Ford 9 inch 3rd member options?
 

I need a streetable third member for my 9" rear. Currently I've got 31 spline detroit locker with 3.89 gears in a Currie 9+ case, 1330 short/wide yoke. I'd like some high gears for cruising. I need some education on cost effective cases and a more street friendly posi.

How well do the Detroit Truetrac units hold up to hard street use? What case do I need? Is anything stronger than a stock N case OK for 650hp? Who's got the best prices on 3rd members now?

I'm thinking of the cheapest aftermarket case from Quick performance, with the aluminum daytona pinion support, the short/wide 1330, 31 spline truetrac and 3.25 gears. Going in a Moser housing/31 spline forged axle combo in my '67 LeMans.

LMAO But check out Speedmaster?

GT

Lmao what? Speed master says they're for lower HP application with street tires. And they use clutch type posi. I don't want to switch between limit slip or straight fluid when I use the locker at the strip.

I’ll be watching this thread as I also have a Detroit Locker w/3.89 gears and considered going to a 3.50 - the 3.89 is a bit steep with the TKO600 2.87 first gear. From what I’ve seen, QP is as competitive as it gets on pricing.

Quick performance for the win

A few random thoughts

The original Detroit Locker is a Helluva lot better than the newer units like the one's sold with the Yukon name.

Factory N case is strong. Weight of the vehicle, the gear ratio along with the amount of power need to be considered. Around 720 HP at 3850 lbs. with a 3.89 gear we would shear the teeth off the ring gear over time. It never was an issue with the 3:50 or 3:70 gears we used even though we were transmitting more power through those gears. The factory N case flexing under power was causing the issue with the gears. With a lighter car this was never an issue. It wasn't a problem with the 3.50 and 3.70 gears either because the ring gears wear thicker and didn't flex as much.

I should add we braced the housing to solve the ring gear issue and that helped some but definetly didn't solve it. An aftermarket carrier did.

We also always used the OEM Ford front yoke and never had an issue with that.... Regular use of a trans brake may exceed the factory yokes limit in short order.

Well, I broke them all with constant drag racing with a lot less than 650 HP. Even a "guaranteed" Detroit Locker. Eventually broke 31 spline Mark Wiliams axles considered the best by many too.
Finally went to Moser Aluminum thru bolt center section. 40 spline axles and full back brace. I did break the 31 spline axles with back brace though.
My housing does have aluminum caps, wish they were steel but its held up fine.
There is one member who runs the Pro gears on the street and gets away with it. He had someone build him a very custom housing that holds a huge amount of gear oil to deal with the heat. But his game is high level street racing. And he runs a spool.
I would never run one on the street. Almost took myself and the car out.

Some of you guys are very tough on rear ends. I've had the same 9" for 25+ years. Nodular case with Soft Locker, 3.89 gears and 31 spline axles. 600+ hp and 3725 lb car. For years it was raced 2 or 3 times a week always with slicks.

I purchased mine in 1986 and installed it in the early 90's. Still using the original housing that came out of a Cougar Eliminator. Thought it would be a good time to upgrade the center section and axles a few years ago when I stepped up to an aftermarket block engine.

Were you running a transbrake to break all those parts? I've currently got no plans for that, esp not with 3.25 gears. Should I stick with an iron case, or can I run an aluminum 3rd member on the street and have it live just as long? It'd be nice to have the weight savings if it would be reliable. What does "thru bolt" mean? Can you not run the housing studs with an aluminum case?

I run a Detroit trutrack in mine with 350 gears. great on the strip but a bit too revvy on the street.

I've gone with a wavetrac. Currie recommended it for dual purpose street/drag use. I went with 35 spline and back braced housing. I like the lifetime warranty.

Quick question - a used set of Richmond 3.50 gears came with the Lemans when I bought it but they don't have any markings for pinion depth or backlash. Does this make the useless for future use or is there a method to determine how they should be set up?

AFtermarket loves to sell 3.50 and numerically higher. I have 3.00s in my pontiac powered studebaker with 9" ford, they came from a 69 mustang. 2.75 was common in ford big cars with big engines.

I was footbraking when I broke the posi type units. Trans brake took out the axles.
Thru Bolt means its not a stud, 4 long bolts goes all the way through the case for the caps.
This is the case I have.
https://www.moserengineering.com/306...ance-case.item
The stuff I have is good to 1600 HP. You are not going to hurt it on the street.

I have a QP housing with an aluminum thru bolt center section. 33 spline moser axles. Tru Trac carrier with 3.70 gears. Excellent rear

Stert with your old shime and adjust from there.

GT

Is one just looking for a good pattern at that point? Is there a higher risk of failure than when one has the factory suggested pinion depth?

one thing you can do is pattern it before dissasembly take pics of it and check backlash prior as well then replicate best as possible

You want a pattern that looks like a football near the wide part of the root of the tooth.
.011-.012 backlash and you are fine.