Tech Tips: September 2019 Edition

by Steve Turchet

COMPENSATING FOR SOMETHING

I just bought a 1967 M35A1 with a multifuel engine. The truck was used on a farm but not very much and is almost completely stock. I am going to restore it to a Vietnam appearance.

The engine runs well, has lots of power and doesn’t smoke a lot, but I started getting advice on military vehicle Web sites to adjust or bypass the “compensator” to get more power. What is the compensator, and should I mess with it?—Zach Whitmore

For some folks, getting an M35 style deuce is like installing a new browser or application program; they seem to want to jump right to “Advanced” or “Custom” settings without first finding out if the default setting might serve them just as well ... or maybe even better.

With multifuel deuces, it sometimes seems to be forgotten or ignored that, except for a few faults and quirks, they were one of the best military trucks in the world. There are many conflicting opinions and advice about compensators on multifuel engines; and with free advice—including mine—one usually gets what they’ve paid for. Anyway...

During the latter part of the 1950s, concerned about the potential devastation of nuclear war and the resulting havoc to supply systems, the U.S. military wanted vehicles that could run on several different kinds of fuels, such as diesel, gasoline, kerosene and jet fuel ... even antifreeze mixed with engine oil.

Although the Reo Truck Company, which had originally designed and built the M35 (or G742 family of vehicles), submitted a design for a multifuel engine, the contract was awarded to Continental, which built so-called “Hypercycle” engines under license from M.A.N. of Germany. There were several models of these engines, and the fuel-injection pumps on most but the early LDS-427 were equipped with a fuel density compensator.

This compensator varied the fuel delivery rate when different fuels were used. Basically, it allowed the engine to perform about the same no matter what kind of fuel was used—though the best and preferred fuel was always diesel.

Normally, no adjustments were needed when changing or mixing fuels. In theory, the trucks were simply fueled with whatever approved fuels were available, including a mix if necessary, and then went out on their missions. In practice, this wasn’t entirely successful; but since (thankfully) a nuclear war didn’t happen, there was usually an adequate supply of diesel fuel so the engines’ multifuel capabilities were seldom tested. In fact, toward the end of the multifuel era, the fuel density compensators on some vehicles were bypassed, and the engines were then supposed to be run on diesel fuel only, although there were usually directions on how to reactivate the compensator if needed.

You will get tons of advice on this topic (and whether or not you want it) some of it good, some of it not so good. Mine is this: If it ain’t broke, don’t try to fix it. If your engine is running well, seems to have sufficient power and doesn’t smoke much, it might be best to leave the “default” setting in place and just enjoy your new deuce. I’m sure you’ll find plenty to work on without messing with the compensator. Of course, get service and operation manuals.

GOOD OLD DEUCES

I enjoy all of your articles and tips. I want to buy a Korean War M35 deuce-and-a-half and restore it. I have seen lots of M35s for sale in your magazine and also at MV shows but most of them are either newer diesel-powered trucks or multifuel powered from the late 1960s and early 1970s. I haven’t seen a gas-powered deuce-and-a-half at any shows I have gone to. Was there something wrong with these trucks?—M. DeFassio

Thank you for your kind words about my writing. For their time—the 1950s and early 1960s—the Reo 2-1/2-ton trucks, first powered by the Reo OA 331 gasoline engine, then the COA 331 (which was the same engine built by Continental) were very rugged and dependable vehicles. The engines had replaceable cylinder sleeves and seven main bearings, and were extensively used in the civilian trucking industry during those times, including in the White Compact, which was widely used as a garbage-packer and starred as the dreaded “Scoops” in the movie “Soylent Green.”

Though underpowered by today’s standards, there was nothing “wrong” with the first M35s, which soldiered very successfully through the Korean War into Vietnam. I personally owned one and have driven many others. If I was going to restore an M35 style deuce it would be one of these.

As you say, there doesn’t seem to be many restored examples, as most folks seem to opt for either the multifuels or later straight diesel-powered units, and it would be nice to see a good example of this seemingly neglected era of M35 history.

SHOW YOUR AGE WITH PRIDE

In your article on Parking Brake Service (MVM no. 131), you mention that turning the brake drum to true up the surface is not recommended because the shoe (or band) might not be able to be adjusted properly. Not to show my age, but in my day when this was done they added a shim behind the lining equal to the amount taken off the drum. However, most shops today are probably not familiar with this procedure.—Leonard McCabe

If folks like you with experience don’t “show your age”, how will younger people learn anything? Of course you are right about adding shims to compensate for a parking brake drum being turned, but you are also correct in saying that many repair shops today wouldn’t know that. Thus, it’s important for those with experience to share their knowledge with younger people who may be just now buying a vintage MV and don’t know the tricks and procedures for keeping an older vehicle in good running condition. Thanks for sharing your experience!

HOT STUFF

I want to install a G.I. issue heater in my 1959 M37. I understand that there are two kinds. One uses coolant from the engine and one burns gasoline. Which kind would be best?—TJ

If you live where the winter temperature often drops below freezing,the gas-burner would probably be your best choice. For more temperate climes, the other type should suffice.