Best options for regearing (or replacing) a rear end

[ratdude747]

This is another back-burner idea/goal for my 1984 Bullnose.

I'd like to (eventually) regear or replace the existing 8.8 open 3.08 with something slightly deeper, no more than 3.55. Maybe an LSD too...

Rationale being the existing 3.08 doesn't agree with the AOD at highway speeds. Most of the time cruise is practically unusable on all but the flattest roads with no headwind or crosswind. Bogs down in OD.... downshifts, races back up, then have to tap the gas or manaully down and up shift to get back to OD, only for it to fall off again. I know most of my torque is low... but 1500-1900 RPM at speed isn't working so well. Per a look through the gear calculator on the site, it seems anything 3.27-3.55 or so will put me in a better spot (2020rpm at 70mph, vs the 1752rpm it shows me as currently getting)...

I've never done a regear before, but have seen many videos and articles on the subject (gear pattern, backlash, pinion depth, etc.). If I bought the tooling (indicator, setup bearing, and a proper press), I'm sure I could do it.

I've seen used parts on ebay... but am a bit apprehensive on such. I've heard (and seen) horror stories of used/"rebuilt" ebay gearing (transmissions, etc) gone horribly wrong.

Then there's the option of finding another rear end entirely and swap it wholesale. If I went that route, would it be advantageous to keep with the 8.8, or would a 9 be better or just as good?

Just tossing around ideas at this point. There's more stuff higher on the list (like fixing the rotted out and bailing wired muffler and tail exhaust)... but it came up on another forum and got me thinking.

[Gary Lewis]

David/1986F150Six with his '86 300 six and 3.08 gears and the 4-speed OD transmission said it does really well on the highway. Looking at the calculator his would be turning 1714 and yours 1618 at 70 MPH. So it looks to me like either you are geared just a bit too tall/high or maybe the AOD's TV cable is maladjusted?

I don't have any experience at all with the AOD, but just scanned the factory shop manual section on it (Documentation/Driveline/Transmissions/Automatic Transmissions/AOD) and didn't find a symptom listed that describes your problem. But, if it were maladjusted it might come out of OD too early.

From what I read any changes to the idle speed or other adjustments to the carb can affect the TV linkage. Have you had work done on the carb?

But if the AOD's TV linkage is correctly adjusted, then maybe a gearing change is in order. I've often thought that 3.55 gears coupled with an AOD's .67:1 OD would be a good combo. With 30" tires you'd be turning 1865 R's at 70 MPH, and should be able to handle most hills w/o shifting out.

[85lebaront2]

In my opinion the 9" truck rear is stronger than the 8.8", it also has either ball or roller bearings that are pressed on the axle. The 8.8" has roller bearings that run on a hardened axle. If you wipe out a bearing on the 8.8" you may also destroy the axle.

Changing gear sets, on the 8.8" you have an integral carrier, meaning either you work under the truck or remove the whole thing and take it inside. It requires the differential bearings be shimmed for preload and backlash, then you run a pattern and if you did everything right your are good to go. Any pinion position changes require disassembly and reassembly of the pinion and bearings.

9", the hogshead is removable from the axle housing and can be worked on by itself. Pinion preload is set once and the pinion bearing carrier is moved in or out with shims. Differential bearings are adjusted with threaded cups, allowing the preload and backlash to be easily adjusted.

[ratdude747]

David/1986F150Six with his '86 300 six and 3.08 gears and the 4-speed OD transmission said it does really well on the highway. Looking at the calculator his would be turning 1714 and yours 1618 at 70 MPH. So it looks to me like either you are geared just a bit too tall/high or maybe the AOD's TV cable is maladjusted?

I don't have any experience at all with the AOD, but just scanned the factory shop manual section on it (Documentation/Driveline/Transmissions/Automatic Transmissions/AOD) and didn't find a symptom listed that describes your problem. But, if it were maladjusted it might come out of OD too early.

From what I read any changes to the idle speed or other adjustments to the carb can affect the TV linkage. Have you had work done on the carb?

But if the AOD's TV linkage is correctly adjusted, then maybe a gearing change is in order. I've often thought that 3.55 gears coupled with an AOD's .67:1 OD would be a good combo. With 30" tires you'd be turning 1865 R's at 70 MPH, and should be able to handle most hills w/o shifting out.

I've been messing with the TV cable as long as I've owned the truck. This issue is the reason why. Any tighter and it hangs in 3rd gear once "kicked down" (it kinda does that as it is now, come to think of it). Any looser and it never gears down in cruise (just bogs down to the point of cruise cutting out).

Carb's been rebuilt once (before I rebuilt the whole motor), then partially torn down later, (after rebuilding the engine) trying to track down "poor" fuel economy... which wasn't ever fixed, I just gave up and figured what I have is normal.

It can do OK on the freeway... but only if there's plenty of other traffic to draft off of. By itself, it falls on it's face in cruise. Out of cruise, it does OK other than the fact that uphill I'll be getting passed by 18 wheelers.

