The Stovebolt Site Forums The Shop Area Driveline What is the verdict on the 3.55 conversion?

Looking for general input on the 3.55 differential conversion. Given the stock torque tube and using a stock four or three speed transmission was the cost worth the gain?

Yes - I like/notice the 10-15% RPM reduction (from R&P change and larger-diameter radial tires).

An added bonus is that the drivetrain is much quieter - partially due to lower RPMs and due to replacing all driveline-wear parts (using the parts kit sold by Classic Parts and others).

Me too. Best thing I've done to the green turd.

Well worth the cost and effort..............no pay back in $$$. The pay back is in increase comfort and noise reduction.

A no brainer for a 4 speed (with granny first gear). If you use the stock 3 speed, you won't be able to pull stumps any more (maybe little ones), but you will enjoy the lower RPM at high speeds.

Tom,

I did that conversion on my restored '36 Chevy PU and I agree with Tim, Craig and Showkey. Mine was way more work than a simple gear change and is described in this article posted by a VCCA member:

https://vccachat.org/ubbthreads.php/topics/334876/1936-chevy-3-55-rear-gear-conversion.html

My '36 is way more pleasant to drive with the 3.55 rear gears and I would enthusiastically recommend it to anyone. It has the 1932-36 version of the synchronized Chevy & Pontiac 3-speed transmission, a crude device compared to the '37 and up units. The 4-speed of that era is an unsynchronized beast of a trans with a first gear ratio that, as Carl points out, is well suited to pulling stumps and probably also for moving 747s around at the airport. They are cheap, however. In fact guys have told me they can't even give them away.

Well worth the effort and cost.

I am using the stock 1941 3-speed with floor shifter Because the truck is basically an around town grocery getter I decided not to go with the 3.55.

I hear you Tom, 90% of my driving is on secondary roads too, yesterday the weather was good and roads were dry and I went for a drive and got out in the country a ways, time got away and I jumped onto the freeway for a jaunt and the 235 is not happy at 65mph with 4.11 and 29" tires, too much engine speed, it sweet spot is 50-55. So I have gathered up the parts for a 3.55 but haven't started the work yet. I have a sm420 so I can start out in first if I have too and good synchros in the trans too so there is always that factor..

Good reasoning, Tom

Will the 3.55 differential give you a workable, usable 1st gear with the SM420 granny low? In other words, does the SM 420 become a true 4-speed for practical normal driving? It is a pretty $$$erious modification to make if it carries no real benefit.

When I finally finish my '53 truck I want to be able to drive it somewhat normally (if sparingly) and not have to be afraid of Interstates.

Three (+) uses/benefits of the 1st gear in an truck with an SM420:

1. Win the slowest-truck race at an old-truck show (I did).
2. Burn rubber in first gear for 20-40 feet (to display smoking rubber, and/or, tire marks - impressive, to some people).
3. Easily start from stop when carrying/towing a heavy load (towing, for example).
4. Drag a VW bug up/out of a deep ravine (I did), without burning the clutch.
5. Never worry about getting stuck in gear when downshifting (as often happens with a worn/mis-adjusted 3-speed linkage).
6. Impress your friends by driving up the side of a barn (this is just a bad joke).

It does not give you lower RPMs at high speeds, compared to standard 3-speeds both are 1:1 in high gear.

Since 1st. gear is non-synchronized, using it as a traffic gear is pretty much a lost cause, as you have to come to a complete stop to engage it. Then a kid on a 10-speed bicycle will beat you getting away from every stoplight. 4th. gear will produce the same highway speed as the current 3-speed does, as neither transmission has an overdrive ratio. On the upside, given halfway decent preventive maintenance, it will outlast the 3 speed about 4 to 1. Chevy 3 speed transmissions could be made out of wet pasta, and it would be a noticeable improvement in strength.
Jerry

With 3.55, your 4 speed granny gear will still be usable for pulling stumps, and starting from a stop up an incline or out of a hole. After that, the gear ratios in 2, 3, and 4 will be very similar to 1,2, and 3 of the 3 speed transmission.
I would go with the 4 speed/3.55 rear end gears before I'd go with the 3 speed/3.55 rear end gears.
Carl

Sounds like it adds more versatility than actual usage on a day-to-day basis. I will keep this a low-priority item for now...

Mine is in now and it was the best thing that I have done to the 53. Cruising on the freeway now. The 1st gear in the sm420 is still very low, only use it at stop signs on hills.
Mine cost me only the parts, I set it up myself, I wouldn't of paid somebody to do it though.
Steve

I can honestly say the two best upgrades done to my 1953 1/2 ton are the 261 swap for the 216, and the 4-speed swap for the 3-speed. Hands down, the floor shift wins, but the compound low 1st gear is useless for road use. I use 2-3-4 for road driving. I've run mine up to about 55, that's the fastest I want to push this driveline and don't feel the truck would be at all safe above that speed. I too have considered the 3.55 rear end conversion, but seeing that mine is not a daily driver nor am I needing to go 65 mph+ in it, so the stock 4.11's work just fine for me.

