The Stovebolt Site Forums The Shop Area Driveline Changing Rear Gears 4:11 to 3:55

I have a 1952 Chevy 3100 (216 engine, SM420 trans, torque tube in place, original rear axle & 6-bolt though with 15" wheels, original 4:11 gearing)-owned since 1973. It has been resting in my garage for 18 years-ish, and now I am returning it to service. Primary goal is to get to safe and to reliable for fun cruising, light hauling, and readied to pass on. I would like to changing the gearing in the rear, as I don't need torque, as much as a need a bit more speed. I have read most of the topics here regarding this conversion, but hoping to keep it simple, perhaps something new going on, or summarize for this old brain of mine-also made in 1952?
I desire at this time to: keep all the equipment/parts on the truck as I can, though I hope to find a later 50s 235. What is the least invasive, least amount of changes and replacements etc to convert to 3:55 ratio? Its not the $$, and I will use a local (if available) experienced mechanic to do the work or help me in the job.
Thank you, I look forward to your replies. Brad

I did the switch and I am looking forward to driving on the highway.The switch was a bit difficult for me as my truck being a Canadian truck has differences in the rear end ,but a few extra steps and the gears fit ,I also got the bearing kit and replaced all the bearings as well as a oakie bushing .There is a lot of imfo out there with 1 Hagarty has a article with pictures showing the swap. 2 devestechnet.com has a article with pictures .There are also lots of threads on this site with issues that other stovebolt members had doing the swap and tips.CurtB if you look up under members in here ,click on his profile and look for his ytube in red letters will show his truck going down the highway with the 355 gears .His truck is also a Canadian truck and his thread on the swap helped me a lot .Kevin

Also adding that member Sweet has a tec tip and was helpful in my changeover .

All depends on terrain you drive in. I've had several different rear axle ratios / transmissions in my '60 C10 over the years and the 4.10 axle w/ T5 transmissions is great here in the PNW where 6% or better grades are common.

You might consider putting this change on the back burner/bottom of list until the truck is otherwise completed and you have driven it for a bit, especially since it is a change that can be completely done without issue on a finished vehicle. This recommendation is not based on $ but on the fact that 20 years on you may view the drivability from a new perspective?
There are rear ratio calculators on line that take tire size trans ratio etc and show mph. They were very accurate when I changed my rear end, I have a T5 and 235 in my 46, went to a to a 4.11 (5th is overdrive) and glad I did all the calcs. Am pleased with the result. My cruising comfort zone is 65mph at most, usually 55-60 is just fine, but to each his own.

Chuck

Thank you for sharing. Brad

The 216 might be a little short on torque to handle a 3.55 gear ratio. Another option might be to switch to an open driveline setup by swapping the rear bearing retainer and output flange of the SM 420 and going to the 3.90 rear end used in the later model stovebolt pickups. With an open driveline, you would also have several choices of newer 6 bolt lug pattern rear ends with a variety of ratios to choose from, while still being able to run original looking 6 bolt wheels.
Jerry

If you go to an open drive line axle you need to provide some way to limit axle windup as the original axle is mounted to the springs with bushings such that the axle can change angle relative to the spring.

You have to weld on spring pads and use u-bolts to attach it to the axle.

I see online "conversion ring and pinion sets" and also the conversion kit with bearings, gaskets, shims,etc. The literature says its just a straight swap conversion, hinting no other modifications needed on driveline, axle, wheels, carrier. That said I realize it is "tricky" to get it all in there correctly to run well.

My question is...Is it truly that simple of a "swap" without changes made or needed on other parts? I just trying to wrap my around this project, before considering options. After reading many threads here and elsewhere, please set me straight. Thank you.

Before doing my 3:55 swap, I read the info in Tech Tips LINK and the information on Deve's TechNet LINK. Also, my kit came with some instructions. All were very helpful and gave me confidence to do it. I have never swapped gears before, but have watched lots of YouTube videos.

Thank you for the link, and the encouragement. Brad

Thank you, all again. 1Ton Tommy and Phak1, I did not know about that concern and fix, thx.

I did this conversion following Deves instructions that were very helpful. It was a bit of a challenge but I did have some previous gear experience. I’ve driven it about 300 miles this year since the swap and I love the difference it made. I haven’t checked rpm but I definitely feel more comfortable with it running at higher speeds. With the speedometer showing 50 mph, I am actually doing 63 mph.

That was fast. I took forever to do mine, but I was scared and learning.

My speedometer is off too - that is, when it's working. I've read that you can get a compensator. You drive at 50mph and let them know what your speedometer reads and they can calibrate something that your speedometer cable connects to. I can't remember where I saw this, but will look for it when all is working with my truck.

I did the same as what John is talking about, but was able to work with a local Speedo shop who supplied me with a speedo adapter. Then just a few years ago I changed my tire size and had to do this again but this time having a cel phone GPS ap such as SPEEDBOX or similar made it easier yet, telling me how fast the truck was actually going versus what the truck speedo was indicating. Now and then I periodically check the truck's speed reading using SPEEDBOX as a comparison. Both are talking same language (at least close enough for my likings.)

Just went thru my truck folder and found that I used PATC of Bossier City, LA to supply my latest adapter back in July of 2014. Sorry to say, no longer in business.

About 3 years or so ago I converted my speedometer to a GPS unit and have really enjoyed it. Quiet, spot on accurate (with any rear end or tire size) and the way I did it, the unit looks very close to original. The main difference is you can not get an electric odometer that looks like the analog type. Or at least I haven't seen one. If you search you can see what I did. Actually I'm thinking about making a few of these to sell this winter. They would be a drop-in unit...meaning you would remove your old speedometer unit, install this one and connect the wires.

Found it. Here's what mine looks like (I added a couple of leds for turn signal indicators):
https://www.stovebolt.com/ubbthreads/ubbthreads.php/topics/1549469/1

I did this swap quite a few years ago on my 1953 3100, 261 engine, 420 trans with torque tube. I had the local machine shop do the actual swap, which he said was fairly easy. My top speed increased to 87 mph, which I did only once.