A few 9" questions
Thread Starter
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Fleet Mechanic
Joined: Feb 2006
Posts: 1,393
Likes: 5
From: MA
A few 9" questions
My truck is a 79 F100 2wd. 460/c6/3.55:1 riding on 30" tires. (Originally a 302/c4/2.75:1 truck).
My questions are as follows
1. Can it be assumed this truck is a 31 spline rear axle truck? (were any ever 28 spline?) I'm looking to buy a rebuilt center section, but, I'm not 100% sure if this is a 31 spline unit.
2. My truck has a God-awful hum/whine on coast/deceleration that seems to go away on acceleration/cruise. I do know the center section has been around, and has an unknown amount of miles. I figure time for a rebuild?
3. I plan on running 27" tires (lowering the truck) eventually. For what the truck is (daily driver/street rod/ may tow once or twice a year a few hundred miles/may see the track once or twice a year), I'd like to keep the RPMs where they are now (seems to be around 2600/2700 at 65 MPH). Performance with the smaller tires should be equal with a 3.25:1 rear gear?
4. Might as well go trac-loc/posi, right?
My questions are as follows
1. Can it be assumed this truck is a 31 spline rear axle truck? (were any ever 28 spline?) I'm looking to buy a rebuilt center section, but, I'm not 100% sure if this is a 31 spline unit.
2. My truck has a God-awful hum/whine on coast/deceleration that seems to go away on acceleration/cruise. I do know the center section has been around, and has an unknown amount of miles. I figure time for a rebuild?
3. I plan on running 27" tires (lowering the truck) eventually. For what the truck is (daily driver/street rod/ may tow once or twice a year a few hundred miles/may see the track once or twice a year), I'd like to keep the RPMs where they are now (seems to be around 2600/2700 at 65 MPH). Performance with the smaller tires should be equal with a 3.25:1 rear gear?
4. Might as well go trac-loc/posi, right?
Elder User
Joined: Feb 2002
Posts: 636
Likes: 0
From: Portage Twp, PA
1) more than likely, yes, it's 31-spline. But for all the harder it is, go pull an axle and count the splines, you lazy punk!
2) assuming it's not a bad u-joint, check your pinion nut. The nut in my '89, for whatever reason, tends to loosen up over time. I tighten it up to 150 or so ft-lbs and go for a while. OTOH, could be the pinion bearing itself...check for wobble on the yoke.
3) general rule of thumb for tire height vs. gear ratio: subtract 0.1 from the ratio for every inch taller a tire is. So in your example, going from a 30" tire to a 27" would effectively get you a 3.85 ratio.
4) silly question
No matter where I go on the internet, Paul is always around SOMEWHERE!
Pat
2) assuming it's not a bad u-joint, check your pinion nut. The nut in my '89, for whatever reason, tends to loosen up over time. I tighten it up to 150 or so ft-lbs and go for a while. OTOH, could be the pinion bearing itself...check for wobble on the yoke.
3) general rule of thumb for tire height vs. gear ratio: subtract 0.1 from the ratio for every inch taller a tire is. So in your example, going from a 30" tire to a 27" would effectively get you a 3.85 ratio.
4) silly question
No matter where I go on the internet, Paul is always around SOMEWHERE!
Pat
Thread Starter
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Fleet Mechanic
Joined: Feb 2006
Posts: 1,393
Likes: 5
From: MA
I know it takes all of 15 minutes of work 
just don't feel like doing things twice/three/seventeen times.
Pat, I think the guy who bought it from my father used it to tow a little boat trailer once.
The vent hose was split at the rear axle tube oozing
I checked to see if the unit was full/had any lube in it tonight..
On level ground, fluid was way above the drain plug (well, puked out more than a drop or two.. light brown.. and very thin..
I bet he submerged it.
Hahaha... you cannot escape me! I didn't know you lingered here too!
just don't feel like doing things twice/three/seventeen times.Pat, I think the guy who bought it from my father used it to tow a little boat trailer once.
The vent hose was split at the rear axle tube oozing
I checked to see if the unit was full/had any lube in it tonight..
On level ground, fluid was way above the drain plug (well, puked out more than a drop or two.. light brown.. and very thin..
I bet he submerged it.
Hahaha... you cannot escape me! I didn't know you lingered here too!
Elder User
Joined: Mar 2007
Posts: 626
Likes: 0
From: Rome, GA
I looked through quite a few truck rear ends to find my 31 spline axles. I thought most were 31s but found out alot had 28s. My 78 had 28s factory. I am about to change my 4.56 gears out for 3.89s the 56s are way to steep to drive on the street with my big block. My tires are 29 inches tall and with the 3.89 it should cruise at 60 miles an hour at about 2800 rpm. www.ringandpinion.com has calculators that will give you rpm according to tire size and gear ration.
