Using 67-72 cabs on newer trucks (like 87-91 or 92-97)

[occupant]

Living in Ohio for the last few months reminded me that RUST is a problem here when I'm looking at vehicles. So I've been working and working and WORKING and taking all my time making a 55% commission on all the tows and roadside calls I do and come to find out one of the other drivers gets 80%. Why does he get so much more? He owns his own truck and pays his own insurance (but it's on the company policy so it's only a small portion of the bill vs the other tow trucks). I figure I make about $1400 every two weeks after about $800 in fuel. This $2200 biweekly is an average of the last 7 months of paychecks. So that means whatever I'm doing, I gross $4000 a week for the company. If I had my own truck it would be $3200 every two weeks, or about a $2000 a month difference. Bonus: if I had my OWN truck I wouldn't have to wait for other people to get out of the truck I'm scheduled to be in. Another Bonus: if I had my OWN truck then I wouldn't have to worry about other people breaking the tow truck so I can't work several days at a time. They've gone through a few clutches and transmissions and PTOs in the trucks when I'm not driving them. When I get in a truck it goes fine for me, though.

Anyway, I've been looking at various diesel F Super Duty rollbacks from 1987 to 1997. The common theme on these is that the fenders and hoods and doors and bodies are rusting out, the frame and bed are solid, the engine and transmission generally work well, but they look terrible, at least the ones I can afford. I'm thinking, why not merge a nice solid metal 67-72 cab with a newer truck? I won't have to worry about the bed or anything, and the F Super Duty trucks have a 34 inch wide frame that is straight rail, which should match up nicely with a 67-72. I found a picture of a flatbed 1971 truck and cleaned it up a bit and this is what I'd want to build:



And this is what I'd likely have to start with, this is a $7000 rollback that drives fine but doesn't look so hot:



I've been looking at several of these, all F450s (F Super Duty they called them), 7.3 diesels, 1992-1994 non turbo, 1993-1994 turbo, 1994-1997 powerstoke, 5-speeds, either white or red, with 19 foot beds and wheel lifts. The truck would remain as useful, it would just look better as a classic truck. And I would fill in empty spaces under the deck with mesh baskets or toolboxes or quarter fenders, and I was even considering a nice auxiliary fuel tank too. The stock 18 gallon tanks front and rear are kinda small and the Aerotank replacements 40-45 gallon are pretty deep and may not clear the bed's hoses and cylinders and fittings.

But should I make it work, it'll be beautiful. Goal here is to buy one truck, work it for awhile and pay off any debt I incur to get it, get a second truck, work it for awhile, have another driver run the first truck, get a third truck, have a driver take truck two while I run the third and get it ready for work, then get a fourth truck and run it, have the driver from the first truck move to the third, and I would tear truck one down to do the cab swap and refresh. LIKELY if the 7.3 is tired at all or the trans/clutch need changed out, I will just do a Cummins 12V swap. No sense keeping a 444cid monster in there.

I will say the 1992 F450 7.3 IDI/5-speed with 5.13 gears is easily able to return 11mpg if I never exceed 55mph. I drive like a grandpa. If I keep it under 45mph it'll do 13mpg. No joke. It needs gears, BADLY. 45mph is 2200rpm! 55mph is 2700rpm! Redlines around 73mph!!!

[sargentrs]

Why not try to locate an F-350 DRW bump, swap in a 6BTdiesel and fit a roll back on it. The whole would probably cost less than what you'll pay just for the truck you're wanting to swap cabs on. There's a couple of diesel flat bed swaps already on here for information you'd need. There was even a really nice F600 posted for sale a while back for a ridiculously low price.

[occupant]

In order to fit a 19 foot rollback bed it would need a minimum 120 inch CA (back of cab to axle) length which translates to probably a 201 inch wheelbase (based on a 2005 Super Duty brochure, I can't find older ones). The 1972 brochures on this site show long wheelbases as 131 inches (8 foot bed) 135 inches (9 foot bed) 159 inches and 164.5 inches (for chassis cabs). That means I would have to stretch the frame 42 more inches and lengthen brake lines, fuel lines, driveshaft, and wiring harness to match. Great if I can afford to pay someone to do that for me at a shop. So not great. I see F600 models were available with a 194 or 212 inch wheelbases but those are not as nice looking as the F100-F250-F350 trucks.

