The Stovebolt Site Forums The Shop Area Engine Shop T-5, New Axel and drive line swap

Someone sent me an e-mail asking about the T-5/open driveline/rear differential swap and I wrote them this back. Can anyone think of something important I might have left out?

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I've got a '51 1/2 ton with a 261 that I'm in the process of building. The original 216 in it when I bought it wouldn't crank even though the seller told me it would just need a little tlc to get it going. So, I bought a 235 and when I got it home it turned out to be a 261! When I tore apart the 216 to get the motor mounts out of it it had clearly been rebuilt and probably would have worked with more tlc than I gave it. Oh well!

The deal with tranny's and rear ends is real straight forward for me. The original 4 spd 1st gear is 7.06:1, so its really a 3 speed with a granny gear for pulling stumps. 4th gear is around 1:1 so, your max highway speed is around 50-55 and that is maxing out the engine's rpm range. Since my tranny is old and leaks a little it's time for a rebuild or to replace it. The best option for me would be a 5 speed, though I would prefer an automatic. To use an automatic you have to do something about the engine mounts because the rear engine mounts are on the manual bell housing and if you swap to automatic you lose the bellhousing. The powerglide bellhouse/adapter doesn't have engine mounts on it and the truck isn't set up for side engine mounts. So, I'd have to fabricate something or buy something and just didn't want to get into that at this point.

GM, Ford, AMC and lots of other people used the Borg-Warner t-5 five speed in lots of different applications. The bolt pattern on the front of the Chevy t-5, where it mounts to the bellhousing, is the same as the pattern on the original four speed or three speed. So it bolts to the original bellhousing and you can use the original motor mounts, a very nice feature!

The 'box' of the t-5 trannys are all the same but the guts, tail shaft housings and shifter box locations are different. What you need is a transmission with a tail shaft that has the shifterbox at the front of the tailshaft/back of the 'box'. The transmissions with the shifterbox at the end of the tailshaft causes your shifter to come up in the middle of the bench seat. So, that won't work unless you are putting in two bucket seats and a console. The GM vehicles that came with the shifter box at the front of the tail shaft are the s-10s, astro vans and blazer's (2 wheel drive). The four wheel drive trucks have a transfer case instead of a tail shaft, so unless you are thinking about going to 4wd those won't work.

The t-5's came behind L-4s,v-6, and v-8, and each had different gear ratios and torque handling specs. For example, the l-4 can take less torque and has higher gears to compensate for the relatively low torque and hp of the l-4. Same for the v-6 and v-8's. There are even distinctions within the v-6 and v-8 tranny's. Some are 'close ratio' trannys with a small difference between the gear ratio's in 1'st, 2n'd, 2'rd and 4'th, which is desireable for a Camaro z-28 for example. Some had lower 1'st gear ratios, like 3.35:1, most of the v-6 truck trannies are 4.06 in 1'st. There are some v-6 trannies with 3.5 1st or 3.76 1st gear, which are more desiarable for our applications.

A high low end torque engine doesn't need high gears in 1'st to get it moving quickly, a smaller engine does. To high and you wrap out the RPM's at low speeds and have a limited top speed, too low and you don't get power leaving the light. Generally an inline six is a torquey engine with low top end RPMs.

How do you tell which gears the t-5 you are looking at has? Check the id tag number or green and white sticker on the top of the tranny. All the t-5's start with '1352' the next number are telling. There is a .pdf list of all the t-5's online somewhere. I had it, printed it out, used it and lost it. You should be able to find it doing a t-5 search in google. Print out the list and take it to the boneyard with you. Ignore the 4 speed tranny's. Unless you've got money to buy two trannies, skip the camaros, firebirds, etc... and focus on the s-10, blazer and astrovans. ALso run a search on Stovebolt and read all the threads. You'll probably want a pre-1991 or 1992 tranny because they used mechanical speedometers, just like the one that came in your truck. Unless you've changed your guages or are willing to throw down some greenbacks for an electrical/mechanical adapter, stay away from the electric speedo trannys. Mine came from an '85 S-10 (1352-145) and has 3.76 first and .72 fifth. Second and Third are close to first and eachother, so it'll be nice and smooth in town and with the right rear end, efficient and fast on the freeway.

4th gear in almost all 5 speeds is 1:1. 5th gear in the t-5's varies depending on the application it was built for. Most of the trucks are .8:1 or .76:1, which isn't bad at all. The camaro ones may be as low as .68 to one. A lower 5th gear ratio means you will be turning less RPMs at highway speeds. So, at 75 you can be going 2,300 rpms and cruising with plenty of the powerband ahead of you to pass or accelerate. Too low rpms and you'll have to down shift to get more power, too high rpms and you won't be fuel efficient, so the rear axel ratio is important and should be determined by the engine and 5th gear ratio.

