The Stovebolt Site Forums The Shop Area Engine Shop Urgent need of knowledge - Tow capacity of 270

I have been restoring a 3/4 ton, 55 1st with the original 270 (rebuilt), the original SM420 transmission and new 3.90 rear gears. At first this was just to be a hobby truck, fun to drive, but also would be used for truck stuff. I have recently bought a pretty large "box trailer" setup for vending and had hoped to use this truck to tow it.

I am now shaken in my faith that this truck, with the 270 & SM420 will be capable of towing this trailer. The trailer is a tandem axel (3500lb each) trailer that will probably come in weighing 6000 - 6500lbs. Should I expect this truck to tow that load effectively?

This is a new business that I will be starting and I am investing $ right now in setting up this truck with the 270. If it will not do the job, then I need to stop spending the $ on the 270 and go with something that will.

So….I need some advice. First – will the 270 and SM420 cut it. If yes – GREAT! If not – then what would make for a good tow motor and transmission combo? If I have to go with a new motor and transmission, I am willing to stray from original – however I would MUCH prefer to hear that this 270 will do the job like the champ I believe it is.

Thank you all very much for all the advice you have already given. I really appreciate your wealth of knowledge and I hope that I can learn from you and share with others as I get more experienced.

God Bless! - Tom

I think I would be more worried about stopping it. Are you using the factory brake set-up? Also how will the hitch be attached to a frame that was never designed for modern hitch applications? I think you are asking to much of the truck just my opinion.

7,000 lb trailer fully loaded, I think you'll be close to happy with it.
If you're on the fence a transmission with more gears and change the rear axle ratio as necessary will help some.

Brakes are always good.

Grigg

As Brad posted, the bigger questions would be the strength and positioning of your tow set-up, your rear springs (should be OK), and your brakes. Your rear ratio is relatively high but you have the SM420 1st gear to get you moving.

The 270 engine was used in larger GMC trucks (at least up to 2-ton), as was the SM420. Of course they will "cut it".

On what types of roads will you be driving and at what top speed?

3/4 ton 270?

Truck was meant to be "used" when it was built.

Back in the "day", not much thought would have been given as to how much weight it could haul , but, how much would it haul.

They were not too fast on the highway, and the brakes were "State of the Art" at the time.

Change to a 12 Volt system which would accomidate the electric brakes on the trailer.

I have installed a dual master cylinder and rerun all the brak lines. I would be using the factory drum setup though. I want discs on the truck, but the 3/4 ton (8 lug) conversions dont appear to be readily available.

The trailer is equiped with electric brakes. I know that is not the end-all solution but it should help.

With regard to attaching the hitch reciever, the previous owner has welded metal like a tank to the frame and created a whole new rear bumper configuration. I was intending to utilize that setup and attach my #3 receiver unit to it.

I LOVE YOU GUYS!!! Keep the good news coming! I need to hear more!!!!!

I will be hauling this thing approximately 10 miles one way (20 round trip) daily in a relatively flat environment. I will go as fast as I can comfortably go but would like to do 55MPH. I am in St. Louis, MO and coming from just outside the city, where the trailer is stored, to the heart of the city on highways for 98% of the drive. Now, I hope to do some events around the area that could lead me to a more hilly environment, but for the lion's share of days, i will be making that same 10 mile commute on relatively flat roads.

Oh yeah, I still have the original 4.57 gears and drop out carrier should I need them, but back when this was to be a hobby driver, it would be nice to have more highway speed so went to the 3.90.

Might be fine with your current plan, but also consider keeping the 4.56 gears and adding an NV4500 5 speed overdrive transmission.

Grigg

Go with the 4.57 gear, and a good weight-equalizer hitch. Watch following distances, and keep the trailer brake control adjusted to a fairly sensitive point so the trailer brakes help out on every stop. Did I mention don't follow too close? Stopping distance is your friend!
Jerry

Yeah, I have the drop hitch with dual sway bar setup and Load distribution bars on this already. Would not consider proceeding without that.

X2

Grigg took the words right out of my mouth. The NV4500 is a great transmission for this application. With the 27% OD the NV4500 offers, your 4.57 rear would feel like a 3.34 rear. The 270 should have no problems pulling this weight on the flat. On a "hilly" road, you will likely have to downshift to direct.

OK, now for some clarifications... You state the truck as a 55.1 with the original 270. The 270 was not offered for 55.1 3/4 ton trucks. The "original" motor should be a 248 gmc. It appears your truck was repowered at some point. Most likely this was done by a previous owner, but it was not uncommon for trucks to get the bigger motor from the warranty service. If the later was the case, there would be a sticker on the firewall that indicates a repower of the vehicle.

