The Stovebolt Site Forums The Shop Area Driveline 12 bolt rear end

Hello. I want to ask you guys about installing a 12 bolt rear end off a 1964 Chevy pickup with 3.73 gears, a guy is selling for $200. in my 1947 3100.
My truck has a 235 engine (0434434T255xA) from a 1955.2nd and a 3 speed original floor shift transmission(591195) and original close line drive line with 411 gears.
Will this 12 bolt help my truck reach highway speeds 55-65 mph?
Or am I better off swapping the ring and pinion to a 3.55 Gears?
Thanks guys

That is a really good price! (if the bearings and gears are in good shape).

Have you measured the spring perches? Cutting those off and rewelding will take more $$.

I plan on a truck 12 bolt in the future too.

Just plug in your tire height, rear gear and play with the engine RPM to see how the MPH reacts. (trans ratio will be 1 in 3rd gear, 1:1)

https://spicerparts.com/calculators/transmission-ratio-rpm-calculator

If I remember correctly I had a 72 gmc half ton with a 250 six and 373 rear gears and I could go about 60 mph after that it would start to scream ok for passing but cruising I think 60 was comfortable.

3.73 and 3.55 are close enough, little difference will be noted.

Ed

Going to 3.73 from 4.11 is about a 9% improvement. The swap will be work and money. You'll need a driveshaft and a transmission with open drive design. Shock mounts and parking brake conversions will take time and work. When you're all done, the rpm needed to drive at 55 won't seem much different. 9% is not much. 3.55 however is about a 14% reduction in rpm at 55mph. So in the first example, you might reduce 3000 rpm to 2730. In the second example you might reduce 3000 rpm to 2580. If you're going with a 12 bolt rear (which I've done already) I would suggest looking into a T5 transmission. Then you can get some real flexibility with rpm, etc. and you're going to have to get a different transmission anyway. Good luck.

The best thing you can do if spend some time crunching numbers over at 4lo.com they have some amazing gear and tire size calculators so you can plug in the numbers and seen what rpm you’ll be at with a given gear ratios and tire size. Takes all of the guesswork out of it.


4lo.com

Thanks for the replies guys. I thought, I could get away with just a rear end swap and driveshaft to get better highway speeds. I have a lot to learn here.

I was planning on rebuilding and using the factory 3 speed transmission with the 12 bolt if I buy it. but I’m not sure that will work now?

After reading some post here on Stovebolt, I’m kinda thinking of the t5 swap now. Seems that most of the guys who have done the swap are happy with the outcome.

Is there a specific T5 transmission I should consider (pre 88 with mechanical speedo)?
Would a T5 bolt to a 216 bell housing? I forgot to mention that.

Jon G it looks like you’ve already done a t5 and 12 bolt swap on your 52. How do you like it? How difficult is it to do?

Bomber contact Lugzutz he is our resident expert on T5 swaps.🛠

Also take axlook in the "Tech Tips". There is a bunch of info on T5s in transmission section.

If GM had come out with a T-5 in 1947, there would be no such thing as any other auto manufacturer. They are that good behind a Stovebolt engine.

Hi Bomber,
Ditto on the Lugnutz comment. Great guy and very helpful. The T5 and 12 bolt change worked out great for me. What I did was to figure out which of the T5 gear ratios when combined with my 3.07 rear end would produce as close to the same overall ratios as the AD truck had when it was brand new. After that, it was a matter of making sure my 5th speed was .86. Anything less than .86 with a 3.07 rear wasn't going to do anything but lug the engine. Plus I now have a fully synchronized transmission, 4th gear is a 25% reduction from 4.11 and then 5th is about 2.66.

The bottom line on the 12 bolt swap was this:
you need a driveshaft. I had mine made by a place in San Antonio and it was about $400. If you can find a used one it will be less expensive, but the wrecking yards in DFW have turned into slumdog apartments.
if possible, get the emergency brake cables with your 12 bolt. You can adapt them to work with the AD setup.
you will cut off the mounting pads on the axle. You can use a cutting torch, but be careful not to get one side of the axle tube too hot. Do it in bits.
your work will be easier if you use adjustable (bolt-on) spring perches and then weld them when your pinion angle is set.
the other brakes are simply a matter of making sure you can connect your flex cable. BIG TIME HINT: keep the hookup (tubes and 3 way connector) from your torque tube rear. It can easily be adapted to the 12 bolt.
shocks...you will need a drill press but you can convert your AD shock mounts to work neatly with the 12 bolt and use the same shocks.
brakes...shockingly similar to the 52 and newer AD type.

Back to the T5, get one of the gizmos out there to raise the shift lever up and shorten the throw. They aren't costly and there are two kinds...one is 1 inch thick and the other is 1.35" thick. The guy who makes the 1.25" thick one is a real gentleman. When you get to this point, contact me and I'll give you his info. IF you can't find one with a mechanical speedo output, don't worry. It isn't difficult to make an electronic (or GPS) speedo look very much like the original. Will it bolt to the 216 bell housing? I think the 48 and up truck 216 bell housings used the same bolt pattern as the T5 but let's see what others say. It works fine with the 235 bell housing. You will need to fiddle with the throwout bearing and use a clutch disk that fits with the T5. Not a problem. Plus you get a reverse light switch with the T5. Nice to have.

The 3 speed with the closed driveshaft will only work with a closed driveshaft. Completely different design, but you can rebuild it, install the 3.55 rear end gears and cut 14% off of your highway rpms. Note: your take-off gear ratio (some call it the starting line ratio) will be reduced to 10.4, so you won't be able to haul 1000 pounds of stuff in the bed as handily as with the stock ratio, but not everybody cares about that. If you hark back to my starting comment, it was important to me that mine still operated just like a stock AD pickup, however. Hope this long winded comment helps. Good luck.

I’m not familiar with all of the different bell housing bolts patterns. In short, a T5 can be made to bolt directly to a bell with a Muncie/Saginaw bolt pattern WITHOUT need for an adapter plate. If the bell housing does not have the Muncie/Saginaw bolt pattern, an adapter plate can be purchased.

As previously mentioned, it is important to match the T5 overdrive gear with the rear axle gear and tire size. An S10 or V6 Camaro T5 will have either 0.86, 0.76, or 0.72 as the overdrive gear. Most V8 Camaro T5 transmissions have a 0.63 overdrive gear.

A 1988 or earlier S10 T5 with a mechanical speedometer output is very hard to find these days. Needless to say, that drives the price up quite a bit. 1989 and newer S10 T5 transmissions have a VSS output signal. There is a man on eBay who can convert the VSS style tail housing to a cable style output.

Contact me by email if you have more questions. I can share more pictures that way.
Jay
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