Mercury Cougar Owners banner

1 - 20 of 27 Posts

·
Registered
Joined
·
315 Posts
Discussion Starter #1
Ok i just bought a third member off ebay. Says its a 3.00 ratio 31 spline out of 79 f100. My cat is a 3.00 but I have a feeling its a 28 spline. Most people say only 427s came with 31 splines. My question is if the axle is the reasoning for the spline number would this chunk still work or would it have to be modified? Hopefully I can just pop it in and everything will work. But if not, I'm guessing either I'm gonna have to change the ring and pinion or the axles
 

·
Registered
Joined
·
315 Posts
Discussion Starter #2
Upon further investigation I found that a 79 didn't come with a 9" but a Dana. Or maybe I'm mistaken. Really could use some help
 

·
Registered
Joined
·
11,950 Posts
Couldn't tell you on a 79.....all I can say is do a stare and compare. It should be fairly obvious I would think.......
 

·
Premium Member
Joined
·
13,577 Posts
You cannot use a 31 spline carrier with 28 spline axles. You would have to buy special axles with 31 splines but with small bearings to fit your 28 spline housing. Also, the splines are not part of the ring and pinion, the splines are in the differential side gears of the carrier. Perhaps you could change the side gears but you are getting into a lot more work (i.e. you have to break down the entire carrier once you get it) in order to do so (if it would even work, I am not saying for sure it would).

You need to buy a 28 spline carrier not a 31 spline carrier.

You might be better off repairing the rear you have, might just be a bad bearing, but perhaps time is a factor, I recall you saying something about the car being a daily driver. Could also be an axle bearing which changing the center section is not going to fix!
 

·
Registered
Joined
·
11,950 Posts
SO you are 100% on the spline count..? I don't know that an f100 is always a 31 spline axle....I'd be surprised if they are
 

·
Registered
Joined
·
245 Posts
It won't be a Dana...Danas didn't have a removeable third member. The Ford trucks that were equiped with 9 inch rears mostly had 31 splines and 35 plines, though I do believe that some did have 28 splines. If it does have 31 splines, then you can aquire the axles you need...though they won't be junkyard pieces, you can get them. Both Superior and Currie can get you fixed up. Just let them know what you have, when you get it.
 

·
Registered
Joined
·
748 Posts
I bought a 79 new that did have a 9" rear. I think that some years had both and 79 may have been one of those years.

If your original ring and pinion is bad you'll probably have to use parts from both to make one that will work. It would be easier to buy 31 spline axles but probably more expensive.
 

·
Registered
Joined
·
1,884 Posts
'78-'79 F150s had 31 spline 9" rear axles exclusively, even with 6-cylinder engines. Dana units are typically found on 3/4 ton and larger models, and sometimes front axles on 4WD trucks.
 

·
Registered
Joined
·
7,552 Posts
A picture here would help us determine what you have,...
 

·
Contributing Sr Motorhead
Joined
·
5,470 Posts
'78-'79 F150s had 31 spline 9" rear axles exclusively, even with 6-cylinder engines. Dana units are typically found on 3/4 ton and larger models, and sometimes front axles on 4WD trucks.
Yup. I used to scour the local junkyards for 6-cyl stick trucks. They had either 3.50 or 3.70 31-spline 9" rears. In fact I still have one (3L50) sitting in the storage room as a 'spare'. Can't find them around here any more since Ford quit using the 9" almost 20 years ago!
 

·
Registered
Joined
·
315 Posts
Discussion Starter #13
I have no idea what the issue is with the vibration Ive got, Ive tried numerous things with no luck. New motor mounts, transmission mount, rebuild entire transmission, replaced driveshaft with new balanced one and new u joints. I still have gear oil leaking from the rear end bad enough to where it pools up over night. I bought axle bearings and am wondering if it would be easier and cheaper to replace them before messing with this 3rd member.
 

·
Registered
Joined
·
1,884 Posts
Looks like a typical '70s truck 9" pumpkin. Impossible to tell from the pics, but highly likely to be a 31 spline unit. If you use it, be sure to carefully compare the yoke with the one on your car now. They vary both in length and in width between bearing cups. Hybrid u-joints are available for making up width discrepancies. But if the length is off by more than about 1/4" you'll need to swap the yokes. If you do that, its highly advisable to replace the crush sleeve, in order to set the pre-load properly.
 

·
Registered
Joined
·
1,884 Posts
Quote: I have no idea what the issue is with the vibration Ive got, Ive tried numerous things with no luck. New motor mounts, transmission mount, rebuild entire transmission, replaced driveshaft with new balanced one and new u joints. I still have gear oil leaking from the rear end bad enough to where it pools up over night. I bought axle bearings and am wondering if it would be easier and cheaper to replace them before messing with this 3rd member.


It takes just a few minutes to pull the axles and check the bearings, and the axles have to be removed to replace the third member anyway, so yeah, I'd check those first.

But if oil is pooling under the center of the rear axle, and not at the end(s), that wouldn't indicate a bad axle bearing. More likely to be a bad pinion bearing, which has in turn destroyed the seal.
 

·
Registered
Joined
·
245 Posts
It's most likely a bad pinion bearing. I had one going, but didn't keep ahead of the leak on my daly driver. I was an hour and fourty-five minutes from my house, when it finally let go. It sounded like a dump truck poured a full load of gravel through the trunk. By the way, you're going to to need new axles, if you use that third member.
 

·
Registered
Joined
·
1,884 Posts
I may have found some 31 axle shafts, out of a 70's f100. Would I need to cut these, or would they even work?>
Length will be wrong for starters, so they'll have to be shortened. A lot of 9" axles have a taper outboard of the factory splines though, so there may not be enough material to have them re-splined. If you get past that hurdle, the truck wheel bolt pattern is different, so they'll have to be re-drilled for new axle studs. Depending on the axles, the outer "hub" that the wheel centers on will likely need to be turned down, as well. This all must be done very precisely, or it'll cause a vibration.
Finally, you'll need to find axle bearings that fit the truck shafts and the car housing, since the factory sizes are not the same.
So while it may be possible to make them work, you'll save a great deal of trouble, and probably $$, by finding axles that actually fit.
 

·
Contributing Sr Motorhead
Joined
·
5,470 Posts
Ford truck wheel bolt pattern for most F-100 and F-150 is 5 x 5.5". Your Cougar is 5 x 4.5". If you really want 31-spline shafts for your Cat, you can order some from any number of Mustnag or Cougar used parts vendors, or just buy new ones from Mark Williams, Moser, Strange.... They're not that expensive. Easiest thing to do would be to change the 31-spline side gears for 28s, but then again I can do that in my sleep. I'd do that when I was rebuilding the used posi (if it has one) anyway, since it would likely be worn out...
 
1 - 20 of 27 Posts
Top