Jump to content

Switching an existing automatic transmission on a 73 Chevy pickup for a 4 on the floor

Guest lancebon

Recommended Posts

Guest lancebon

I want to change from automatic transmission to standard 4 speed w/ low,low.

the transmission I'm being offered is from a 70 Chevy truck with a 327 V8 engine.

My pickup is a 1973 short bed step side c10 with an inline 6.

Other than the clutch assembly are there other things to modify, like the drive line, existing flywheel, are there converter kits, are there books on this subject or do you have first hand experience. etc. Please let me know.



Link to comment
Share on other sites

With all due respect, the amount of labor and getting appropriate parts to do this conversion would be more than the truck could ever be worth!

Considering that the '73 platform went all the way to the '91 Suburban/cab and chassis trucks, you could probably find a 4-speed 3/4ton pickup in a salvage yard, somewhere. THAT would get you all of your clutch linkage items. You might also be able to upgrade to the later hydraulic clutch set-up, which would greatly simplify the process! BUT, you'd need the clutch linkage for a 6 cylinder, which can be even harder to find.

The clutch linkage has a pivot stud on the side of the engine block. This is where the linkage pivots on its way to the clutch fork and throw-out bearing. I suspect that would be "engine-specific". I suspect the actual pushrod for the clutch fork (where the adjustment nuts are) is universal for all manual transmissions back then.

Of course, you'll need to replace the automatic transmission "flex plate" with a genuine manual transmission flywheel. Get this new--period--from a reputable parts source. Then add a new pressure plate and clutch plate. You'll need new flywheel bolts (different length than the automatic ones), too. Plus a new pilot bearing (get the newer needle bearing model rather than the earlier bronze bushing . . . they all go in the same place. BUT . . . as your engine was a factory automatic transmission spec motor, you'll FIRST need to see IF, repeat IF, the end of the crankshaft has a deep enough hole in it to accept a pilot bearing/bushing in the first place! In some cases, the crankshafts were specific to which trans they went in front of, so check this FIRST before proceeding with ANY sort of conversion!!!!

If everything checks out, you'll also need to determine if the NewProcess transmission's bell housing bolt pattern is the same as what you currently have. If you were doing a V-8 to V-8 deal, no problem, but the 6 cylinder might accidentally throw a kink in things. Might need a 6 cylinder bellhousing for that "granny" 4-spd.

Once all of the trans mouting and linkage issues are "done", you'll also need the center hump in the floor pan! The 4-speed one is taller and also has the hole for the trans shifter tower to stick through. Either a bolt-in or tack/braze-in situation. Plus the appropriate under-dash clutch and brake pedal items. You can probably find an aftermarket shift boot, I suspect.

Then there's the issue of the transmission mounting crossmember. Best to source this from the salvage yard. Unlike some other brands, GM used crossmembers which were probably "over-built" with many bolt-together pieces and braces to make ONE trans crossmember. But the whole thing can probably bolt-in as an assembly. Building one from the parts book makes you aware of these things.

If your truck has a two-piece driveshaft, you'll need the front section modified at a reputable driveshaft shop. Probably a length alteration and front yoke difference, I suspect. You might get a new center carrier bearing assy while you're at it, for good measure.

BEST thing to do would be to find a donor truck to use for your parts source. It can also supply or give you an idea of what sort of speedometer cable changes would be needed. The ideal thing would be to have BOTH trucks side by side for "the surgery".

Certainly, what you're considering is a factory-based production situation, just that you'll be adding it "after the fact" than on the assembly line as the truck is being built. Anything is possible, especially for the weekend morning PowerblockTV guys and their multitude of fabrication tools and expertise. BUT I suspect, witih all due respect, that is not necessarily YOU. Certainly, this can be done in an unused slot in your driveway, carport, or garage, too, with enough time and energy.

