Swapping a '3rd member'

[motzingg]

I've got 3.73 ( i think) gears in my bump and i'd like to get 2.70 gears. I've heard/read that the 9 inch carrier unit can get swapped out without having to disassemble it, mess with shimming or all that mumbo-jumbo. Ideally turning it from something that would require removing the axle and a weekend of messing around into a hard saturday afternoon (12 pack) kinda job to affect the changing of gears.

My question is this: am i totally misinformed? how 'swappable' are these and what are the variations i have to look out for. Ideally i'd like to find someone with the exact same truck who has factory 2.70 gears and wants 3.73's and would be willing to just trade 3rd members.

Are the axle shafts going to pop right into this thing? I know there are different axles for 9 inch, but is this more of a custom/aftermarket thing, or did ford change spline/diameter/etc for different years, GVW, etc?

Would this cause any driveshaft length or U Joint problems? I'm swapping in the MR5 pretty soon here as well, so if its something where i'll have to have someone make me a driveshaft, i'd like to get it right the first time.

[1972hiboy]

I can't answer all the wuestions but I know there are different spline counts for the axles and a few variations of third members. To pluck the 3rd member out I recall unbolting the backing plates with all the brakes attached still and sliding the axle out a bit on both sides then removed the driveline and the 3rd member retaining bolts and the unit popped right out.

[motzingg]

well, a bit more digging turns up some good info, not sure why i didn't just do that in the first place.

here is a great site about all the 9" choices:
http://www.kevinstang.com/Ninecase.htm

so the question is: 28 or 31 spline axles in a 68 F100... they say passenger cars had the 28, since i have a 5 bolt hub you'd think that would mean 28 with the smaller bearings?

thats actually good news, the 28 might be easier to find in a tall gear ratio.

all this talk has me wanting a stock locker, probably uncommon to have those in trucks with the tall highway gearing though, right?

[sargentrs]

Yes, you have 28 spline axles in a stock '68 F100. You can swap the whole 3rd member in an afternoon but to just change gears you're better off pulling the 3rd member out and taking it to a drive line shop, unless you're proficient at setting the gears and have the needed equipment.

[motzingg]

ha, i should just start asking you questions direct, you seem to always come up with the fastest, most direct answers, thanks!

i'm thinking maybe i'll try to trade someone or find a different 3rd member at a JY. Seems like that would be the cheap/easy way out, and people who are looking for speed/acceleration always seem to be wanting lower gears. nobody seems to want crazy tall <3 ratio gears.

just walking into a gear shop is going to set me back 3-500 bucks by the time i buy new gears and have them set me up. I could probably figure out how to do it myself if i had to, but i'd rather not. there are gear sets used on ebay that are ~$50, but it seems silly to do all that when some dude across town is having someone take his 2.75 gears out and putting in 3.73's...

so the answer is- yes. i need to find someone who has 2.75 gears with a 28 spline axle. May find one in a 70's era f100, van or large car if i hit the junkyard.

Awesome! Also just found my transmission, $150 bucks from buddy craig, already pulled.

[sargentrs]

Yeah, I've gotta get a life. I spend way too much time on this forum. My truck originally had 3.25 gears in it which locked up a couple of years after I got her. I swapped in a set of 3.89's out of junkyard slick. They really woke her up! However, at 70 mph you couldn't hear yourself think, much less a radio, she was screaming so loud. I took the 3.00 pumpkin out of a donor truck to go back in her. Decided to go ahead and put new bearings and seals in it while I had it out. I'll never do that again. First time I ever tried to set gears and still don't know if I've got it right. But since it was basically free, if I toast the gears on the first burnout I'm not out much. Hopefully it won't take out my PowerTrax No Slip diffy when it does Next time I'll take it somewhere and have it done professionally. Sounds like you have a good plan. Good luck and congrats on the tranny find!

[ultraranger]

You won't find any stock 3.73 gears in a Ford 9-inch rear. That's a ratio common to an 8.8" but not a 9-inch.

Any Ford 9-inch 3rd member (car or truck) will fit any 9-inch housing. The only way a pre-73 Ford rear would have 31-spline axles is if it has a 9-3/8" 3rd member.

The majority of bumpside 9-inch rears will have 28-spline axles & differentials. Most passenger cars (that have a 9-inch rear) will have 28-spline differentials.

Only performance cars like a Boss 302, 351, 429, Torino Cobra Jet, etc. Would have 31-spline axles/differentials.

Not a lot of Ford cars or trucks had locking differentials. Ford stopped putting 9-inch rears in the cars by the end of 1980. The last 9-inch to be put in the trucks was at the end of 1985. Any Ford produced rear ends, axles and 3rd members/differentials have been drying up in the wrecking yards for a good many years now. --not that you can't get any of this stuff any more. It's just much harder to find Traction-Loks and such.

