The Stovebolt Site Forums The Shop Area Engine Shop SM420 or the T-5..?? What's your choice?

I'm just throwing this question out to the Stove Bolters to get a feel for the general consensus on transmission preferences.. I currently have a SM420.. I'm running it with a 235 motor and a 3:07 R&P.. So far so good.. But I have heard lots of talk about this T-5.. I'm wondering if I should consider it when it comes time to rebuild or replace my current transmission??
I kind of really dig this SM420 I have.. And I'm certainly not complaining about it.. If I could change one thing about it I sometimes wish I had an extra gear on the freeway.. But for now I just drive slower..
My application is light duty and daily driver..

It depends on what you want and/or need. The SM420 is a very sturdy box that was in production for about 30 years and found service in everything from light trucks to busses and military vehicles. It was, and remains, an excellent piece of equipment. But for all practical purposes it's "only" a fully synchro 3-speed with an extra low first gear if you need it.

The T-5, on the other hand, has 5 fully synchro gears including an overdrive if you have a need for expressway driving.

So, if you find yourself wishing you could downshift into first or upshift into overdrive the T-5 seems to be the weapon of choice. Otherwise the SM420 is going to give you excellent service and outlive you and your kids.

I want my truck to be a 1951 Chevy truck that's fun to drive on local roads and around town so I had my SM420 overhauled and will keep it.

Sounds like your interested in putting miles on your truck and I would highly recommend the S10 .72 OD mechanical speedo T5. As Woody pointed out, retaining the SM420 is going to give you a bulletproof box that would be great for local driving. The T5 would expand the versatility of your truck. However, there's more to it than just bolting on the transmission.

If you haven't already upgraded your brake system you'll need to give that SERIOUS consideration, as well as making sure that your front suspension/tires are ready to roll along at 70mph. The 3.07 diff you're currently running would definitely be too tall for the .72 T5 so you'd have to consider the .86 T5 found behind the 4 cylinder S10 engines. Also a good transmission, but it has a 4.03 1st gear, which has you short-shifting almost immediately.

I'm currently running a .72 Mechanical speedo T5 with a 3.42 diff and my cruise is 2000 rpms at 70 mph. Great for longer trips but puts me on the lower end of the torque curve when putzing along at 50-60 mph. A 3.73 would be better for maximizing engine efficiency/mileage. My engine will accelerate from 45 mph in 5th gear with traffic, without a downshift and will easily cruise at 75-80 with rpms to spare.

The modifications to the T5 are modest but you'd be wise to consider the total cost/work involved in upgrading your truck for today's open road speeds. I documented my upgrades in my webshots albums and this might give you some perspective of the process and the end results.

Dave

I have a '62 with an sm420 in it. I have already purchased a t-5, the one with the 3.76 first, and the .72 OD. The OD is not the only consideration. The extra usable gears below 4th help alot. Acceleration is very much improved, and the wieght is reduced as well. I'm sure that shifting will be easier as well. I'm certain that the t-5 will be the single best improvement I'll make to my truck.

If you are running a 3.07 behind a 235 you are not going to have enough hp to do any good with an overdrive. Back in their day 235's were run all day long at 60 mph with 4.11 rears and it didn't hurt anything. You just have to get used to the engine noise. You say when it comes to replacement time. SM420's are so tough that you will probably never wear it out. My only objection to them is that reverse is so slow, but I live with that. My school car in the late 50's was a 50 GMC with a SM420 and I abused it. Everything broke but the transmission. I still have several GMC's with SM420's. I don't even dream about doing anything to them. I am building a GMC with a 302 that is getting a Borg Warner 3 speed overdrive behind it, but its getting 3.90. I am very partial to the BW overdrives. I ran many of them behind Y-block Fords and I still have 3 Y-blocks with od and 3.89. As far as speed goes, when I was a teenager I used to peg out my GMC at 80. My parents always told me not to drive faster than my Guardian Angel could fly. I must have had a pretty fast Guardian Angel.

There are quite a few guys here that run T-5s and 3:08 gears. I shift into OD at 50mph usually and it will pull a modest hill without downshifting. On steeper hills just downshift into 4th. I don't have a tach but the engine sounds happy at 60-65mph and you'll still be able to accelerate on flat ground without shifting.

I myself would prefer the T5, however look into a T5 Hybrid using the Camaro V8 front section and the S-10 rear which includes the shifter. That way you do not have to extend splines, or cut down the pilot or bearing carrier as GM has done that for you already. Another option would be a 3/8" spacer and then cut down the bearing carrier (or use one from the V6 camaro T5)

If you want another gear for the highway, then I think you should consider the T-5.

i have a 62 1/2 ton pickup with a 235 and 3 speed that screamed at 50..i've decided i'm going with the t-5.reading this forum it seems like a very popular upgrade for the bolts. the gearing should be ideal for the 12 bolt 3:73 axle i just put in

Just a point to clarify......lots of guys are running T5's with a 3.07 R&P.....but they're running the .86 T5 with the 4.03 1st gear, most likely from behind a 4 cylinder S10. A .72 T5 from behind a v6 with the 3.76 1st gear will not work as a 5 speed with a 3.08 R&P and a 28-29" tall rear tire. It'll be great through the first four gears but you'll need to be running like blue blazes to maintain speed in 5th gear. You need to determine WHICH T5 you're buying and this is determined by the tag on the drivers side of the transmission or the bar code sticker. The casting numbers won't help one bit. I've outlined a manual way to spin the transmission to determine the ratio in my webshots "shoebox" album. BTW....I have a nice 3.08 diff as a reminder that a .72 and 3.08 won't work together.

In fact, I would go so far as to recommend a 3.73 coupled with a .72 T5 for max efficiency in the 55-65 cruising range. My .72 T5 with 3.42 R&P runs at 2000 rpms at 70mph and my vacuum comes up higher at between 70-80 mph. I would suggest that the key to maximizing engine efficiency is to gear so that your normal cruise speed is at 2100-2300 rpms. That still gives you plenty of throttle for higher speed cruising should the need arise.

Scott hit the nail on the head with the Hybrid T5 with the S10 tailshaft and shift forks. The ratios are much friendlier and you'll be able to stay right in the center of your 235's torque curve all the way through the gears. I'm building one now for a 40 Street Rod. Driving the 235 with a T5 is just plain FUN!!!!!!

Dave

An SM420/SM465 does not compare favorable to the T-5, except if you are stump pulling. I am not trying to be a joker, but for driveability, the SM420, at highway speeds (60-75 mph) demands some high revs. out of the engine.

I have and prefer the SM420, because I am a stocker/vintage/purist (you get the picture), when it comes to my 4-5-6 Chevy trucks, but if you are looking for performance (over the SM420), a small upgrade in trannies, will leap-frog the SM420 and the T-5 is a popular/viable option.

Cool beans.. Lots of good info as usual from all you Bolters.. When I first joined this form I didn't know half as much as I know now about trucks.. I thought to myself, "all these guys on this forum need to get girlfriends or something.. They have to much time on their hands to be all excited about these trucks"..
Since then I have realized that I needed to get rid of my girlfriend and learn more about Chevy/GMC trucks..
I have been a happier man since..

i have the t-5 in my 54/3100/235. love it but sometimes in traffic especially when i get caught in jam i wish i had put in an automatic.

Or get a girlfriend who owns an old truck!