Rear end 1990P-30 Diesel

pilacs · 02-22-2010, 01:27 PM

Hi,
I'm new in this forum just recently bought a 1990 Bluebird P-30 with a 6.2 diesel in it.
My Q is how can I identify the rear end on my bus? (Dually)
I'd like to swap my rear end to one with higher gearing in it,but can not figur it out what kind I have.
Any info appriciated.
Thanks.

the_experience03 · 02-22-2010, 02:38 PM

I have reason to believe this is the axle you have. It would be a 14 bolt GM.

The only other option I suppose it would be would be a Dana 70

pilacs · 02-23-2010, 05:16 PM

Hi,
Today was a good day to investigate under my bus.
Here is some pics I took.
Please help me identify my set up.

pilacs · 02-23-2010, 05:21 PM

Here are some more pic's.
What is that brake drum look alike on the front of the drive shaft?
Thank you.

the_experience03 · 02-23-2010, 06:19 PM

Holy surprisingly large axle! That's cool. As you may have detected from the tag it's a Rockwell model 103 axle with 5.13 gears.

It's actually an axle you see some of the offroad guys play with. It has a monsterous 12 inch ring gear (monsterous relatively speaking anyway) and 32 spline axleshafts. Near as I can tell by looking at my books there are Dana hubs that fit it so you could theoretically change the lug pattern to the same as a 1-ton truck or even a half ton with aftermarket stuff...not that that really matters. It looks like you might have disc brakes too. If you do I believe the calipers are interchangeable with a Dana 60 front...again...cool. I just can't fully break down the number you have to say for sure, but I can tell you that if there is truth to that the parts are available just about anywhere.

Swapping out that third member to something more highway friendly should be easy enough. I see parts listed down to 4.56 but I can't promise it will work for sure, again because of my numbers, but I don't see why not.

That big brake drum looking thing on the back of the transmission is just that...a brake drum. That's your parking brake because many trucks of that size did not have a "Park" function in the transmission and risk sticking the pawl if it's used on a hill anyway.

Do you have a plate in there somewhere telling you what the GVWR and GAWR's are? I'm a little curious now. I bet the GVWR is at least 16,000 lbs.

pilacs · 02-23-2010, 07:17 PM

Hi,
Thanks for the great info.
Since I'm new to this game what sould I look for if I go to look around a Junkyard?
Sould I get a new rear axle or swap gears in mine?
Can you just explain it as plain as possible what to get or what is going to be the easyest swap to gain more highway speed?
Since I'm in Florida I'm looking for at least 65-70 cruising speed.
Thank you again for your help.
Here is another pic I took from inside

the_experience03 · 02-23-2010, 10:06 PM

Steps to change gear ratio in your bus.

1. Find proper third member from salvage yard.
2. Chock wheels
3. Put bus in neutral
4. Drain gear lube from rear end.
5. Remove the 5 bolts from the center of each hub (at the wheels).
6. Remove each axleshaft by sliding it out at the wheel. You might need a slidehammer to break it loose after pulling the bolts out.
7. Remove the driveshaft from the differential (4 bolts)
8. Remove all the bolts in the circular pattern around the third member under the bus
9. Take the old third out, replace with new one
10. Slide axleshafts back in and bolt them in place. Expect to have to turn pinion on diff to get them lined up.
11. Reconnect the driveshaft. Expect to have to turn the whole shaft to get it lined up.
12. Refill with gear lube.

Seriously, it's that easy. I think 12 steps was really stretching it out as you don't even have to jack the bus up to do the job.

jkindt · 02-23-2010, 10:12 PM

Quote:

Originally Posted by the_experience03

Steps to change gear ratio in your bus...