I also want the disc brakes front AND rear, rear ABS, 15000# GVWR, HD springs/shocks, 10 lug wheels, and all that was offered on the 1987-1997 Super Duty. I believe the highest GVWR on the 67-72 F350's was a mere 10000#. Actually I'd be happier with a truck that can have a 19500# GVWR like the 1990-2001 Chevy/GMC 3500HD's. I'd even use one of those chassis (also have 34" frame rail spacing)

[sargentrs]

Well, then I guess that won't work. I know there's a thread on here of somebody mounting a bump body on a 2002 F150 chassis so anything is possible.

[occupant]

Well, then I guess that won't work. I know there's a thread on here of somebody mounting a bump body on a 2002 F150 chassis so anything is possible.

That's the one I'm hoping will help me and it's bookmarked on my laptop. I figure that's an F150 half ton frame so the F450/SD frame will be even straighter and as I recall he had more trouble with his bed than anything else, and I won't need to mount a bed because the rollback body will be back there

I may also be doing a Cummins swap at the same time, but it would also require a gear change. Those SD axles are Dana 80s in a unique carrier. The only two ratios you can get for them are 5.13 (which the company's rollback has) and 4.63 (which is evidently harder to find). The other Dana 80 axles from other makes can swap in ring/pinions of much better ratios. S110/S130/S135 axles also have better ratios down to about 3.07 which would be great. Cummins engines like to cruise at a lot lower rpm (about 1400-1500) than say, a 7.3 which likes 1800-1900rpm.

[sargentrs]

Sounds like you know what you're looking for and a plan for how to get it done. Be sure and start a build thread for us. I'm sure a lot of people would like to follow that. Good luck!

[guhfluh]

I wouldn't say a 12v Cummins likes to cruise at 14-1500rpm. I would say it likes 1800-1900rpm, where it is designed to make peak torque. It is less efficient below and above that rpm. Changing the static timing around can move the rpm range though. Also, from what I've seen, it isn't going to be easy to fit a 6BT Cummins and the required fan and coolers in front of it for it to be used as a workhorse without overheating or melting down from high EGT's. Cutting into the firewall is probably going to be required if you want to fit enough radiator and intercooler and fan in front of the engine to be able to use a Cummins hard. If you opt for a non-IC, rotary pumped 6BT, and keep the fuel down near stock, it should live a happy life and give good fuel mileage, but be down on power compared to later IC versions. You should be able to fit a good radiator and fan in there to keep it cool, without cutting also.

Just my opinions.

[occupant]

Exactly, Randy. And once I get a rollback of any year or make, I'll be whipping out the tape measures and phone camera to figure things out. Whatever it is, I'm sure a bumpside cab and front clip could be wedged on there some way. I'm also considering just getting a rollback to work the heck out of and another older simpler one for lighter duty stuff, at which point a 10000# GVW would be close enough. I could upgrade the brakes to discs all around, change wheels/tires to 19.5, replace the rear springs with something beefier, probably change front springs as well, all this using parts from 14000-16000# trucks, so I would be comfortable running around 12000#. To put it one way, the 1992 non-turbo rollback we have scales 6480 with no passengers or fuel (but with all the tools and equipment in the truck that's normally in it). Add 34 gallons of fuel and 18 stone of me, and that's running about 6900-7000 without a car on deck. I doubt there is much weight different between the cabs. So if I claim 12K GVWR then I can easily have 5000 on the deck and that's enough. I'd probably run that without a wheel lift since it would add more weight in back. But the main truck needs to be rollback with wheel lift, 15K GVWR minimum (F450 or 3500HD). I'd love to make a 94-02 Dodge rollback but the GVWR is usually 10500 or 11000 because they are class 3 trucks (the 4500/5500 models didn't come until way later), and those later 3500 models only go up to 14000, still not quite high enough to cover 5000 on the deck and 3000 on the lift if it's a 7000lb tow truck.

I've read a LOT about Cummins engines and fuel mileage and there's a lot of great advice out there for Cummins owners, whether 12V, 24V, CR, or 6.7's. Here's a few snippets:

"The 5.9L Cummins H.O. inline six peaks its torque at 1600 rpm and redlines at 3400 rpm. In my experience, optimal fuel efficiency on the highway with this engine has been in the 1600-1900 rpm range, the best mileage achieved around 1600 rpm when not under load. Overall, for fuel mileage, the shifts points for this engine should be 1400-1600 when unloaded, 1600 if possible when loaded. There are times, of course, when the 1600-1900 rpm range is necessary to keep a load moving, and even higher rpm may be necessary for acceleration and climbing grades."