It depends entirely on what you want. For many the T5 conversion is a nice alternative but you have to give up the old torque tube and adapt a newer style rear end in there. It isn't all that difficult, actually. Just takes time and time to study which gear ratios you want to end with. If you're careful, you can have nearly the exact same ratios as the stock 3 speed in gears 1,2,3 and then 4th becomes your first overdrive with a 25% reduction in rpm and 5th becomes your second overdrive, useful on relatively flat terrain and with a roughly 14% reduction from your 4th gear. If you have a 261, the T5 should be a natural with it and gears 1-5 are synchronized. I agree going 65 or over in the AD truck isn't wise or safe...although there are some in the audience who will jump up and say "oh I drive mine at 70 or 75 all day long." The truck was not designed to drive at that speed. As Jerry once said "it handles like a hippo on a wet mud bank" or something close to that.

The 3 significant changes that I have made on my 1954 Suburban, in order of value & performance & enjoyable drive-ability are:

2000: 3.55 R&P conversion (and ALL new bearings/bushing/seals - venders sell kits - double your costs and spend the money)

1972: 1960 261 engine (cheapest available engine I could find, when my original 235 died)

1975: 4-speed SM420 rear end (when I gave up on the 3-speed jamming/locking in gear)

No regrets on doing any of them, but, I wish I knew then how to deal with 3-speed shifter-linkage parts and adjustments.

Geez Tim, you are a pioneer, doing all that back in the early 70s, back in 72, I hadn't even discovered girls yet and I'm pushing for 61 now.
Good stuff though, I'm building my 261 now, already an sm420 convert and the 3.55 I put in a few months back.

Steve

You will appreciate and enjoy that 261, Steve

Did you get a full parts kit for the 3.55 conversion?

I found my amplifier-box Suburban to be as quiet as a new vehicle after doing the 3.55 conversion with all new bearings/bushings in the rear end and rear axles. I guess the sound difference might/will be much less in a standard cab?

On the 3.55 conversion, I bought the mother-trucker gear set direct, then found the 4 bearings NOS on fleabay, then sourced the pinion shims from a member here. Bought the pinion bearing spacer tool/and Okie drive shaft bushing/seal from the Filling Station Worked out quite a bit cheaper, that way.

Kids these days deal with road noise by eliminating it by making more noise inside via subwoofers,lol

Tim, you didn't miss anything. As much trouble as I had getting mine to work reliably, I believe that it probably took a few years off the end of my life.
Carl

I can't believe the crazy price that the big vendors are demanding for the bearing & seal installation kit.

Just like Steve did.....You can locate all of the associated parts needed for the 3.55 conversion on your own for a fraction of the price.

Deve has documented the individual part #'s for the installation kit in his article on the 3.55 gear conversion.....

More info. Here

^^^^^^^^^^Plus if the current rear end is in good working order............the bearing can be reused.
Following the DEVE instructions......
Bolt on the new ring gear to the carrier. No need to place the side bearing if they are good.
Slip off the pinion bearings off the old pinion , reuse the bearing. Often a press is not needed.
Set the pinion depth with new shims.
Set the gear backlash with the side bearing adjustment.

Done

You talk about reusing bearings, no disrespect, but when I started disassembling my rear, I found that one of my pinion bearings had lost its cage and was missing one ball bearing on each side of the bearing. I found cage remenants but no sign of the ball bearings, now, everything should of been found between the carrier in the torque tube side because it is trapped, but found nothing, leaves me to believe this bearing was manufactured minus 2 balls and has been running on them since new. My rear end had 36K on it and I thought it was in top order too. I would not stake an expensive rear end rebuild on a used 65 year old pinion bearing, discounting the amount of work, those bearings can be had for 30-60 bucks if purchased right. I just cant wrap my head around cutting that corner.

Here is a parts list - I simply bought the kit from Classic Parts.. Good luck saving time and money on buying those parts otherwise (if you do, please let us know).

Also, if you buy your own parts - buy rear axle bearings and seals. This is a simple procedure, and the cost is not much. You might need to also buy “speeding- sleeves” if there are grooves on your axle. You can now talk at normal voice level as a rear seat passenger in my Suburban. I attribute this to new rear axle bearings.

According to Deve... he did the entire job for $600. { the gears cost him $350...so he was able to source the install kit for around $250 }.
Most of the big vendors are selling the " installation kit " alone for $500.

Quote from Deve's article :
"Besides the actual Gear Set, there will be select Bearings, Seals, Gaskets, etc. that you may need. If you have a local BRC Bearing, or Murdoch Supply, or any company that specializes in Bearings and Seals, you can get them cheaper by eliminating the middle man. All you need is the part number which is supplied here. My estimate on total cost is about $600 for most installations TOTAL including the Gear Set. If there is nothing wrong with your Bearings and Seals, maybe much less."



There is no need to pay for a high $$$ "installation kit" and there is no need to replace parts that are completely serviceable & can be reused.

Instead of paying out the wazu for the complete installation kit.... you are better off waiting until you get everything disassembled & only replace the individual parts that need to be replaced.

.

As mother stated above........it’s best to disassemble and carefully inspect all the parts. Then order whatever is needed. If one or more bearing are damaged .......they all will likely be damaged by the debris.