The Ford locking differential through 1968 would be the Ford Limited-Slip. From 1969-1985, it would have been the much improved Traction-Lok differential. From '73-up, most Traction-Lok differentials have 31-spline axle gears.

Common truck ratios are 2.75, 3.00, 3.25, 3.50, 3.70, (pre-'67 commonly had 3.89s), and 4.11s. Some bumpside trucks had 4.57:1 gears.

Some common 9-inch passenger car ratios were 2.47, 2.50, 2.75, and 3.00:1. Some Ford performance cars had 3.25, 3.50, 3.91 and 4.11:1 ratios. The lower passenger car ratios (higher, numerically) are not very common.

Unless you have an engine with LOTS of low-end torque, going to a high ratio like you are talking about, is going to make the truck very sluggish on take off or if trying to pass someone. --if you're going to be racing it at Bonneville where you have several miles to get up to speed, then it might be great.

Otherwise, if the truck is just being used for street/highway use, a lower set of rear gears coupled with an overdrive transmission would be a much better way to go.

[Bvh56]

Motzingg, I'm planing on doing the same thing as you. Im either going to run a 2.75or a 3.0 in my 68. I ran a 2.47 gear in a 77 f150 with a c6 and it had plenty of take off power. Let us know how it turns out.

[motzingg]

cool thanks for the advice.

yeah my bad, my axle is an '09' with 3.70 gears.. not 3.73. not that it really matters.

I put together a spreadsheet to calculate my rpm's and shift points.

well shoot, its going to take a lot of screwing around to get the graph to load, this is the best i can do :
If anyone wants a copy of this spreadsheet, email me and i can send it to you. Its designed to be easily configured for any tire size, tranny, rear end, etc.

this is an image of the spreadsheet using the MR5od trans and the stock gearing- 3.7
https://docs.google.com/file/d/0B2HqNHL ... sp=sharing

This is the MR5od trans using the 2.7 gears
https://docs.google.com/file/d/0B2HqNHL ... sp=sharing

I'm going to be doing a lot of highway driving, some city driving and mild to moderate towing. I'd like to be able to haul my boat (~2000 lbs 17 foot fiberglass I/O) and tow a car trailer, small camper, etc.

The Mazda transmission looks like it will give me a pretty good 3/4/5 split, i can run 3rd gear at 2500-3000 rpms to maintain 55-60 mph under heavy towing, uphill, etc. I can shift 3/4 going from 2000 rpm down to 1500 when accelerating between 45-50 mph, my fourth gear can wind out all the way to 70 if i need it to, and i can use 5th as highway only when i'm cruising.

The setup that i have in my subaru truck, which i really like, is a 4 speed with overdrive, thats basically what i'll end up with here, only 5th will be like a super overdrive so i can lug along at 1500 rpm doing 70 mph.

I'm used to driving heavy trucks, which are universally underpowered compared to cars/light trucks, so i'm not too concerned with speedy acceleration, mostly just looking to break 20 mpg on the highway and be able to hold 70 mph (not necessarily at the same time) for long stretches without revving the crap out of the motor. Also i think that going to the tall gears will make better use of the 2/3 split in the mazda. If you have 3.70 gears you are shifting 2/3 at 20 mph going from 3000 rpm to 1500, with the 2.7's you go out to 30 mph and shift at 2500 going down to 1500. In my usual driving style that would mean with the short gears i'd be skipping eithr 2nd or 3rd in daily driving.

As it is right now with the ford 3 speed and the 3.70's i dont ever use 1st and shift 2-3 at 25 mph. pretty silly to have a whole gear and a half wasted.

[motzingg]

as for having enough power: i'm also still factoring in the 2 year plan involving mounting a TD-04e-16T turbocharger for peak torque at 1800 rpm and peak power around 2800...

if i have to drive it in second gear until that gets sorted out, i guess that will just motivate me to finish it faster!!

[motzingg]

looks like there is some big car swap meet in charlotte NC about 1.5 hrs from me sept 19-22... probably gonna try to source my 3rd member from there.

[trebor12081968]

Hi, little late to the conversation... I have a 1979 Ford f100 with a 9 inch rear end running a 270 gear, I believe is 28 spline. I am wanting to go to 4:11... But AM wanting to pull my third member out and put a complete new third member in with 411 gear. Anybody out there want a 270 gear for a 4 11 gear ....swap third member for third member?

[ultraranger]

Hi, little late to the conversation... I have a 1979 Ford f100 with a 9 inch rear end running a 270 gear, I believe is 28 spline. I am wanting to go to 4:11... But AM wanting to pull my third member out and put a complete new third member in with 411 gear. Anybody out there want a 270 gear for a 4 11 gear ....swap third member for third member?

There weren't any 2.70 factory gears for a 9-inch but there was a 2.75 9-inch factory ratio.