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I am in the planning stages of replacing my 7.20 : 1 single speed rear axle gears with a 2 speed 6.40 :1/8.40 : 1. My truck is a '57 5700 with the GM 15000 lb. axle, another member has a doner truck, a '56 6400 with the GM 15000 lb. axle with the 2 speed pig. I have measured the bolt spacing on both pigs, they are the same distance from each other as far as I can tell with just a tape measure, it is difficult to see if the shape of the mounting flange is the same but they are close. My axle housing has square tube ends, the doner axle has round tube ends, spring center to center specs are the same, center of housing to center of U-joint are the same. Can I swap the pigs or are the housings not compatible, anyone here have experience with this procedure? Thanks for any advice.

I would see if you can just swap the axles outright that's what I am gonna do on my 67 c50 because the wheels are illegal and it's cheaper to just put a new axle with normal rims under

Are you doing this for speed or for a lower low for pulling heavy loads? If for speed, it seems like a lot of work for maybe a 5 mph gain...if for work, it's a big plus!

If you go that route, make sure you get the speedometer cable gear reduction unit off the back of the speedometer. Also don't forget to get the vacuum shifter/cable/valve assembly off the donor truck.

Mike B

If the pig/pumpkin won't interchange, then I will swap the complete axle. I am hoping to stay away from that only due to the fact that my current original axle already has great wheels/tires/brakes/bearings/springs, etc. It would be less work to remove both pigs and swap them, inherently no gear set-up would be needed, no brake swapping/bleeding, no custom U-bolts, no worrying about pinion angle, etc. I'm looking for a little more road speed, it'll hit 50 - 51 mph but that's against the governor and I mainly want to reduce the rpm at 50 - 55 mph and keep from holding up traffic. All of the 2 speed shifter parts are still in place on the doner rig so I will have those. What do you think? have you swapped pigs or know anyone that has? At this point I'm only looking for info on compatibility.

What is your wheel bolt pattern? Other options may be available depending on that!

Ed

Hi Ed, my original and the doner axle/hubs are 6 bolt Budds, 22.5 inch rubber. I want to keep close to the 15000 lb. weight capacity 'cuz I do use the truck to haul stuff, probably will never see the 16000 max again but I don't want to reduce its possibilities too much.

I've never done this with a Corporate Rear, but did do it with a Eaton. The only difference I had in those two was the ring gear was taller on the 2-speed and I needed a small slot ground out of the bolt flange surface so the ring gear would slide in the opening. I confirmed it would fit by drawing a picture showing each mounting bolt/stud and then measuring between every bolt and compared. If they all work out it should fit. I also made 2 long install studs out of concrete anchor bolts found at HD or Lowes with the ell's cut off to guide the pumpkin into the housing. I did it all with the truck sitting on the floor using just a floor jack...the entire job only took an hour or two. You will need to shorten the drive shaft as the 2-speed is longer (yoke to mounting surface) than the single speed.

Mike B

Thanks Mike, I am imagining a similar experience with this swap. Once I get the doner scraped clean of the built-up debris I can get a better measurement between the mounting bolts, mine is pretty clean 'cuz I replaced the pinion seal a few years ago and power washed the axle before hand. For now it looks like it should match, I'm going forward with it and will report back with the results. I think my driveshaft length is fine, same dimension from the center of the U-joint to the center of the axle housing and mine is a three-shaft truck with a slip-joint on the rear shaft. I did what you did, drew a picture of each with bolt spread dimensions, I think they are the same. We will all find out as I progress.

I went from a 7.17 single speed rear to a 6.14/8.36 2-speed and gained about 10 mph. The truck (1957 10500 Fire Engine) would run 63/64 mph all day long at about 3800 rpm...if I pushed it to the governor it would run into the high 60's.

You'll be happy having the donor truck to get all the little pieces from as you swap it over.

Keep us posted with pictures, I'm interested in seeing how it all come together for you!

Mike B

I figured it might get 5 - 10 MPH more, 5 would be fine and allow me to back off the throttle occasionally. The 2 main roads that I have to travel to get anywhere are somewhat heavy traffic, I can miss rush hour stuff, but I don't like to make people mad. One road has very limited passing opportunities but it's only a 50-mph road, the other road is 55 mph but folks want to go 65 - 70.

Measure axle length carefully, my single speed axles were different lengths, two speed I added were both the same!

Ed

OK, thanks Ed, the manual and Heritage site say that mine are equal length, I hope that both trucks are the same but I'll have to figure out if the splines match and the length. That should be a "no-brainer" once I pull all the axle shafts. The doner owner and I are conversing now on the next step, he needs some of the rear suspension parts so I think the doner axle may as well come out of the frame and then work on it in the open.