"Cummins actually recommends 2100-2400 rpm for peak efficiency in commercial use of the ISB diesel engine. They would like to see 2100 rpm at 65 mph and no operating below 1900 rpm under load at highway cruise speeds. In stock form, 1900 rpm netted approximately 69 mph. Switching to 4.56:1 gearing, 2000 rpm (with overdrive at 0.69:1) nets close to 65 mph. True to my expectations, my peak fuel efficiency is now at 65 mph or lower, a calculated change."

"If you have a 5.9L Cummins and would like to experience better fuel efficiency, watch your tachometer. If I creep over 1950 rpm, the price will be a linear increase in fuel consumption. By 2100-2200 rpm, fuel efficiency, reflecting load as well, begins to drop like a rock..."

I think the 1800-1900 sounds better. That also seems to be what the 7.3 likes.

Good news, while I was writing this post, the owners decided to put that rollback back in the transmission shop (since it's damn near impossible to downshift through 3rd or 2nd now) and while it is in there it will get a gear ratio change. They are changing the carrier too. 3.73 if they can find one, 4.10 or 4.11 if not. Anything else isn't worth the money to do a ratio change.

[guhfluh]

I was speaking of 12v P-7100 pump 6BT's static timing liking 1800-1900rpm. From what I've seen, 2100rpm for it is way out of its efficiency range unless modded. VE pumped 12v 6BT's may like a little different(lower rpm) and get better mileage while unloaded from their variable(with rpm) injection timing, very large turbine housings(some models) and no IC plumbing restrictions(some models). Commonrail, ISB, ~2003+, electronic Cummins are a different animal that I'm not very experienced with on mileage and efficiency rpms, etc., but it makes sense that by basic design ~1900rpm is peak torque and efficiency. They also have some reliability issues that make a lot of die hard Cummins fans want to swap back to a 12v. For a workhorse, I'm not sure I'd want a Commonrail or VP44 Cummins. For a daily driver, play toy and fun truck that I'm not wanting to work hard for a living, I'd love a Commonrail, 5.9 or 6.7.

[occupant]

Yup, that was a common rail most of those quotes came from, a guy who had a 4x4 with a lot of upgrades (lift, tires, etc) and he was pushing 9000lbs without a load. I figure the rollback will weigh 6500-7000 empty and typical vehicles I haul long distance are one of four kinds:

a) Mini Coopers (2300-3000lbs), 3/5 Series BMWs (3100-3900lbs), Audi A4/A5/A6 (3400-4000lbs), and Volvo sedans (3500-3800lbs) for a dealer when they finish fixing a car and want it reunited with the owner (typically over 100 miles away)
b) ridiculously priced exotic cars like Ferraris, Lamborghinis, Aston Martins, Maseratis, and Bentleys, which weigh anywhere from 3000lbs to 4500lbs and then Bentleys are just heavy as heck, 5000+lbs
c) people who break down and need towed all the way home and these usually end up being heavier SUV-type vehicles like Pacificas, Durangos, Navigators, Tahoes, and the like
d) people who had a rental car break down on them and need a replacement out in the middle of nowhere, these are usually light cars, Focuses, Cruzes, Altimas, Jettas, Captivas, Elantras, Impalas, other small SUVs, sedans, and minivans, etc...but there have been a couple times I needed to haul a rented Suburban, Expedition, or 12-passenger E350 van and those are 6000lbs or slightly over it

so I want to have that extra GVWR to cover those 6000lb vehicles...

I would be happiest with the simplest Cummins swap out there, and as far as that end of it I've already decided 12V is the way to go, and I while I'd want the VE pump to both make power for towing and be efficient empty, the P pump gives me more headroom for power should I need it and can be efficient enough. All it has to do is be better than the 7.3 for fuel economy empty and loaded. 24v and CR just complicate things. Actually would probably run the 7.3 as long as possible so I can gather all the Cummins swap parts over time. I know I can get 11-13 with horrible gearing on a 260K mile 7.3 and once I get a chance to drive the '92 rollback with changed gearing I'll be able to see how good it can get the way I drive, I'm expecting 12 loaded 15 unloaded if they can get it to 4.10/4.11 and possibly 16 unloaded with 3.73 gears.