It's also entirely possible that my engine is an anemic boat anchor compared to what it's supposed to be... although without another truck or past experience I can't say. It's better than it was when I bought it... but at that point it barely ran and had 5/6 upper compression rings busted with trashed bores.

I'll have to see what 1986F150Six is running... stock EEC-IV feedback carb like me?

 

[Gary Lewis]

I've been messing with the TV cable as long as I've owned the truck. This issue is the reason why. Any tighter and it hangs in 3rd gear once "kicked down" (it kinda does that as it is now, come to think of it). Any looser and it never gears down in cruise (just bogs down to the point of cruise cutting out).

Carb's been rebuilt once (before I rebuilt the whole motor), then partially torn down later, (after rebuilding the engine) trying to track down "poor" fuel economy... which wasn't ever fixed, I just gave up and figured what I have is normal.

It can do OK on the freeway... but only if there's plenty of other traffic to draft off of. By itself, it falls on it's face in cruise. Out of cruise, it does OK other than the fact that uphill I'll be getting passed by 18 wheelers.

It's also entirely possible that my engine is an anemic boat anchor compared to what it's supposed to be... although without another truck or past experience I can't say. It's better than it was when I bought it... but at that point it barely ran and had 5/6 upper compression rings busted with trashed bores.

I'll have to see what 1986F150Six is running... stock EEC-IV feedback carb like me?

David's truck has plenty of miles on a stock 300 six w/a carb. However he's running a carb from a 70's truck to get away from the feedback system. But otherwise there's nothing special about it.

[ratdude747]

David's truck has plenty of miles on a stock 300 six w/a carb. However he's running a carb from a 70's truck to get away from the feedback system. But otherwise there's nothing special about it.

Dunno then. I'd rather not piddle away money on a regear when my actual issue was engine performance.

Next weekend I'm driving it 7 hours each way to deliver a couple server racks to a buddy in Missouri. I'll see about getting some tach readings at speed... although I question how accurate the tach is (the speedometer is 5mph slow, I know that for a fact).

[Gary Lewis]

Dunno then. I'd rather not piddle away money on a regear when my actual issue was engine performance.

Next weekend I'm driving it 7 hours each way to deliver a couple server racks to a buddy in Missouri. I'll see about getting some tach readings at speed... although I question how accurate the tach is (the speedometer is 5mph slow, I know that for a fact).

A leak-down test is the only good way I know of to determine how solid an engine is. It will tell you if the rings are worn, if there are leaking valves, or if there are head or block cracks.

If I were in your shoes I'd do a leak-down test and then decide what to do. You could spend hundreds re-gearing only to find that the engine is worn out.

[ratdude747]

A leak-down test is the only good way I know of to determine how solid an engine is. It will tell you if the rings are worn, if there are leaking valves, or if there are head or block cracks.

If I were in your shoes I'd do a leak-down test and then decide what to do. You could spend hundreds re-gearing only to find that the engine is worn out.

I rebuilt the engine last spring (about 5,000 miles ago, remember the stupidly long "lugging under load" thread?). Unlike before the rebuild, there's no blowby to speak of... so I'm doubting a compression issue. By "engine" I meant the fuel/air/ignition system too.

If anything, I'd suspect something wonky with the feedback controls. Issues that weren't resolved and I ran out of diagnostic tooling/ability to troubleshoot (such as it sometimes giving lean codes during KOER tests, but the O2 voltage indicating a rich condition). Come to think of it, I haven't checked for codes in a very long time... with no CEL, it's easy to forget about.

[Gary Lewis]

I rebuilt the engine last spring (about 5,000 miles ago, remember the stupidly long "lugging under load" thread?). Unlike before the rebuild, there's no blowby to speak of... so I'm doubting a compression issue. By "engine" I meant the fuel/air/ignition system too.

If anything, I'd suspect something wonky with the feedback controls. Issues that weren't resolved and I ran out of diagnostic tooling/ability to troubleshoot (such as it sometimes giving lean codes during KOER tests, but the O2 voltage indicating a rich condition). Come to think of it, I haven't checked for codes in a very long time... with no CEL, it's easy to forget about.

I'd forgotten, so thanks for reminding me. Yes, if you are having carburetion or ignition problems that could easily lead to poor enough performance that it can't maintain speed when it should.

[Rembrant]

I've often thought that 3.55 gears coupled with an AOD's .67:1 OD would be a good combo. With 30" tires you'd be turning 1865 R's at 70 MPH, and should be able to handle most hills w/o shifting out.

I thought all (or mostly all) trucks with the AOD trans had 3.55 gears. I know every one I've looked at had 3.55's.

I'm running the M5OD 5spd w/OD and 3.08 gears and I like it, but that's is with fairly low profile tires (27"?). However, I know a lot of people would not like this set-up, so it depends on the user I guess. A 5spd with 3.08 diff was a pretty common combination in the 1988-1996 trucks. I've seen lots of them.