Back in their day, 270s pulled 36' van trailers with a legal gross of 58,000. Pulling a box trailer, wind resistance will be just as hard to pull as a load. Especially with a side wind. I would consider a sway bar because 55 year old suspensions weren't as stable as what we have today. And good trailer brakes are a must. I would prefer the 4.57 because you will need the rpm's for power and to keep the engine running cool. I would also consider a thicker core radiator. Overheating was the biggest killer of GMC engines.

Stopping will be your biggest concern, because you will want to or be pressured to keep up with the traffic. all of the newer vehicles can stop faster and better than your truck. I have cracked brake drums before pulling a trailer with drum brakes having to stop fast. I had the boost at the pedal but not the equivalent at the wheel. that got remedied with disca and bigger rears.
A good hydraulic brake system for that trailer would be advisable.
You did state that if you didn't have the power to move it, you would solve that problem. treated with respect that 270 will do the job; so if you don't have to power up then you should put that change to slowing down, which would have been more so if you had added power
ron

Is there a disc brake conversion kit out there for 3/4 ton GMCs yet? I know I could do a new front end and new axels, diff., etc. but that is getting quite pricey and I am not sure where the point of deminishing return is.

The trailer does have Electric brakes, sway bars and load/weight distribution bars.

I will be considering the re-installation of the 4.57 set, but before the rebuild, that set would only get 45 MPH without a load. I am kind of fond of not getting run over, so I was hoping for 55 MPH and stay in the far right lane!

If you keep the 4.56 and still want to keep up with traffic you need an overdrive transmission, that's why we recommended the NV4500

For disc brakes there is no kit available, but there are ways of fabricating something to work. I'm figuring and calculating on that right now for an AD 1 ton truck.
What is the wheel mounting surface to wheel mounting surface measurement on your truck any how, we might have a nice solution already (but it's not quite narrow enough for an AD truck)

Check this thread, although it wanders around there are some good thoughts and ideas in there. That's what I'm working on now.
https://www.stovebolt.com/ubbthreads/ubbthreads.php?ubb=showflat&Number=603612#Post603612

Then if you want a newer rear axle check this thread
https://www.stovebolt.com/ubbthreads/ubbthreads.php?ubb=showflat&Number=599879#Post599879

Grigg

Looking around for these NV4500s. Price seems to be $900-1000. Is that right? if I was to go down this path, am I looking at major issues with finding the appropriate bell housing, transmission mounts, other mechanical stuff? Sound like some of you guys have experience with this. Is it a simple swap, like the rear pumpkin?

Tom

I believe you should stay with the higher rear end ratio and stock transmission if dollars are also concern.

This way truck will do well without trailer.

With trailer you will just have to use lower gears on hills and startup. You may not be able to do 55 on some portions of road - welcome to practical trucking. I would not even think of stopping this with truck brakes but would rely on trailer brakes first.

Dan Bentler

With a 235, I successfully towed a 1 ton car hauler with a 3500 pound 1995 Trans Am on it. However, that said, when a tire blew on the trailer, it almost pulled my truck off the road...which is exactly why Uhaul no longer rents out car haulers unless the towing vehicle weighs at least as much as the entire weight that it is towing.

Plan on about a realistic 3500 pound tow capacity for your truck. It's not that the 270 wouldn't handle more...it certainly would, if my 235 did. It's just that any more than that and you have an unsafe condition.

there is a fellow in arlington, wa. that may be able to help you with frt disc brakes. he would be willing to make the setup. he says that there is no one making disc brake setups for 3/4t. he makes them for 6 lug but sounds like he knows what he is talking about. just google, "buffalo entreprises". phone only
ron

You need to find a 1994-1997 Dodge 2wd NV4500. A newer Dodge NV4500 can be made to work, but it you will have to get a digital to analog converter for the speedometer.

Why the dodge NV4500, and not the GM NV4500?
The early dodge version can work with a Mechanical speedometer. The pilot shaft of the dodge unit is 1 inch longer than the pilot shaft of gm unit. Since there is no version that will bolt directly up to a stovebolt bellhousing, a 1 inch adapter plate is used. By using an adapter plate, most of the stock clutch components can also be used.

The NV4500 swap is still not quite a drop in swap. Additional machining is needed to bolt it all together. The adapter plate that Advanced Adapters sells is indexed for a bellhousing with a center hole of 5.125 inches. The bellhousing on your 270 is indexed to 4.686 inches. What this means is you either have to turn the indexing lip down on a lathe so it will fit your bellhousing, or have your bellhousing bored out so it will fit the adapter plate.

Next is the bearing retainer on the transmission. The throwout bearing rides on the bearing retainer of the transmission. The Dodge bearing retainer collar is a little wider than the sm420 collar. This part can be removed from the transmission and cut down on a lathe so the stock throw out bearing can be used. You may be able to use a crossover bearing and avoid this extra machine work. I machined mine so I could use off the shelf parts and still look it up by the year and make of my truck.