There are two types of t-5's. World Class and Non-World Class. As the names imply, one is better than the other. The World Class has better bearings and a different type of syncros. The Non-World Class is still a great transmission but doesn't have all the latest design advantages, not that you'd really notice once its in and you are driving, unless you are racing... None of the trucks came with WC T-5's. None of the WC t-5s had the shift box at the front of the tailshaft. Want a WC t-5, buy a WC and Non-WC, swap the tail shafts and move the speedo gear to where it needs to be on the tail shaft and then sell the left overs to a tranny house as a core, or keep it as a doorstop. Don't feel like going to the boneyards? Call the big tranny houses in your area and buy one already built with just the gearing and speedo set up you want.

Putting in the transmission isn't a big deal either, you just unbolt the rear cross member on the old tranny and either relocate it or get rid of it alltogether cause without the weight of the torque tube you probably don't need it. Then, as discussed in several posts to Stovebolt, you may have to grind the input shaft of the new tranny to make it just as short as the original one. Same thing with the input shaft housing, which is the sleeve around the input shaft at the front of the tranny. I took of about 3/8" on the input shaft and about 1 inch off the sleeve. MEASURE TWICE, CUT ONCE. Took me about an hour to do those mods 'cause I'm extra cautious. The only other modifcation to the transmission is to widen the mount holes on the tranny to fit the original bolts on the old tranny. Some of the t-5s have smaller holes. Run a drill through the T-5 holes to make 'em the right size and you are on your way. Then use the original flywheel, pressure plate and throw out bearing and use the clutch plate from the t-5 if its the same diameter or slightly smaller than the original one. If not, find the right one using the number of splines on the t-5 clutch/input shaft and the diameter of the old pressure plate.

Once you find the tranny you are going to work with THEN you start looking for a rear differential. The rear differential you'll choose will depend on three things.

First of all, the bolt pattern of the wheels. You'll probably want the front and back to match so you only need one spare. Sticking with 6 bolts? No problem there are 6 bolt differentials out there too.

Then width might come next, though I'd argue rims come next. Width is tricky. Your axel might be 62 inches from wheel mount to wheel mount, but what is the backspacing on the original rims? Is the mount plate right at the middle of the rim (50/50). Do you have a 6 inch wide rim and thus 3 inches of back spacing? Well... then you'll have three inches of wheel and rim on the inside of hte backing plate and 3 inches on the outside. What if your new rim is 8 inches wide with 6 inches of back spacing? That means you'd have six inches on the inside of the mounting surface and a 62" wide axel will be too narrow because the inside of your tires will rub against the frame and inside of the wheel well. For that reason, it is better to find the bolt pattern you want (i.e. to match your front axel OR what you plan to do to the front axels in the future, such as a disc brake conversion or Mustang 2 IFS). Then buy the rims you will actually put on the back of your truck and measure the backspacing. Then measure the original axel and do the math, taking into consideration the back spacing. My original axel is 62 inches, I need a 65" axel due to backspacing on the modern rims and tires I'm using. Don't forget that tires count too. Your rim may have the right backspacing but if the tires extend beyond the rim width it must be considered.

Once you have the width figured out and you know the gear ratios in the transmission you can figure out what gear ratio you need in the rear end so that in all gears you'll have shift points and rpms your engine can handle, taking into consideration it's power band (the rpm range it has the most power at and can handle (usually 2200-4000 with a 3/4 cam), and what you want to use the truck for (commuting, hauling a trailer, strip racing...) I'd guess you'll probably come out somewhere in the 3:43-3:73 range. You decide what gear range by calculating the tire height, transmission ratio, speed you want to travel, and rpm range you'll be comfortable with, which is easy using any number of online 'calculators'. My 235-75R15s are 28 inches high. In the back I went to a 275 60 r-15, which is wider and still 28" high. The height of the tires impacts the effective gear ratio, a smaller tire has a higher numerical gear ratio on the same axel with taller tires. So, 25 inch tires are 3:73 and 28" tires are 3.4x (roughly...)

Now you know the width of the axel, the bolt pattern and the gear ratio (s) you are looking for. Go to the junk yard and start looking under cars. Ideally, in my opinion, you'll find a positraction or limited slip differential that meets the characteristics you are looking for. It doesn't really matter if its set up for coil springs because you'll have to cut off the existing mounts and put on new leaf spring mounts (aka perches) right where you need them anyhow, since most axels won't have the mounts in the right place or the mounts won't have the right pinion angle to match your new transmission. I bought a 4" grinder from Harbor Freight for $15, a pack of cut of wheels and grinding stones and went to work cutting everything off the axel tubes and grinding them down. It took me about 3 hours to do it all, including cutting the 'ears' for the coil spring locator/tracking bars on the pumpkin and painting the whole thing with rattlecan Krylon black.

