Geoff Weeks

My white one, the cab is so far gone that the upper cab is loose from the lower (supposed to be welded together) I have another cab for that one. The winch truck is as close to "all original" as one can get. The wire type used in that period deteriorates over time, and making new is the best way, eps if you can get it out in one piece and copy. You have the choice of modern wire or modern wire that has an older looking fabric outside for period look. I have never done the matching numbers/colors wire type "restore" but my hats off to those that do.

Clean looking for its age.

Can't hurt, air tool oil, but I never did.

Best thing that can help is a picture of the rear end from the front, any casting numbers you find on the drop-out (center section, hogs head, pig or what ever slang is in use in your area) and same for the transmission. We need to ID these two items in order to see what can replace the current gear sets with the least amount of changes. A check on line of the Mack model numbers leads nowhere for either the trans or the rear. If you are lucky, Mack didn't make them, but bought a vendor supplied unit, and we have to figure out who that was and what they call that model.

Trailering might be the cheapest way to go. If distances are long the fuel bill is going to be big even if you could drive it. I spent some time looking through a diff catalog. There are less that 1/2 dozen front loading diffs with a 5.13 ratio. SO if it is front loading AND we get some pictures, we MIGHT be able to find out if there are other options. Trans change is likely going to be part of the equation also, with that come possible bell housing and most likely driveshaft changes. All would be hidden underneath and would look stock from the outside. I agree I wouldn't want to cut up something like that, and things like tread width can vary leaving you with an axle that sticks out too far if the whole rear is changed. Any change is going to a an expensive undertaking, but knowing what might be possible will help make the choice of what to do easier.

Actually no most of the hyd braked 2 spds are in the 6.14 high to may be mid 5's 17K (or there abouts). Once you jump up into the 20K axles most are air braked. If you could find a hyd braked axle in that range, than changing the gearing would be easy. The 20 K axles use 16 and 16.5 inch ring gears and are also in common with the 38-40K tandems, The lighter axles often found in Loadstars and the like are 15" ring gear. There are much more limited range of ratio's in the smaller size. The carriers do not have the same bolt pattern and you can not substitute one for the other in the axle housing. The reason is most gassers (which would use hyd brakes) have a gov speed in the 2600 to 3000 range. I had an 2 spd out of a Loadstar and I know. The heavyer axles, like the 20K are the same center drop-out as the Eaton/Dana tandems in the 34-40K range. Far more ratio's were made for those axles than for the lighter one. That said even in the heavier axles very few were made with the high range in the 3's, they are out there but not common. th When I put 2 spds in my Marmon I had to re ratio to get what I wanted. None could be found already with the correct ratio. I agree a two speed would be the ideal way to go, and a photo of the drop-out center section (and any casting numbers on it) may show if that is an option. A five speed with O/D in top gear of around .73 would also do, as long as its lowest starting ratio keeps the overall reduction about where it is now. Both options may require new driveshafts, which can get expensive. Mine for my K just set me back over $800.

I took a look though my old (er) truck manuals to see if I could find something where the trans ratios were the same. I was unsuccessful, I did notice that your trans ratios are fairly close together. Raising the rear ratio may make it hard to start on a hill unless a different trans with a lower starting gear is used. Most 4 and 5 speeds have from the mid 5's to the low 6's for the bottom gear. Granted a gasoline engine in the 700 CID range has a lot of bottom end torque, the clutch could take a beating if the overall reduction in bottom gear is raised significantly, as would be required to gain 15 mph of road speed. The "massive" torque available from a 700 cid gasser may be why a trans with deeper reduction was not used, too easy to blow it apart with deep reduction gear sets. If you did find a trans with deep reduction, you would have to be careful in lower gears not to rawhide it. For the above reason, I would look long and hard at finding a different trans to use and not try and do all the change with the rear ratio alone. Keep in mind changes to the trans will effect the pump drive as well. One problem is my manual collection is more recent, from the times of diesels and road tractors primarily. I do have a set for my forties K IHC's and nothing matched up there for a 4 speed.

I come up with 4.05:1 @2100 would be 60 MPH with 9.00x 20 4.19:1 with 10.00x 20.

Before you pull the trigger on a rebuilt head, look and see what can be found in "running core" or used engines. If you have a place to work and store indoors, a spare engine is always handy. I purchased a running core that needed a water pump and a little freshening for less than the head cost. It did turn out the engine needed a fuel pump (855 Cummins) but it ran up and down the road as installed. Point being having a whole engine, even if it needs work, can be helpful to have a ready at hand set of cores to send off or repair yourself. I spent the last 30 years, being a sole proprietor, and almost exclusively doing my own work. Having back-ups so to not disappoint customers when the un planned for happens can be worth the cost.

Doing a quick internet search, I am not getting a hit on either of the Mack numbers. My guess is they are not Mack mfg'd but vendor supplied. I have no documentation to support this belief, but it is more based on it being of the age and size truck that there were plenty of suppliers that they could use. The Mack mfg axles that I know are heavier than that would have, same for transmission, 5 speed is the fewest number of gears that I know Mack made. For example: My IHC K-7's have a IHC number for the rear axle, the housing has the IHC logo on it, but the guts are Eaton 2 spds. If we can determine what the guts are, we MAY be able to come up something. I qualify that a bit because there may be a limited number of ratios made. 5.13 was a fairly "fast" ratio for the day, and there may not be any faster. Higher road speed was achieved with O/D trans, aux trans and larger tires.

