1972 K20 towing capacity

[DJ_]

So problem is I have 1972 K20 with a 5.3/4l65/np241 but the previous owner took off the axles and put 1/2 ton axles and so you can call it a k10 lol . but I plan on putting back a 14 bolt and d60 in front. and putting 4.10s because right now I have 3.73s and I have a 5 inch lift on 35s. But my goals is to tow a 5k boat during the summer and a 7k-8k inclosed trailer during the winter and I want to know if after I swap the axles and put 4.10s will my truck do this comfortable? In the future i would love this truck to pull a 10k-12k tow hauler because that would just look great driving up to Glamis and Pismo beach. What would guys do to make a truck like my truck tow good?Like sway bars front and back, disc brakes all the way around? sorry for sounding like a newbie Im just new to this old truck thing lol

[ranger danger]

First off, nice truck! That 5.3 is just barely enough motor for 7-8k towing. Remember a 5.3 works out to be a 323 cubic inch motor! I wouldn't think that 4L65 is going to like 7k for long either! My concern would not be the axles so much as the amount of lift and tire size for towing. I'm not saying it wont handle it, just that you will want to use extra caution when towing with a truck that high. Replacing the 1/2 ton stuff with 3/4 ton would be up there on my list of things to do. 4.10's would also be much better for towing that much weight.

[Ed F]

I have never owned a lifted truck but I have towed trailers since the late 60's with about everything I have owned. Common (or maybe uncommon) sense prevails when towing anything. I hear people say," I can't even tell it's back there" . Whenever you tow, you better know its back there. One of my first vehicles was a new at the time, '70 C20, 307 with a 3 speed manual,410 gears, running stock 7.50-16's. It carried a slide in camper and trailer behind just fine if you drove sensibly. As for the 5.3, my current daily driver is a '13 GMC 1/2 ton 4x4 with the 5.3 and 6speed automatic. That engine/transmission combo is amazing! I restore and show antique wood boats weighing in the 5000-6000 lb range and have towed them from Canada to Florida and from New York to Idaho with that truck. All of those trailers have surge disc brakes. I also towed my '70 C10 on a tandem car hauler trailer about 400 miles. That trailer has electric drum brakes. I limit myself to around 70 mph maximum whenever towing and that truck does the job effortlessly. A lot of the time, stopping is more of an issue than going. With your tires I would be a concerned about traction on wet roads. I have never used an equalizer hitch but with your lift I would recommend one. As for the drivetrain and tires you are running, I think you will be just fine as long as you use your head and drive according to conditions and load. The truck will handle it just fine if you handle the truck within reason. Use special care in wet and or windy conditions.

[72MARIO]

I would be serious considering 4:56 gear with 35" tire. I have a 6.0/4L80 in my 1968 Suburban 27" tire 3:42 gear. With that setup to tow in od comfortably I need more rpm. I plan on switching to 3:73.

I personally think comfortabe towing capacity for your driveline with 4:56 gear should not exceed 7000 lb for any great distance.

Check out randys ring and pinion for rpm calculator l.

[jocko]

Your engine/trans combo is the standard 1/2 ton 4x4 from Chevy today. It is not your weak link even if you used your original K20 specs for towing/GVWR (after you revert to K20 axles etc). Consider what it is in place of - a 350/TH350 only. No 402s or TH400s in K20s. Good point above considering gearing because of the big tires. If you're buying this to have a cool tow rig, might make sense to lose most of the lift and go back to more reasonable tires. Then a 4.10 gear would be about right.

[wbc409]

You will have a very hard time finding a draw bar for your hitch that's rated high enough with the lift and tires you are currently using. All the draw bars I have seen get seriously de-rated when you get more than a 4 inch drop. The other issue I see cropping up is getting tires in the correct load range in that size.

[jeffahart]

I've used WD shanks with extra drop length on a few trucks. Never any issues. And you are pulling a trailer, so your axles on the trailer are carrying weight. So, brakes are a big issue, on the truck and the trailer. I pulled a boat and gear many times with my K20 back in the 80s with 35s and a 4" lift. No issues. It had the original 350 sm465 and np205. But now has 4.56(which I want rid of) I loved the 4.10s. Any how. The weight issue used to come into play with me when loading Sand, Gravel, pallets of bricks, blocks or stone. Loading the bed down to the axle. Get's a bit unnerving with the 35s. But I usually buy materials local to the project so no highway, or very minimal low speed highway.

