MY 1969 PETE ON YOUR STEPDECK? SHOT IN THE DARK!!

Dunno, that was on there when we bought the place.

 

Cash actually, without much argument too.

 

Cool beans.

 

How much did you take out of that?

 

That's a profile pic if I've ever seen one. BAMF

 

5 inches.

 

You have a 13x4. No larger than a 12713 or so on the 13-speed, without seeing a data-plate or even a box, it's near impossible to say the precise type of 4-speed you have.

Post a photo of the driveshafts.

Being a 13x4, the truck is set for heavier work, it may have had a winch at some point or another, as the 4-speed would allow for a power-tower, or an auxiliary-mounted PTO that you could leave the mainbox (13-speed) in gear and control the speed (be careful doing this).

The shift pattern for the 4-speed should be an H-pattern with a horse-collar, like so:

DU (1) OD (4)
|_____|
| |
UD(2) Dir (3)

Gears are DU: deep-under (1st), UD: Under-drive (2nd), Dir: Direct (3rd), and OD: Overdrive (4th)

You can find out, without a data-plate, whether or not it's a proper deep-reduction deep-under box by shifting from deep-under to underdrive. If it has the ultra-low deep-under (2.37:1) hole it'll be about a 1000-rpm shift. If it's a relatively shorter shift, it's likely a progressive auxiliary.

Because your's is mated two a 13-speed, it's most useful to effectively, if doing heavy work, to treat the auxiliary as a two-speed rear, shifting between underdrive and direct.

Deep-under, whether in this set-up or a 6x4 like my truck, you don't use once you're out of it. It's strictly an ultra-low starting hole gear. If you were in a higher gear in the main, trying to downshift to the deep-under hole would be the equivalent of dropping two gears on the 13-speed.

There's two problems with utilizing the overdrive gear all that much with your set-up: bearing-speed and driveshafts (this is why it'd help if you posted a photo of the driveshafts).

Assuming your 13-speed (the mainbox) is a 12713, it's going to have an 0.81 overdrive gear, more than likely, the overdrive in the aux will be between 0.90 and 0.80, giving a compound overdrive gear in the neighborhood of 10% higher than modern 13-speeds (top two OD, 0.81 and then 0.73).

That means two things: the bearings, and the driveshaft, will be spinning immensely fast in overdrive. If this was a retrofit, which I imagine it is, then the driveshafts might not have been updated. I wouldn't imagine you have anything heavier than standard-wall 1210, MAYBE 1410 driveshafts. The lighter driveshaft won't like spinning that fast, as you'll be looking at in excess of 3000-rpm in the big hole at 2100rpm.

The bearings in the auxiliary will be spinning at immense speed, as well, in that hole.

The backside, generally you don't want to utilize OD in the auxiliary as much when you're in underdriven gears in the main (anything lower than direct gear). Here and there won't kill it, but, a sharp torque load from the engine with the main (13-speed) in a lower gear where it's multiplying torque could potentially wreak havoc on the auxiliary if it's in OD. If you're going to put it in OD in the aux while in a lower mainbox gear, don't just stand on the throttle, walk into it (you should always drive this way anyway). Direct gear on the main (top hole, splitter in DIR) and the OD on the aux won't notice a thing, no multiplication of torque.

Make sure you square on the corners on the horsecollar shift (2nd to 3rd), if you don't you'll bind the linkages to the auxiliary, and you'll need a bar to pop them loose. Considering the age of the tractor, the linkages are likely worn, meaning it's more susceptible to this, square the corners like you're drawing three sides of a square.

 

That 13 may not be an overdrive, but an RT 6613, or RT 9513.

 

Also, I doubt it came with a 13 x 4, as there is way too much overlap in the gears. I'll stick with it it originally having had a 5 speed main, maybe even a 4.
We'll find out when Bigzilla gets the info from Peterbilt.

 

If it's an early enough 13-speed, that'd be true, as 3/4 and 7/8 would be reversed and it would have a horsecollar shift, as well. For the OP's knowledge, on older truck transmissions, OD was typically up against the dash.

If, however, it was a direct-drive, I'd presume either a two-position splitter and t-handle or the Low/ Int/ Dir splitter. Of course, a Low/ Dir/ OD shift knob could easily be swapped.

For the OP, when parts hunting:

I'm 95% certain you have Rockwell rears on that truck, SSHD's or SQHD's would be my guess, very common rear end years ago. The end-cap of the hub (which is actually the face of the axle shaft) has the distinctive Rockwell two half-round embossings on it, not to mention the housing looks to be Rockwell. If you want a positive ID of a Rockwell rear, look at the what looks like a washer behind the lugs on the hub. If you find a small split in the "washer" it's a Rockwell rear, and it's actually not a washer, it's a cone that acts similar to a lock-washer.

If they're Rockwell rears, LOTS of fun getting the axles out if you ever have to, a fun little process involving an impact wrench, screwdriver, hammer, and a 15-lb sledgehammer.

Edit:

I'm going to hazard a guess, unless we're talking a lot of conversion going on, that it isn't a 6613. It'd be an unusual choice unless it was the only transmission lying around, as these were most commonly in gassers due to the lower torque-rating of the 6613. Not saying it's impossible, but, I'd be surprised if it was retrofitted with a 6613 once the original mainbox came out. Even the baby V8 Detroits were over 800lb.ft. of torque. But, it's not unfathomable that some backyard swaps might result in one of the little 13-speeds behind a bigger hammers.