The Stovebolt Site Forums The Parking Lot Tons 'o Fun 1956 Dually rearend help

Hi Im new to ask a question about a 1956 dually. What type of rearend is it? How to find out and can i hook up a 700r4 to it. What speed can i go with that humongous rearend which i will never want to have to give up. Thanks for your help.

Hy 57DanR, please remember to tell us the size (series designation) of the truck your are asking questions about. There are many truck sizes which have dual rear wheels. If the truck is a one ton (3800) then the rear axle is either an HO52 or HO72. The highest speed gear ratio available for those rear axles is 4.11 to 1, there are many threads on this website about swapping this gear set into your housing if you do not already have that ratio of gears in your rear axle. As to the 700R4 swap, that could be a bit more complicated, the original engine/transmission was supported at the front of the engine and the rear of the engine. Engines with a 700R4 attached were supported on the side of the engine and at the rear of the transmission, so to swap in the 700R4 and do the job with the least strain on the components you should install engine side mounts on your chassis and purchase/ fabricate a transmission support crossmember to support the rear of the transmission. You should be able to find an output yoke for the 700R4 that will take the same "U" joint as your stock driveshaft, then it is just a matter of getting the correct length of driveshaft made up. A word of caution with the 700R4, this transmission requires a very specific throttle pressure adjustment and travel through the range of throttle movement, failure to have this adjustment work correctly will allow the transmission clutches to slip and ruin the transmission in short order, hope that helps

Thank you so much!! With the 4:11 is it a posi/limited slip rearend? Would a 400 transmission work or 350? I would like to keep the rpms down on the freeway here in the twin cities. Will i be able to keep up with traffic @ 65-70 mph? New to this truck. It has a 1974 350 police interceptor engine in it currently. Thanks for your help.

Hy 57DanR, all the transmissions you mentioned will present the same mounting problems as the 700R4 you initially asked about and the 700R4 is the only one with overdrive which will help with fuel mileage. The 4.11 to 1 gear set can be installed in an open carrier or a "posi" type if you can find one, I installed a Detroit Locker in mine, but I don't know if they still make one for that application, try looking on EBay or contact a good rear axle shop in your area to see what might be available if you want to go the "posi" or limited slip route, hope that helps.

Thank you for your help and ideas! I may go with the 700r4. I like the giant pumpkin rearend and if i can keep it and install that protax posi in it, ill have to look into it.👍

Gotta chime in...

Any transmission that is not an overdrive is 1:1 in high gear, be it automatic or manual, so unless you're going 700R4 or a 5 speed manual, you won't be changing your RPM any. Since you just clued us in that it has a '74 350 in it, then does it have the mounts at the front by the timing chain cover, or did they make the mount brackets to use the '74 engine mount location? Original would have the two rear mounts at the bell housing. If you use ANY late model trans, you must go with mount brackets to fit the side mounts for the '74 and make a new cross member to mount the transmission tail shaft housing. For the time and effort you'll spend, you might be better off trying to locate a "Brownie" auxiliary transmission, ( 5831 B) or a Gear Vendors unit. Brownies are a bit hard to find, but they're out there. Expect to spend between 2 and 3K for either one.

As to 70 MPH speeds, just remember that there weren't any interstates back them. Most road conditions prevented you from exceeding 50 - 60 MPH, and that trucks brakes were designed accordingly!

Thanks so much for your reply. It does have front mounts on the 350. I plan on fabricating and welding up a special trans cross member. Thanks for making me think of stopping power!

When I was teaching auto mechanics to a bunch of rowdy high school kids, I made it a habit to insist "We're going to make it stop and steer before we even think about making it go fast!" The one's who didn't like those terms had a hard time getting their cars within 50 feet of the shop door!
Jerry

Thanks for that reply. You are 100% right.☺

Some of the hardcore 4x4 crowd have found you can install a GM 14 bolt carrier into a Eaton Drop center rear axle. (with a little clearance work) They claim it is such a marriage a bit stronger in because of the Eaton's one piece construction design and has a little better ground clearance than a 14 bolt and a little lighter. All Positives in a stump-jumper, but for your application that swap would open a whole wide rage of carrier and gear options to make that big 'ol AD fly