So what can it do, put the boat on it and one trip down the grapevine and back up you will know.

I've gone over the grapevine many times in my K20 back in the day pulling a boat to ski sites, mostly the deltas in Stockton. No issues. You will slow down coming back up the grapevine for sure. And you will need good brakes going down it... for damn sure! I never used a trailer brake with my boat back then either. But I would to day, so get a trailer brake.

These old small blocks ain't nowhere like a modern diesel. But if you have patience you can make it work. Just try what you have and slowly morph it into what you need.

You will never turn it into a modern tow rig... unless you spend big bucks!

Did I say get a trailer brake, if not... get one!

[Coley]

Originally Posted by wbc409

You will have a very hard time finding a draw bar for your hitch that's rated high enough with the lift and tires you are currently using. All the draw bars I have seen get seriously de-rated when you get more than a 4 inch drop. The other issue I see cropping up is getting tires in the correct load range in that size.

Excellent points.
Lifted trucks seem primarily to be for offroading/clearance purposes or general preferred appearance on 'the street'. Given the paint/appearance of the truck shown here....I'd say mostly street stuff.
For best towing you want your center of gravity as low as possible if you can get it.
Sometimes you have to pick your application when setting your equipment up......because you can't have it all.
All Good
Coley

[jeffahart]

I've towed plenty with between 2" and 4" lifted trucks. There is no lift kit for trailers that I know of, so you're wind issues are not, unless you're getting a big cab over camper... totally different animal than a boat or travel trailer. Most of my modern 4x4s without a lift sit nearly as high at the tailgate as my K20 with a 4" lift. If you're getting a fifth wheel or using a goose neck then maybe a 4" lift will matter. Else, take it out with your boat and you will get an idea of what it can do.

I've had quite a few 4x4s that have seen nothing more than dirt roads. But have towed many miles. Lifted and not lifted. And, I've never lost a trailer, been forced out of lane from a trailer commanding my trucks, not been able to stop, had ill effects on the trailer from wind due to the tow vehicles lift. I'm no expert, but I've pulled a lot of stuff up the road.

The only way your going to know what your truck is about is to give it a try. Pull that boat to Palmdale and Back, that's a good test. Nice hills going through Acton. You will get a good idea of what you are dealing with. And there's usually plenty of wind going through Palmdale on the 14. And if you don't like how it behaves on your test run, you can bug off the 14 onto Sierra Hwy.

Trailer brake, get one and set it up right!

[67 chevelle]

As is Id say you are good for 5000 lbs , upgrade your axils , and pop in a 4l80 and maybe 7000 , more than that and Id be looking for another truck , really anything over 7500 lbs , and everything works better with a diesel , 250-350 , or 2500-3500 , my biased opinion . I have a bit of towing experience .

[cleszkie]

This is how I built my K5 Blazer to tow a loaded car trailer (7k+ lbs) or my 27' Bayliner cabin cruiser (8k+ lbs) through the mountain passes in southern California:

1) Built a 400 SBC with 475 ft-lbs torque and 450 hp @ 3000 RPM to provide the grunt.

2) Swapped out TH350 auto tranny for SM465 manual 4 speed. The TH350 would never survive. The SM465 is bomb-proof.

3) Installed Gear Vendors over/under drive to be able to split gears to keep engine in RPM power band. All the torque rating in the world is no good if you cannot keep the engine in its power band.

4) Installed 14 bolt full floater and Dana 60. Disc brakes on the 14 bolt and 60. 4.10 gears with 35" tires, 2" lift.

5) Installed Hydro-boost brakes. This is a must if you want real stopping power while towing, even if the trailer has brakes.

6) Installed Air Lift load assist bags in the rear to support extra trailer tongue weight.

7) Installed BeCool 2-core aluminum radiator with dual electric puller fans, and one additional auxiliary pusher fan out front.