It is no big deal to bolt the new perches in there, line everything up, mark the axel with the right location and right pinion angle, tack weld the perches in place or mark the axel and then pull it out and weld it or take it somewhere to be welded. Setting the pinion angle is easy too. The idea is that the angle of the tranmission/crank shaft is the same as the angle of the axle. Basically they are paralell, or within a few degrees. This is necessary for the u-joints on the new drive shaft to work right and not wear out too quckly. It's very easy to set the pinion. Measure the angle on the tranny, put the 'inclinometer' ($10 from sears) on the axel and turn it til you get the right angle, mark the axle (or tack weld) and then make it permanent. Don't feel comfortable doing that? Bolt the axel in and drive it to a driveline/spring shop and let them do it for about $50.

Having spent a fair amount of time digging through the boneyards, I can tell you that I found lots of 5 on 5 posi rear ends (5 bolts on a 5" diameter bolt circle) under Caprices. 5 on 5 is a standard ford bolt pattern too, so if you've got (or are going to have) 5 on 5 in the front, consider the the famous ford 9" differential because it is easier to swap gears in than the chevy rear ends. The 9" is also a stronger rear end because it has bolt in axels instead of the c-clip system chevy uses. The 9" also came with 5 on 5 rear discs under lincolns. Another advtage to the 9" is asthetics. Since it has a pumpkin that bolts in from the driveshaft side, from behind it looks like the original axel.

The most common Chevy rear axel you'll run across is the 10 bolt. A true 12 bolt is better but harder to find in a 5 bolt posi configuration. There are faux 12 bolts to watch out for that have 10 bolt ring and pinions inside, generally they are newer and not under chevy vehicles. There are also diffences in the GM 10 bolt rear ends that you should be aware of. They don't all use the same diameter ring gear and some diameters are more expensive to replace/repair than others. Check speedwaymotors.com for replacement ring and pinion sets to figure out what the desirable diameters are and then figure out which year differentials those diameters came in, and skip cars that aren't of those years.

Another thing to keep in mind is the rear brakes, do you want rear drums or rear discs? If you go with drums try to find 12" drums with the widest shoe you can find. 11" drums are nice too. Stay away from anything with less than 11" diameter because it is easy to find 11" and why take less rear braking than you can. If you are looking in the bone yard, mix and match if they'll let you. For example, you found the axel you want with 11"x2 3/4 drums that have grooves in them. Find another set of 11"x2 3/4" and switch them before you pay. Another thing you might keep in mind is the e-brake cable. Measure the one on your truck and look at it. The cab end of it is threaded. GM makes a cable like that but it doesn't come under every car. Find the ones you want and put them on the axel you are going to pay for before you buy it at the boneyard so that you get everything in one fellswoop, rather than just grabbing the axel and getting home to discover the brake drums need to be replaced and you've got to buy e-brake cables. Keep in mind that brand spanking new drums are only about $40/apiece from napa. Wheel cylinders are cheap too, as are springs, shoes, brake hardware and brake lines for newer brakes.

How can you tell if an axel is posi? If both wheels are off the ground spin one, if the other one spins in the same direction its POSSIBLY a posi, or someone has welded the spider gears inside the axel. If they don't both spin in the same direction, it might be a posi but it is probably worn so badly that it'll need lots of money to repair it. If both tires spin in opposite directions, it definately isn't a posi. Another way to tell is if the differential has a tag on it saying 'use only limited slip fluid'. Those are often missing so no tag isn't a guaranty it isn't posi. By the way, there are two types of posi's, conical and clutch. The conical is rarer and harder to repair. The clutch type is much more common and fairly easy to repair, relatively. Some people argue the conical or gear driven version is 'better.' I'm sticking with clutch driven so I can find parts in another 20 years.

Is there anything wrong with a non-posi axel if it has good brakes, the right bolt pattern and the right width? Nope. Just like the brakes though, if you can find a posi for the same money why settle for less. There are lots and lots of posi's under sedans and station wagons and cadillacs. Look at the caprices and trucks, if they've got the right width, brakes and ratio.