Don't forget the step where you change the gear in your transmission that drives the speedometer so that it matches the new differential ratio.

pilacs · 02-24-2010, 12:09 PM

Hi,
First of all thanks for the great and very useful info.
Sorry for the lack of my knowledge,but I have more Q?
I have find some third members in different sizes and ratio.
if I go 4.56 how much different is gonna make about crusing speed? (10-20% less rpm?)
Would be a good idea to go to a off road shop and ask if someone want to trade mine to one with lower gears?
Can I go lower than 4.56?
If I go to a junk yard what model or make vans-trucks I have to look for?
All the 12" Rockwells are the same? (Bolt on)
Thanks again for all the great info.

Will · 06-26-2010, 11:08 PM

I thought that was going to have the Dana 70 HD with 10 lugs and those huge axle tubes. I'm really surprised to see that Rockwell thing in there. You just have to start calling yards and asking about C103 Rockwells with 4.56 gears.

There is a sticker on your air filter housing. I'm guessing that is a 155hp 6.2. It needs to rev to make that power. If you don't have hills, maybe a gear swap will be okay. You will lose 11% of your torque in 3rd gear and lower the rpms by 11%. or raise the speed you wcan comfortably tolerate by 11%. This is not a vehicle designed for the highway. No schoolbus is. I would find a turbocharger kit and then lower the axle gearing by even more.

I'm guess you can go 65 (3,600rpm) with the pedal to the floor and 55 (3,000) is uncomfortable with the amount of engine noise. 4.56s will lower the rpms at those speeds to 3,100 and 2,600 respectively. With a turbo acting as a muffler (it's a great muddler) 2,400rpm is sustainable living with these. 3.73s would give you that at 60mph. But that is giving away 35-40% of your torque and I don't know if that engine would pull it. Second gear with 3.73s turns those 3.73s into like 6.25s which is worse than keeping the 5.13 if the truck won't maintain speed. A turbo kit first, than a gear swap. I paid $900 for my Banks kit on EBay. Very worth it.

If you keep the bus, and they don't have 3.73s, you might consider a Gear Vendors overdrive-splitter. 4.56s, which I know you can get, and the GV ratio puts you right there. And you don't need a transmission downshift for a hill because you can tap it down about half a gear and not lose speed on a hill. I love my splitter and its really close ratios. I wish it were strong enough for a big bus.

Jarlaxle · 11-23-2010, 08:34 PM

Very important question: what transmission do you have? 1990 could be a TH475 or the 4L80E overdrive. Without OD...realistically, you are looking at cruising at 55MPH, tops, even after a swap to 4.56 gears.

Personally, I'd keep the 5.13's and replace the TH475 with a built TH-700 overdrive automatic.

pilacs · 02-02-2011, 07:28 PM

Hi,
Thanks for your suggetion.
I have a question on the tranny swap to a different one.
If I can locate a good TH-475or a TH-700 how hard is it gonna be to get it fit to the motor?
Also it is the same lenght so it can be just replaced with one?
There is adapters for it if needed?
Sorry for all the Q?'s,but I'm new the the whole schooli world and would love to put my bus back to the road for the spring-summer.
Thanks in advance.

Allenf · 05-28-2016, 10:12 PM

Quote:

Originally Posted by the_experience03

Steps to change gear ratio in your bus.

1. Find proper third member from salvage yard.
2. Chock wheels
3. Put bus in neutral
4. Drain gear lube from rear end.
5. Remove the 5 bolts from the center of each hub (at the wheels).
6. Remove each axleshaft by sliding it out at the wheel. You might need a slidehammer to break it loose after pulling the bolts out.
7. Remove the driveshaft from the differential (4 bolts)
8. Remove all the bolts in the circular pattern around the third member under the bus
9. Take the old third out, replace with new one
10. Slide axleshafts back in and bolt them in place. Expect to have to turn pinion on diff to get them lined up.
11. Reconnect the driveshaft. Expect to have to turn the whole shaft to get it lined up.
12. Refill with gear lube.

Seriously, it's that easy. I think 12 steps was really stretching it out as you don't even have to jack the bus up to do the job.

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