8) Built drive shafts with 1350 u-joints on both ends to take the added stress.

9) Installed a rear differential cover that holds an additional quart of gear oil to help keep things cooler out back.

All these upgrades allow me to tow with relative ease in the hardest of conditions. However, I wouldn't even try to tow 10k -12k lbs with my set up. That seems like it may be a bit of a stretch for anything other than a nice 1 ton diesel dually.

[jeffahart]

Originally Posted by cleszkie

3) Installed Gear Vendors over/under drive to be able to split gears to keep engine in RPM power band. All the torque rating in the world is no good if you cannot keep the engine in its power band.

Cleszkie nice post.

Can you tell more about your Gear Vendors. I'm thinking seriously about one when I go through my 465 and np205. Do you have it behind a 205. How do you have it setup, meaning do you use manual much. Do you need to give parameters to GV or can you set it up for your engine and gears yourself. I can't see going with a 5 speed when mating a GV seems so simple. Cost/headache seems like a wash compared to a 5speed trans swap. Is there something I'm missing?

[Aruba1]

At the end of the day it's about drivetrain and luck. I pulled a beefed-up tandem trailer with a 1 ton 1996 GMC dump truck on it for a combine weight of probably over 11k (trailer and the truck on it). The drivetrain never missed a beat. If it was not for the suspension and weak brakes (at that time) it would have pulled a 14-15k load easily. After I got a replacement MC and Booster, I pulled 6-7k on the back of my truck. The suspension was almost fully bottomed out but the thing shifted and stopped perfectly like there was no weight on the back.

I started with a 1972 C20, added a 2001 5.3L with 2005 GTO intake/electrics, headers and a 1996 NV4500.

The suspension, rear end, all around brakes came from a 1997-1998 GMC 1 ton dually.

Then I added 225-70-19.5 all around, Super Springs in the back (they don't do magic), all aluminum LS radiator (2 row 1-1/8" core I believe), steel flat bed, lift gate, GN hitch, digital gauges with GPS speedometer, flowmasters etc.

In the garage I have a set of 5000 air bags and on-board compressor that I will be installing soon and that should take care of the rear suspension. I also have a hydra-boost system, p/s pump, etc that will go on her soon. And on the list of items I hope to install a light duty dump bed and 245s all around :-). And who knows, convert it to 4x4? Hope so. I love seeing the face of people in their new truck when they try to race old Whithie as we hit 90-100 mph.

[DJ_]

Originally Posted by jeffahart

I've towed plenty with between 2" and 4" lifted trucks. There is no lift kit for trailers that I know of, so you're wind issues are not, unless you're getting a big cab over camper... totally different animal than a boat or travel trailer. Most of my modern 4x4s without a lift sit nearly as high at the tailgate as my K20 with a 4" lift. If you're getting a fifth wheel or using a goose neck then maybe a 4" lift will matter. Else, take it out with your boat and you will get an idea of what it can do.

I've had quite a few 4x4s that have seen nothing more than dirt roads. But have towed many miles. Lifted and not lifted. And, I've never lost a trailer, been forced out of lane from a trailer commanding my trucks, not been able to stop, had ill effects on the trailer from wind due to the tow vehicles lift. I'm no expert, but I've pulled a lot of stuff up the road.

The only way your going to know what your truck is about is to give it a try. Pull that boat to Palmdale and Back, that's a good test. Nice hills going through Acton. You will get a good idea of what you are dealing with. And there's usually plenty of wind going through Palmdale on the 14. And if you don't like how it behaves on your test run, you can bug off the 14 onto Sierra Hwy.

Trailer brake, get one and set it up right!

Thanks for all the info and ideas very much appreciated!!! AND thanks to everyone else for the advice!!!!

yeah I knew from the start about my truck towing in the future a 10k toy hauler was most likely not gonna happen, but for now I just need to be able to tow a 7k inclosed trailer and a 5k ski boat during the summer. And yeah I will for sure put trailer brakes on my truck!
For now Im gonna put a big trans cooler so the 4l65 can be a little happier lol.

[DJ_]

Originally Posted by jeffahart

Cleszkie nice post.