A custom pilot bearing will need to be used. This will be a bronze oillite type bushing. The Dodge NV4500 has a pilot tip of 0.750 inches. The sm420 you would be replacing has a pilot tip of 0.590 inches. The ND7109 ball type bearing you currently have will not work for this. So the pilot bearing is the only clutch component that has to be changed.

The Dodge NV4500 comes with either a 10 spline 1 1/8 pilot shaft, or a heavy duty 10 spline 1 1/4 pilot shaft. You want the 1 1/8 pilot shaft as that is the same pattern as the sm420. If you find the heavier duty diesel transmission, the pilot shaft can be swapped. You will also need to swap the bearing retainer for the lighter duty gas version. If using the 10 spline 1 1/8 pilot shaft, the stock clutch disc and pressure plate can be used.

The shifter for the sm420 is centered about 4.5 inches from the front of the transmission. The shifter for the dodge NV4500 is centered about 10 inches from the front of the transmission. When using a 1 inch adapter plate, the shifter will be pushed back approximately 6.5 inches from where it currently is.

The NV4500 is a longer transmsion than the sm420. You will likely have to have a driveshaft made. It may be possible to get rid of the 2 piece drifeshaft and have a single piece driveshaft made. I have a 59 1/2 ton long bed and was able to have a single piece driveshaft made.

Lastly, a transmission tail support will be needed. I had a local metal shop fabricate a crossmember for this purpose.

John

PM me if you have any questions. I'd be happy to answer any questions over the phone.

how long will you be doing this? what are you pulling the trailer with now? like others have said stopping is the key. id prefer a late model one ton that has the power for the speeds you want to run, along with greater and better stopping power to do the job. id use the 270 for touring but thats just me. seems like a lot of time/money for what you are looking to gain. but then that all depends on the answer to my first question.

Did the AD 1-tons even have the capability to reach 55?

in which respects to have the capability
ron

We aren't talking about an AD 1 ton, we are talking about a ND 3/4ton. That 270 can be wound to 3200rpm all day.

3200 rpm with a 5.14 gears (1 ton rear) and 31"tires will give you 57.4 mph.

3200 rpm with 4.56 gears (3/4 ton rear) and 31" tires gets you 64.7 mph

Ok, lets add a NV4500 with a 27% OD in the mix...
3200 rpm will push the 1 ton up to 78.7 mph, and the 3/4 ton to 88.7 mph.

The numbers above are hypothetical. I wouldn't expect a 216 to be able topull these tyhpes of rpm, Nor would I expect a 228 for this duty. A gmc 248, 270, or 302 should have no problems.

If you are going to be pulling a box trailer, I don't think you will want an overdrive. GMC 3/4 tons that were born with a 270 and 4.56 ran 60mph+ all day long back in their day. The speed limit was 60 in Texas until 1962 when it was raised to 70 and we used to get speeding tickets in stock GMC pickups. And it didn't hurt the engines a bit. The 55 speed limit came about in the mid 70's to save gas, but the vehicles were built to go faster than that.

I have my biological family, I have a church family, and now I have a Stovebolt family - at least that is how I feel. You guys Rock! Thank you so much for all the information! I am excited to get this old beauty back out on the road and doing what it was meant to do!

If you are interested in just what I am pulling, feel free to check out & LIKE on facebook. http://www.facebook.com/gatewaydoghouse

We will be opening up on 3/6 with a soft opening for a Mission Fund Raiser at church and then be selling to the St. Louis, MO downtown public on 3/7. Wish us luck, Like us on FB, and most of all - come down and share a dog with me!!!!!

Tom (Papa Tom's Gateway DogHouse)

51 Ashton

I will be doing this daily for as long as the city of St. Louis will renew my permit. I am just starting the business, so I am not currently hauling the trailer with anything on a regular basis. The Truck is at the body shop getting prettied up - paint, chrome,etc. and they have also done some wrench turning for me as well.

The truck will be practical and pretty, at least that is the idea. I need a good hauler for this 10mile haul each day, but I also want the attention/atraction that it will bring (advertising).

In a couple of months I will have a "fleet" of tricycles (2 wheels in front) doing delivery for me and I am fabricating a front end from fiberglass for these bikes to cover the carrier/cooler that will look like the GMC - Lights, hood, fenders, all of it. Talk about an eye catcher. I will be uploading lots of photos as I get these on the road!

Sorry I am rambling, but I can do that when I start talking about the truck, the trailer or the business!

I agree. My '52 GMC 3/4 ton with the Stovebolt and 4.56 used to go around 70 on the interstate...but that was only after changing out the original carb for a 2bbl carb. With the original 1bbl, I was lucky to get up to 60. Just didn't seem to have the available horsies at Denver's high altitude.