VCCA Home Forums TECHNICAL - 1945 AND EARLIER 1937-1945 Differential gears on 1940 Special DeLuxe

I was checking to confirm if I have a 4:11 rear end on my 1940 4-door. I check the cerial no. on the housing which appears to be 593021 then a big 5. There are no letters. Below the numbers are GM M letters. The grandson of the original owner was thinking his grandpa may have changed the differential.
Thanks for your help guys!

The number you are looking for is stamped into the center carrier, right side, above the senter horizontal rib, close to where the carrier and torque tube meet. This is not a raised pad but STAMPED into the rough casting.
For 1940 it could be 2258238 or B-70492 for the 4.11.
The 3.73 would have CB-70492.
Or could be B-70492 for a 4.11 or CB-70492 for a 3.73
The reason for all the numbers is because some differentials were made in in Detroit and some in Tonawanda.
The 1940 torque tube and carrier assembly will fit into a 1940 only due to length. The only other was to be certain of the ratio would be to remove the diff. cover and count the teeth on the ring gear. ,,,as just the ring gear and pinion could have been changed. The 3.73 ring gear will have 41 teeth nad the 4.11 had 37.
The 3.73 was a factory option for the Special Deluxe but I am sure very few were ordered that way.

Gene

Can you supply the same info for a 1937, or weren't they stamped?

The 1937-1938 and 1939 will be in the next G&D....The numbers were stamped the same as the 1940 and would have no prefix letters or a perfix letter of B for the 4.22 and The 3.73 would have a prefix letter of CB or C.
The letters/numbers are stamped into the right side of the center carrier,above the center horizontal rib, ....slightly below and about $' to the front of the right upper set screw/lock nut for the pinion bearing.

I wouldn't assume the numbers stamped on the housing are an accurate indication of the gears installed now. The different available ratios would fit the same housing and a lot of years & owners have passed since the factory build.

You can check to see what the present ratio by jacking up the rear tires and pulling back the U-joint housing, turning the wheels while in neutral and counting the revs of the driveshaft.

You could also remove the pumpkin and count the number of teeth on the R&P.

Perhaps the easiest way to tell would be to install a temporary tachometer (has to be a 6-volt unit) and check engine RPMs at a given speed. With 6.00x16 tires and a road spped of 50 mph in 3rd gear, a 4.22:1 ratio will have an approximate engine speed of 2621 RPMs and a 2.78:1 ratio will have an engine speed of 2314. Of course, this method would require that the proper speedometer drive gear matching the installed R&P was also installed. Check road speed for speedometer / odometer accuracy by driving a measured mile.

Looking for advice on differential ratios form my 1940 w/stock 216. I've not driven it as it is largely apart. My understanding is that with the standard rear, I'm assuming 4.11, but will verify with above helpful info, that a typical cruising speed without pushing the engine would be about 50 mph. Does this sound right? If so, any input on the use of a 3.55:1 set of gears (sold by both Chev's of the 40s and The Filling Station). Any concerns with around town driving or highway driving with 85 hp? Concerns with strain on clutch, other driveline components? Thanks for any thoughts. ragtop40

Commenting to watch the discussion and also offer if you're north on 95 at the Delaware state line you can try my 41 SD that's all stock so you can see what 50mph is like. I don't think 50 is too bad....60 isn't even too bad. Much more than that and it feels like she is working. How much/far are you planning on driving?

Scott, thanks for reaching out. Good question about what my usage will be like and my wife and I look forward to doing some cruising, mostly off the highways....but we live in Fairfax County Virginia so we will need to travel interstates to some degree....95, 66, 81 and our infamous beltway, all of which can be racetracks. But your experience does sound like what I could expect....I'm thinking your '41 is also the 85 hp version 215 with the stock 4.11 is that right? Actually, we aren't that far apart time wise....95 north from here being part of the interstate racetrack! We have passed your way often as my wife is from NJ. Have you heard of discussion by others, pro or con, on use of taller gears? Cheers.

The 3,55 will reduce the power mostly on hills or in the mountains. Around town you will be using 2nd gear more when speed drops below 20 MPH. The ideal ratio is a 3.73 which was used in the leaf front spring models from 1937-1940....an option from 1941-1949.
The 3.73 will reduce engine speed 10 percent and the 3.55 15 percent. That means adding 5 MPH at 50 miles per hour and 7 MPH with the 3.55.
With the 4.11 55 to 60 is a safe engine speed if the engne is in good condition.

Hi Gene, do you happen to know what gear ratio was in a 35 Master and what speed is safe for the engine? Mine seems to be fine at 50-55 but have not checked the speedo calibration. Sure do appreciate your knowledge and willingness to share with us newbies! :)
--John

Gene, thanks very much for the above. I had no idea that Chevy used a 3.73 in some of the '40 models, plus available for another nine years as an option, and it does seem use of these gears might be a good compromise in lieu of the 3:55s. They would make for a bit easier highway cruising, while not being quite as fussy for around town driving. Also, since they were mated with the 84 hp 215 version by the factory, and I assume the same clutch and torque tube parts, there should be no major concerns with proper compatibility with those components, I wouldn't think. On the other hand, using second gear at 20 mph or so wouldn't seem to be an issue. I looked by through my parts books (Chev's of the 40s and the Filling Station) but the only have the 3.55:1 on offer. Would you know of other vendors/suppliers I could check with for 3,73 gears? Thanks

John,
Your 1935 had a 4.11 ratio and with the 17 inch wheels 60 MPH is 3000 engine RPM'S....And 55 is a safe long distance crusing speed providing the rods and mains are not loose. Will not hurt anything to hit 60 and 65 for short distances.
With my '34 I found 57-58 a comfortable crusing speed how ever did run at 60 a bit.. I had aluminum pistons, the lighter pistons are easier on the rod bearings.