Can you tell more about your Gear Vendors. I'm thinking seriously about one when I go through my 465 and np205. Do you have it behind a 205. How do you have it setup, meaning do you use manual much. Do you need to give parameters to GV or can you set it up for your engine and gears yourself. I can't see going with a 5 speed when mating a GV seems so simple. Cost/headache seems like a wash compared to a 5speed trans swap. Is there something I'm missing?

One more thing are you running sway bars on your truck? and if you are how did you put them on? because my truck has no sway bars.

[DJ_]

Originally Posted by Aruba1

At the end of the day it's about drivetrain and luck. I pulled a beefed-up tandem trailer with a 1 ton 1996 GMC dump truck on it for a combine weight of probably over 11k (trailer and the truck on it). The drivetrain never missed a beat. If it was not for the suspension and weak brakes (at that time) it would have pulled a 14-15k load easily. After I got a replacement MC and Booster, I pulled 6-7k on the back of my truck. The suspension was almost fully bottomed out but the thing shifted and stopped perfectly like there was no weight on the back.

I started with a 1972 C20, added a 2001 5.3L with 2005 GTO intake/electrics, headers and a 1996 NV4500.

The suspension, rear end, all around brakes came from a 1997-1998 GMC 1 ton dually.

Then I added 225-70-19.5 all around, Super Springs in the back (they don't do magic), all aluminum LS radiator (2 row 1-1/8" core I believe), steel flat bed, lift gate, GN hitch, digital gauges with GPS speedometer, flowmasters etc.

In the garage I have a set of 5000 air bags and on-board compressor that I will be installing soon and that should take care of the rear suspension. I also have a hydra-boost system, p/s pump, etc that will go on her soon. And on the list of items I hope to install a light duty dump bed and 245s all around :-). And who knows, convert it to 4x4? Hope so. I love seeing the face of people in their new truck when they try to race old Whithie as we hit 90-100 mph.

That sounds like one mean monster you have there lol. Can you post a pic of that thing? But how do you like the 5.3? I want to make mine a little torque monster maybe bore it out to a poor mans ls1 I want to get a cam for early torque in the rpms too. Right now i have aluminum radiator and dual electric fans. But now from what I hear I think its a good idea to put hydro boost brakes on my truck.

[Aruba1]

I was running mine with my copper radiator until one day it threw a belt and temp rised to 230-240 and the engine died. By the time I installed the new belt and get her back running, she was pouring coolant from all corners of the radiator so I got an aluminum one. I wanted to cam the motor but they are too expensive. I like the 5.3L but if I build another one it probably will be a diesel or a 6.0L LS. I like the torque but the 5.3L does pretty well. Let me see if I can get some pics...

[67chevy_hotrod]

My dads 2006 1500 has the 5.3 and I can tell you from using it to haul my old Chevys, it feels like it's just barely enough motor for my 1/2 ton on my tandem axle flatbed. And that truck is bone stock. Don't get me wrong it gets the job done, but I'm sure it would have issues doing something that heavy more regularly.

[Aruba1]

Originally Posted by 67chevy_hotrod

My dads 2006 1500 has the 5.3 and I can tell you from using it to haul my old Chevys, it feels like it's just barely enough motor for my 1/2 ton on my tandem axle flatbed. And that truck is bone stock. Don't get me wrong it gets the job done, but I'm sure it would have issues doing something that heavy more regularly.

Gearbox and differential ratio plays a key role :-)

[jeffahart]

Originally Posted by DJ_

One more thing are you running sway bars on your truck? and if you are how did you put them on? because my truck has no sway bars.

No, the truck came without anti sway bars. But I get no sway, even with the skyjacker soft rides. Which I will be swapping out for HD leafs in the front once I get my front bumper situation sorted out.

Once you put on your D60 14bolt you will feel more planted to the road with the extra unsprung weight. But if you get good shocks and springs you should not need an anti sway.

When I owned this truck back in the 80s I had an HD 4" lift and I used to drive decker road like a bat out of hell at least three times a week to dive Harrisons reef at county line. The truck really hugged the road.

Decker road is where Westlake blvd turns into a tight canyon(Decker) south of the 101 freeway.