1st check the present tire/rim clearance on rear springs and tie rod ends. Make sure there is room to go up in size before buying and finding out you have a rub. Also check the dual spacing you have now, to be sure the wider next size up will not "kiss" going down the road. You can get 9.00 x 20 radials and 10.00x 20 radials, tubeless would be 10x 22.5 and 11x 22.5 respectively. Tire size will only get you one step in rear gearing, like going from 3.90 to 3.73, not much change, and not enough to get what he is looking for. A lot of mfg "re badged" units they did not make, with their own model and parts numbers. Unless someone with more Mack experience can tell us more, the next thing would be to photo the trans and rear axle to see if it can be ID'd. I wouldn't be surprised to find a Fuller trans and a Eaton or Timken rear.

Considering it had an SAE bell, it would be nothing to convert to an over center industrial clutch for its second life. I hauled some stuff for Vemeer that had an Eaton 5 speed used for reduction on a mud pump. I guess it was cheap enough to buy a whole trans already set up to the engine for reduction rather than build something else that had to be stout enough for the torque. IIRC it had a Cat engine in front of it. All brand new.

I am wrong! According to Wiki (and we all know they are infallible) the B20 came with en 291 gasser. at 107 hp, so a diesel of the same aprox displacement might have been installed in the smallest trucks. I stand corrected!

Only one place on the net did I find any spec's and it looks like 2100 rpm could be the max rpm, which is what I guessed at, as that is what larger engines like the 855 and 3406 max is. so a .83 or .73 O/D would get you near 55-60. to go more than that would require more changes.

Ok that is good news, you should be able to find a 5 speed OD that can be fitted. question is what trans model is in there now? Mack more than other mfg tended to use their own proprietary stuff, so someone with more Mack experience could be of help here. 9.00x 20 tires would raise the rpm @60 to 2657 rpm.

Back of the napkin math; assuming 10.00 x 20 tires and direct in 5th trans gives 2570 rpm@ 60 mph. I have no idea what the Mack engine top speed (gov speed) is, so that would be needed before we go further. A .83 O/D would drop that to 2133 rpm@ 60 So I am a bit confused by the 45 MPH max, either the tires are smaller, there is some other gear reduction or my assumed tire size is wrong.

The ideal would be a new center drop-out for the rear axle you have now. We would have to know what axle model is in the truck now to know what is possible. If you don't know or can't find out, a picture of the front of the axle may help ID it. Also where is 5th gear in the trans?

Is it air braked or juice brakes? If hyd it will be hard to find a new rear fitted out for hyd brakes that will meet all the other requirements.

Being a pumper, it just about eliminates and aux trans, as the midship pump drive takes the same real estate. That leaves tires (not much change available, looks to have 10.00 x20's), trans or rear ratio. Trans, is 5th up against the dash or down toward the seat? If toward the seat, a O/D trans may be an option, but be aware, that could change the speed to the pump as well. Have to make sure everything plays well together. Rear ratio, we would have to know what is there now to know if a different ratio can be fitted or if a whole new rear axle would be required.

Plot out any changes you are considering on paper first. Note how the truck does now over differing RPM, where it seams to pull the best, and how it does starting (esp under load if you intend to still pull a load). When I re ratioed mine, it took several sheets of graph paper and trying different ratios, looking at engine speed and road speed in each gear, both at speed and at starting. Lots of math, but when I was done, the truck preformed just as the math said it would. Seat of the pants, guesses often lead to unhappy results. Tire Rev/mile can be obtained from the tire mfg catalog, often available on line. If you have spoke wheels, you are limited to rims sizes for those spokes, to go bigger requires a hub change and that can be expensive and, on an older truck hard to find. Disk wheels you can jump more sizes, but still some rims (24") can be hard to come by and some tire sizes also. A simple O/D of .73 should put you around 55 MPH top speed, but check the driveline 1st, you may have to make changes to keep it from critical speed. The problem with gearing up in the front only to gear down in the rear, is much of the energy goes into heat in the boxes. In a low powered truck, you could easly end up with something that could go fast on paper but not in the real world. Air resistance increases to the square of speed, double speed and the air resistance goes up 4x. Add to that, the inefficacy of changing the driveshaft speed faster to slow it back down, and all the gains you "thought" you'd get can be eaten up in friction and air resistance. All this says is to do the math before you buy the first thing or turn the first wrench.

It has been well established, that it is a Herc, re badged with Mack cast into the manifold. Hp rated at less than 100HP .

1st take stock of what you have. tire size, power, transmission even tube size and length of driveshaft. As I found out, some of the old truck were driving their driveshafts into critical speed. Tires will only gain you a little. Rear ratio changes can make the most difference, with a small amount of change to the driveline. Adding an aux trans can be another way but you have to replace the driveline and set it up carefully not to overspeed the driveline. Of course this all assumes enough power to drive the truck faster. I have one truck that will top out at 68 MPH on paper but doesn't have the power to do more than 55. Little gained by making changes if the power isn't there.

The low output would make me doubt that. In the 40's my K-7's were 21000 GVW and had more HP than that. I just can't see Mack putting less than 100hp in any truck they made. I admit I have no paperwork to back my statements on what Mack did or didn't do. What were Mack gassers in terms of Hp?

Yep, they are easy to get out. Often when the pin is worn out so is the bushing. Since the ones in the springs are new, those should be fine. The fixed one on the frame may need to be replaced. IHC used threaded in all positions, others may not.

Is this what you are looking for? Not cheap, but quick way out. https://www.vanderhaags.com/detailview.php?part=24898211