conversion 3 speed to 4 speed

[Biggs68C10]

I'm a newbie to the Chevy Duty forum. I hope to learn a lot from you guys on this forum.

However, prior to me posting my question I’ve used the search link to find my question and found no info on this conversion.

I have a Chevy 68 c10 pickup with 3-speed on the tree converted to the floor. Now I want to install a 4 speed. So, over the weekend I just purchase a 69 Chevy suburban 4-speed tranny. My question is what do I need to make this happen? I notice that the yolk on the 4-speed is larger and it seem that it will not fit my current drive shaft.

Your answer would greatly be appreciated.

Steve

 

[61Apache]

[updated:LAST EDITED ON Sep-25-05 AT 10:49 AM (CST)]No big deal, look in your local yellow pages for a driveline shop, they will almost be certain to have the correct yoke laying around to swap onto your existing drive shaft. I would install the tranny first, and then measure the driveshaft, as you may have to have it shortened by the drive line shop anyway. And just have them install the correct yoke at that time. Good time to get new U joints also.

Next question is, what type of four speed is this? Does it have a top loaded shifter? If it does you will likely have floor pan clearance issues, which you can solve by getting the tranny hump out of the suburban the tranny came from.

The last question, what are you hoping to gain by swapping in the four speed over the three? Keep in mind, unless that fourth speed is an overdrive gear, it will not get you any better fuel economy as the final ratio is still 1:1.
Not trying to be a smart a**, just trying to save you some time and $$ to reach your goal.....

 

[Biggs68C10]

Thanks for the quick response.

The 4-speed has the shifter on top. Per your advice I went by today and pick up the larger hump cover for the 4-speed.


I was hoping by concerting to a 4-speed I can drive it on a daily basis, however I didn't know both of the tranny had the same gear ratio.

Since I already purchase the 4-speed tranny. I guest I will install it anyway for now. After the installation I will look for a driveshaft store to match the new joint to my truck.

Thanks for the info.

Steve

 

[61Apache]

Well, The final drive ratio is the same, 1:1. But isasmuch as the tranny has a top loaded shifter, this indicates to me that it is a SM 420. Which means that the first gear is a "compound low gear" aka "creeper gear", "granny gear" etc. For daily driving, you will never use that gear, you will shift only with 2nd, 3rd, 4th. In effect, giving you the same three speed tranny you already have.

That first gear is great if you are towing something heavy, just to get the load moving, or for pulling stumps, or pulling Fo*ds sideways down the road:7 It is in effect an "underdrive", exactly the opposite of what you were probably going for.

IMHO, you'll be spending $ and time on something that will not do a thing for you. Find a 700R4 automatic overdrive, or a "world class" T-5 manual O.D. The SM420 is a desireable tranny to the rock crawler, 4X4 crowd, and you could likely find a taker on E Bay

Again, I'm making a few assumptions here as to what you are really trying to accomplish, and I apologize if I'm off base. I'm just trying to prevent some disapointment for you down the line. Good luck.

 

[Biggs68C10]

Thanks for the info..

I was planning to just used the truck for daily driving and maybe towing.

Since you had mention the T-5 tranny. I found a web site regarding this tranny at http://www.5speedtransmissions.com/tr-3550.html, this is a nice setup to have. But it cost to much for what I need for now. x( Maybe down the line.

Steve