79 9 incher!

[boosted12a]

ok, real general question here. im looking at a 79 f-100 302, 3 spd auto with a 9 inch rear. is it possable that the rear would have close to the same gearing as the 9 inch in my 68 ranger 390 with 3 on the tree? the 79 is in pretty good shape and cheep so im gonna get the disk breakes and the ps to put in my truck, but if i can swap the rears id like to do that too cause mine has about 3/4 inch front to rear slopp on the pinion. any other stuff i should swap with my truck?
jake

[FORDification]

Well, the rearend itself won't fit in your truck because the frame on the '79 is 4 inches wider, so the rearends spring pads and shock mounts are relocated. The third members will interchange between the two, but the axles won't, because of differences in their length and spline count.

[boosted12a]

ok, so ill have to remove both 3rd members, take my diff out of the 68 and put the 68 diff in the 79 3rd member and put the 79 3rd member back in the 68 rear. sound about right? a littl more work, but doable.... back to the original question, would the reer gearing be about the same?

[DuckRyder]

No way to say about gearing without looking at the data plate.

Its possible the gearing is the same, but its also possible that it isn't.

The later trucks frequently (always?) have 31 spline axles, while the early trucks frequently (always?) have 28 spline axels.

Better see what they both have before you yank the chunks.

[boosted12a]

thats why i was gonna keep the diff with the axel and just swap the 3rd memeber

[FORDification]

OK, let's try this again:

1968 differential - short 28-spline axles, housing brackets for narrow frame
1979 differential - long 31-spline axles, housing brackets for wider frame

Because of these differences, the pieces between the two cannot be mixed and matched. The only swap that you can do is put the '79 third member into the '68 housing....but you can't use the '79 axles with it, because they're longer (so they won't fit in the housing) and have a different spline count (so they won't mate up with the third member).

[Docholiday72tx]

Fellows, That 31 spline count isn't always true on the late model trucks. I've seen some 73 to 79 models that came with small blocks or inline 6's that had 28 spline axles. My 73 originaly came with an inline 6 and it had 28 spline axles. I have a couple of parts trucks (75 and 78) that came with small blocks and have 28 spline axles also. My current rear end came from a 75 model that had a big block, and it has the 31 spline axles. So I would highly recomend pulling an axle out of the 79 and looking at it before jumping to any conclusions.
As far as the gear ratio: There is a little metal plate that should be on the drivers side on one of the studs that hold the third member in that will have the gear ratio stamped on it. The plate will be about an inch wide and maybe 3 inches long. It will have 2 rows of numbers. Look at the bottom left side of the plate. This will be your gear ratio, and will also tell you if it is an open rear axle or if it is locking. In that bottom left corner one of my parts trucks says 3.25. On the original rear end for my truck, it said 3.50.
The parts truck is a 3.25 ratio that is open. The original rear end for my truck has a 3.50 ratio that is also open. On the current rear axle that I have in my truck, the bottom left corner reads 3L00. This means it is a 3.00 gear ratio that is locking. Look at the tags first and see if it is a gear ratio that you want to use. If it is, pull out an axle and verify the spline count. If they match up your in luck. If you change out the third member, do not buy a gasket. I bought about 3 of them and always ended up with a leak. Clean the mating surfaces realy well, and squirt a bead of blue silicone around the axle housing and then just bolt in your third member. That's how I did mine, and haven't leaked a drop in over a year and about 15,000 miles. Any other questions, fire away.
Hope this helps. Mike

[FORDification]

Fellows, That 31 spline count isn't always true on the late model trucks. I've seen some 73 to 79 models that came with small blocks or inline 6's that had 28 spline axles. My 73 originaly came with an inline 6 and it had 28 spline axles. I have a couple of parts trucks (75 and 78) that came with small blocks and have 28 spline axles also. My current rear end came from a 75 model that had a big block, and it has the 31 spline axles. So I would highly recomend pulling an axle out of the 79 and looking at it before jumping to any conclusions.... Mike

That's true...heck, you might get lucky.

[DuckRyder]

I'd also add: pull them and count them, don't rely on the appearance of the axel ends, as it is not a reliable indicator on our trucks at least.

[Docholiday72tx]

Duckryder is right, make sure to count the splines one by one. It only takes a few seconds and will save a bunch of frustration.
I would like to add this: If your going to change your 3rd member any way, you may or may not want to go with the same as what you have. You can actualy pick and choose the gears that would work the best with your engine, transmission, and the type of driving that you do which is important especialy with todays fuel prices. For example: when I switched to the 302 and c6. I still had the 3.50 gears. That worked out good because the short stroke doesn't make that much torque. When I went to the 390, The 3.50 gears were eating my lunch on fuel. I switched to 3.00 gears and lost all my bottom end pulling power. I went back down to 3.25 gears and had good pulling power and still had reasonable fuel mileage....about 14 MPG. When I went up to the 460 and C6, fuel mileage once again went in to the toilet. I went back to the set of 3.00 gears and I was getting about 12 mpg. I recently put a new carberator on (edelbrock on....Holley off) and I think I may be back up to 14 MPG. I'm going to fill up one day next week to verify that. If you want some advice on which gear set to use, tell us exactly what you have, and what kind of driving you want to do as far as primarily in town, on the highway, whether you plan on doing a lot of hauling or towing or just joy riding. Bear in mind when you change the rear gears it will change your speedometer. That is simply a matter of changing the little plastic gear on the end of your speedometer cable which takes all of about 5 minutes and maybe 6 bucks.
Mike

[boosted12a]

heres another question. can i remove the pinion without pulling the entire 3rd member? imguessing the crush sleeve or mabe one of the berings are messed up cause like i said it had front to rear play on the pinion, other than that the rear seems to be in good shape, no noise at 55 unless you let off. then you get a realy nast groan froom the pinion shifting, it makes the same noise in reverse, just no as loud

[boosted12a]

its not a roar, it drives quiet, well as far as rear end noise its quiet. its more of a moan, only when you let off the gas, not during a steady speed, but when you let off and let the engine slow you down, or if you downshift to slow down, and when you nail it in reverse, normal backing up its fine, no noise. its a 390, 3 on the tree. im gonna rod it out a little, eventualy lower it but for now i just want it to be reliable so i can drive it oncein a while and take my trash to the dump. my eventual plans for the engine are a cam(not sure which one) edelbrock intake, headders and im gonna redo the exhaust a bit bigger, it has duels, but they are only about mabe 1 7/8 diameter with flowmasters. it already has an edelbrock carb (650 i think)

[willowbilly3]

Once a gearset has worn into sloppy bearings, you can never get them set up "right" although you can get them to function if not too worn out. You just have to compromise on the contact pattern and backlash. You can pull the pinion carrier (I think) and change the bearings, but you have no way of knowing how the ring and pinion are meshing when you do that. You have to pull the punkin to do the proper set up.

[Docholiday72tx]

Boosted, I'm not sure how to fix your problem without actualy pulling out the third member and looking at it.
What I can tell you is this. I run a 390 in my truck for a while with a Holley 600, edelbrock performer intake, headers, dual 2 1/2" pipes, and flowmaster mufflers. I had a new process 4 speed transmission in it. 3.00 gears were too high. 3.50 gears made my truck a gas guzzler. I tried 3.25 gears and they worked great.
Mike

[boosted12a]

bringin it back from the dead! well, after many months i finnaly pulled my diff....to my suprise my only problem was that the pinion nut came loose...since i had it out i had a buddy put new berings in it and reset the lash. after he de greased it he found the pinion bering carier was cracked, since he has a 4x4 shop and a bunch of extra 9" parts he slapped a good used on on. now i just have to get her back together....