How do you figure out the gear ratio and how do gear ratios work? Gear ratios are the number of turns on the drive shaft to get the wheels to turn once. 4.11:1, the engine turns 4.11 times to get the tires around once. Higher numerical ratios are better for higher rpm engines with lower torque. Lower numercial gear ratios are good for highway driving and higher low end torque engines. Just like the tranny, too high and you are wrapping out the engine, too low and you don't have enough power. A 2.08 rear end behind a four cylinder might not even move, a 7.6 rearend (like what comes under dumptrucks) is good for moving slowly with lots of weight. There are two ways to figure out the ratio: First, mark a spot on the inside of one drum and the backing plate. Turn the pinion/driveshaft enough times to make the tire turn ten times. Divide the number of turns on the pinion by 10. (i.e. 34 pinion revolutions to 10 tire revolutions is 3.4x:1). The most sure way to determine the ratio is to pull the cover and count the teeth on the ring gear and pinion gear and divide. What about the ID numbers stamped on the passanger side of the axel tube? You might decode them online, but it'll probably take you longer than if you just do the math. Can't you get the rear end width/brakes you want and change the gears? Changing ring and pinion gears is alot of work and requires special calibration tools, unless you are talking about a ford 9" which has a removable pumpkin. You can buy a pumpkin with the gears you want and swap it into your axel in about an hour, really. Thats why the ford 9" is popular with racers! They can change gear ratios depending on the track/competition/etc... I"m using a 12 bolt posi with 3:73 gears.

Last but not least is the drive shaft. Now that you've got the tranny and differential in place you measure the length from the back of the tranny to the pinion of the axel. Go to a junk yard and get yourself an axel of that length with a transmission yoke that matches the one on the tranny($15 around here). Take it to a driveline shop and get the u-joints replaced to match your pinion and then have it balanced. Bolt it in and you are done. You could save yourself a little work by getting the yoke that came on the transmission, but if you didn't it's no big deal. Also, you could make sizing the drive line a two stage process. Go get a yoke, make sure it fits, measure the distance from yoke to pinion and then find a drive shaft and have it all put together. It is important that when you measure the length of the driveshaft you have all the weight in the truck it'll usually have, ie the engine, front clip, cab and bed. The weight on the spring will affect the height of the front and back of the truck which will impact the distance between the tranny and the axel.

Once all the pieces are in place, don't forget to hook up the hydraulic brake lines, bleed them, hook up the e-brake cable, adjust the brakes and test them before going for a spin.

I havn't measured the driveshaft length on my current project because I'm waiting to finish putting together the engine and mounting it. So, I don't really have any pictures ready. Once I do, I'll send them to you. However, lots of people have done what I described here, like Mongo's Garage, and they have pictures of the t-5 and rear end coversion process.

I spotted a mistake:

Higher numerical rear axel gear ratios (called lower gears for some wierd reason) are better for lower rpms and lower speeds.

Lower numerical rear axel gear rations (called Higher gears, for some strange reason) are good for higher RPM engines, higher speeds and fuel economy, which is why they are called 'highway gears'.

For example... 2.06:1 might be great behind a 572 CID V-8 that can turn 8,000 rpms no problem and is looking to do 150 mph, but ain't gonna work so good with an inline 6 that max's out around 4,000 RPMS and is only marginally safe stability and handling wise at 75-80 MPH.

Conversely, if you want to go 75-80 on the freeway 7:06:1 gears won't work too well for you unless you've got an absurdly numerically low 5th gear ratio, like .3:1.

Forgot to give an example of the relationship between transmission and rear differential.

Say you've got 28" tires, the transmission ratio is 3.76:1 in 1st and 2.18 in second, and the rear end is 4.11:1 and you want to know how fast you'll go before you have to shift gears. If you know your powerband is between 2,220 and 4,000, using algebra or an online calculator ( http://ultimategto.com/art29.htm or http://www.csgnetwork.com/multirpmcalc.html
you'll figure out that:

In first gear when you get to about 20mph you'll be turning almost 4,000 rpms have to shift to second.

With the .72 overdrive at 65 MPH you'll be at 2,300 rpms and just at the beginning of the power band, however at 80 you'll only be up to 2,800 RPMs, have power to spare to get up hills and not be burning too much fuel. If you are anxious to get up to 75mph (or want to downshift and get some power to pass) you can safely stay in fourth (at 1:1) unitl 80 without breaking 4,000 RPMs!

If you had lower gears, say 2.73:1, in 5th gear at 80 mph you'd be at only 1,800 RPMs and have to down shift to pass or get up hills. So, 2.73 is too high of a ratio.

Basically, you have to multiply the rear differential ratio by the final transmission ratio. So, if the transmission is .72 and the Diff is 3.73, it'll take 2.7 revolutions of the engine to get the tires to go around once. With a 28" tire that means 2,578 revolutions to travel at 80 MPH.

Very, very good job! I would say you have a very good understanding of the given project and I think you have covered the subject well!!

A+

Joe

Just want to say "thanks" for this helpful message.

Dave