Back in the later 1960s and early 1970s, many farmers usually bought their pickups with the 4-speed "granny low gear" transmission. That's what they'd always done. They liked the ultra-low low gear for creeping along in the hay field, for example. BUT in normal driving, they used ONLY the top three gears! We have a few 1950 GMC pickups with that transmission . . . it's been around THAT long. In my earlier driving days, I didn't understand the "start in 2nd gear" situation. But one of the times I tried to use all 4 gears, I soon found out that the speed range of 1st gear was very short. About the time you got some rpms up, it was time to shift . . . BUT due to the inertia in the low gear set, by the time you could actually shift it, the vehicle had slowed down a good bit, negating any jump off of the line from the deep low gear. Much better to start in 2nd gear! So, just creeping around was all it was good for, especially with the also-low gear ratio in the rear axle. Point is . . . if you were trying to beat your best friend in a foot race for 50feet, he'd beat you if you wanted to use "granny low"!

But a funny thing happened in 1966. One of our associates in the bulldozer business showed up with a new 3/4 ton Chevy pickup . . . with a PowerGlide in it. Everybody wanted to know how it worked, compared to the old 4-speeds he used to buy. He raved about how easy it was to drive out in the field, in loose ground. Much easier power delivery, he noted, rather than trying to slip the clutch to do the same thing. That was one heck of an endorsement! Add a PosiTraction rear axle and he could go pretty much anywhere a 4wheel drive pickup could go!

So, Lance, my recommendation to you would be to change the rear axle to either a 3.55 or 3.73 rear axle ratio. Both were factory options back then. Not only would this give you some of the lower-rpm and lower-speed capabilities that you seem to desire, BUT it would also be less expensive and something that many mechanics and such can do in a few hours of time and a few hundred dollars. If you want to go with GM parts, the gearset can be close to $500.00, but aftermarket gearsets are also available. Add the bearings, too, for good measure. This would also be a good time to add a PosiTrac unit, too. Add an external trans cooler and you'll be good to go.

Even if you're looking for an alternative to rebuilding the THM350, it would be best to rebuild it with at least OEM-quality/spec parts. Upgrading to a THM700R4 automatic OD trans could be done, which would give you the deeper low gear, but it would still need the 3.73 rear axle to be able to use OD on the highway at normal speeds. Still, an OEM production-based situation AND probably more readily found in the salvage yards for '87 GM full-size pickups. You can rig the lockup torque converter with a vacuum-level sensor, as the factory set-up were, too. Then do a few "wake up" things to the 6 cylinder and you can surprise people at the red lights.

In my current employment, when I went to work there (now 35 years ago!), I drove a '76 C-10 "Spirit of America" trimmed Scottsdale. It was a six cylinder, 3-speed manual trans truck. Rear axle ratio was 3.73 with P235/75R-15 tires (LR78-15, back then) on 15x8 Rallye Wheels with the 6001lb GVW option (non-cat, regular fuel). It was a nice looking truck and from a dead stop, it would really move in low gear. There were some times I wished it had a "car" 4-speed rather than the 3-speed . . . certainly NOT the "granny low" 4-speed. With the wider wheels and tires, plus a front sway bar, it was a ball to drive. It would run all day at 90mph (4000rpm) on the highway. It'd run faster, but it didn't like it. 14mpg no matter how I drove it. Oil level never budged from "full", either . . . even after I calculated that I buzzed it to about 6000rpm one day in low gear, in a freeway merge situation. The rpm was so high that acceleration was gone, so I quickly shifted to second and kept on going. It put up one big white puff out the exhaust pipe and never missed a beat! One GREAT truck!

Just some thoughts,


Edited by NTX5467 (see edit history)
Link to comment
Share on other sites

Keep in mind too that since your truck came from the factory with an automatic, that it's not going have the bracket on the frame for the clutch bellcrank. It maybe bolted or riveted on the other truck. You'll have to see how the bellcrank's pivot ball would have went on your engine and if the linkage will work on the engine and the frame.

Link to comment
Share on other sites

I'll be the dissenting opinion here. This is a completely bolt-on swap, as the trucks are configured to go either way. The problem is that you need to find a correct 1973-1980 donor truck for all the small parts that you'll need, otherwise you'll spend a ton of time and money chasing them down. Clutch linkage, backdrive linkage, bellhousing, etc are different from 72-earlier to 73-up. Driveshaft will be different depending on wheelbase and rear axle. Also, these trucks could have come with a car-style 4spd in the light duty models or the granny gear truck SM465 in the heavier duty models. You want the trans that matches your truck. Finally, the 73-80 trucks have a bolt-on hump for the shifter. You can get this repro.

Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
  • Create New...