I would spend my time and money sorting out the suspension other than a sway. My two cents.

[cleszkie]

Originally Posted by jeffahart

Cleszkie nice post.

Can you tell more about your Gear Vendors. I'm thinking seriously about one when I go through my 465 and np205. Do you have it behind a 205. How do you have it setup, meaning do you use manual much. Do you need to give parameters to GV or can you set it up for your engine and gears yourself. I can't see going with a 5 speed when mating a GV seems so simple. Cost/headache seems like a wash compared to a 5speed trans swap. Is there something I'm missing?

The GV bolts to the back of the NP205. With a manual tranny, you have to use the manual mode. I put a push/pull switch on my shift lever to activate/deactivate it. It does automatically deactivate below 10 mph even in manual mode. No parameters to give GV. It just provides 30% overdrive to all forward gears. A GV unit is very simple. Its the cost that scares people away.

Originally Posted by DJ_

One more thing are you running sway bars on your truck? and if you are how did you put them on? because my truck has no sway bars.

No sway bar up front. Only Softride 2 inch lift springs. The air bags in the rear really do a good job of controlling body roll.

Originally Posted by Aruba1

At the end of the day it's about drivetrain and luck. I pulled a beefed-up tandem trailer with a 1 ton 1996 GMC dump truck on it for a combine weight of probably over 11k (trailer and the truck on it). The drivetrain never missed a beat. If it was not for the suspension and weak brakes (at that time) it would have pulled a 14-15k load easily. After I got a replacement MC and Booster, I pulled 6-7k on the back of my truck. The suspension was almost fully bottomed out but the thing shifted and stopped perfectly like there was no weight on the back.

I started with a 1972 C20, added a 2001 5.3L with 2005 GTO intake/electrics, headers and a 1996 NV4500.

The suspension, rear end, all around brakes came from a 1997-1998 GMC 1 ton dually.

Then I added 225-70-19.5 all around, Super Springs in the back (they don't do magic), all aluminum LS radiator (2 row 1-1/8" core I believe), steel flat bed, lift gate, GN hitch, digital gauges with GPS speedometer, flowmasters etc.

In the garage I have a set of 5000 air bags and on-board compressor that I will be installing soon and that should take care of the rear suspension. I also have a hydra-boost system, p/s pump, etc that will go on her soon. And on the list of items I hope to install a light duty dump bed and 245s all around :-). And who knows, convert it to 4x4? Hope so. I love seeing the face of people in their new truck when they try to race old Whithie as we hit 90-100 mph.

You might be able to pull 11k on relatively flat ground in Alabama. But its a totally different game here in southern California where we have 5,000+ foot high mountain passes with long steep pulls and long steep downhills to contend with

[57taskforce]

If that's what you really want to do than this is (12 valve Cummins) the first thing I'd look into after a 14 bolt and front 60. It's a fair amount of work to swap but will do the job from the power stand point much better.

[trac209]

I would say the cummins is the best towing engine out there bar none. I've towed a 15k trailer with them and they hardly seem to notice the load.

[Aruba1]

[QUOTE=cleszkie;7781625 You might be able to pull 11k on relatively flat ground in Alabama. But its a totally different game here in southern California where we have 5,000+ foot high mountain passes with long steep pulls and long steep downhills to contend with[/QUOTE]

That is a true statement. Although sometimes I think m yneighborhood should be called Little California. It's a good 3 miles from the highway with a lot of step inclides and declines. When it snows (once every 2-3 years) nobody leaves because you will end in a ditch somewhere. Heck, my wife lost control twice and ended in a ditch. My sister-in-law did it once. If I have a heavy load my truck will hit 215 degrees by the time I reach my house. lol

Now that I have a military 6x6 truck I can't wait for it to snow, hheheh.

[67chevy_hotrod]

Originally Posted by Aruba1

Gearbox and differential ratio plays a key role :-)

RPO GU6 is a 3:42 rear ratio in a 2006 1500. Truck has the 4l60e

My longhorn has a lame 350 to an original th400 with 3:54 Dana 60. And that's all old stuff. She still is more comfortable to tow with then my old mans. However my brakes are far inferior lol