I haven't researched changing gears yet. I got my car from my family already restored just last year. I've spent my spare time working on reliability (as my car sat unused for the past decade).
I've made a few highway runs along 95 in Delaware (only a about 20 miles as you know)...otherwise mostly local roads. Again...you're welcome to take it for a spin on one of your trips thru

The best way to find the 3.73 gears is from a car being converted to a street rod. They usually go to an open drive shaft, etc. You would want the torque tube and center carrier with gears. The 3.73 is not reproduced the the 3.55 is usually used when a larger 235 or 261 engine is installed.

Scott....thanks very much and I'll keep you kind offer in mind as I would surely learn from the experience, not to mention enjoy seeing you car.

Gene....thanks....just to clarify....if I could find a 3.73, I would need to change out my torque tube as well? The gears would not swap out in my diff with my stock 4.11? That would be an advantage to the 3:55 gears available in the catalogs.

No, the 3.73 gears alone would fit into your differential. It is just that it is next to impossible to find the gears only and more common to find a used assembly.

40ragtop,


I completed this conversion from 4.11 factory gears to 3.55 on my KA2102 in 2017 and have driven +/- 6,000 miles since that time. The conversion was good for how I use this car. I have no regrets.

If you want 3.73 it would be much easier and cheaper to swap in the whole torque tube and pumpkin. You could do just the ring and pinion, if you could find 3.73 (not likely IMHO but you never know...). You would probably have to buy the whole thing anyway as leftovers from some street rod. Building a differential from just a ring and pinion always costs a lot of money, sliding in a good pumpkin, maybe not so much. I believe 3.73 came in the cheaper cars (Master 85?).

Check this link and maybe this is still available: https://vccachat.org/ubbthreads.php/topics/423523/re-1937-216-engine-torque-tube.html#Post423523 Actually just realized this may not work in a 1940??

This is often what you will find as people don't want to take these apart. I bought one and the seller was kind enough to sawzall the torque tube and drive shaft a few inches away from the pinion so he could just ship the whole pumpkin and I can take it apart.

Thanks to all for the thoughts/input. Got me thinking of the search challenges in finding a 3.73 option, and good to know that they will swap into the stock diff without torque tube issue.



32 confederation....if I could ask a bit more about the driving experience with the 3.55 gears (I've have always thought highly Canadian judgement after first experiencing a Molson Ale, a fine adult beverage if ever there was one). Sorry that I'm unfamiliar with KA 2102....but could you advise your vehicle's engine type/hp, highway cruising speed you feel is a comfortable fit for your engine, and how the driving experience around town changed from your 4:11 experience? And seriously, thanks for you insights. Bob

40 ragtop, here's some observations! I drove a '51 216 with the 4:11 rear end back in the late '50's and early 60's. I found that to keep up with the new cars of that era the '51 was labouring - above 55 mph it was noisy. When I took over my uncle's '51 in the mid 60's I later switched out the 4:11 for a 3:55 from a '51 PowerGlide - the local garage did it for me with no problems. You first notice the difference pulling away from a stop - you'll probably stall it a few times until you get used to handling the clutch . Once you're on the open highway it's sheer ecstasy being able to sit on 60 or 65 and not hearing the engine screaming so loudly. Then comes a steep hill!! Where you used to top it no trouble, you'll likely find the speed drops off much more than before, or you finish the climb in second gear. Would I do the change again? Definitely!! I'd be looking at a 3:73 perhaps - would like to drive one so equipped to compare with a 3:55. Hope this helps!!

A very good evaluation. About what I ovserved with my 1939 going from 4.22 to 3.73. Made 65 + on the interstates. Also gas mileage at speeds over 60 MPH increaed from 15 MPG to 17 MPG.

"KA2102" is just a quick way to type out "1940 Special Deluxe Town Sedan".

This particular car has its original 216 c.i., 85 HP engine and transmission (with a working vacuum assist shift) and was purchased pretty much stock. We drove the car about 1,500 miles with the 4.11 stock gears to get a feel for the drive characteristics of the car from the factory (very good - if I may add - thank you Back Roads).

For our 4,000 mile tour to Newfoundland and back, the 3.55 were installed and several test drives were completed before departing. The modifications produced predictable results: slower acceleration at low speeds; a fewer more downshifts from 3rd to 2nd gear to keep engine revs from getting too low when travelling below 20 mph or going up steep grades. The cruising speeds at 50-60 mph was much quieter with less wear and tear on all involved. Any speed at or above 20 mph, on a reasonable grade, the car was in high gear and we could stayed there all day if we didn't need to stop (much like driving our '54 210 PG).

If the majority of your touring is planned to be "day" tours within the 30-45 mph range, consider whether the expense of the conversion is right for your intended use.

Cheers,

Thank you Kaygee, Gene and 32confederation, all much appreciated, especially all the info on driving experiences and comments on thinking through intended use, cost/benefit. Very good advice indeed. Digesting all the comments, I plan to look for a 3.73 (14X41) gear option and, for a plan B if my search comes up empty, move forward with the 3.55 …...keeping in mind the expense of it all.


Gene, in your judgement could I use 3.73 gears from any year in the 39-49 range in my '40? Thanks, Bob

No,,,,3.73 ring gear and pinion from 1940 and up.
The torque tube its self is 1940 only but the gears, which can be removed, 1940-1949