2007
Posts: 3,030
West Coast, USA
jbc426 Offline
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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.


1970 Plymouth 'Cuda #'s 440-6(block in storage)currently 493" 6 pack, Shaker, 5 speed Passon, 4.10's
1968 Plymouth Barracuda Convertible 408 Magnum EFI with 4 speed automatic overdrive, 3800 stall lock-up converter and 3.91's (closest thing to an automatic 5 speed going)
Re: 3rd member options? [Re: jbc426] #2837611
10/26/20 02:24 PM
10/26/20 02:24 PM
Joined: Jan 2003
Posts: 27,796
Oregon
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AndyF Offline
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They recommend conventional gear oil only with the helical units. No synthethic.

Re: 3rd member options? [Re: AndyF] #2837658
10/26/20 04:12 PM
10/26/20 04:12 PM
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Bend,OR USA
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Cab_Burge Offline
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I had a customer who I furnish a new , from Cass up, 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 rant whiney
My message is to make SURE you use the correct gear lube for your type unit scope up


Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)






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