Tech Tips: Humvees go on, M135 engine swap and more

The most common vehicle currently operated by the U.S. military is the High Mobility Multipurpose Wheeled Vehicle, or Humvee. The Army owns the majority of Humvees, which number more than 100,000. The Marine Corps is next with 24,000. The other services together operate about 10,000.

The Humvee is also popular overseas; and more than sixty foreign militaries operate about 250,000. The company that builds the HMMWV, AM General, recently received a contract to produce up to 11,500 more in a variety of configurations for nine different U.S. allies and partners.

There is a plan to replace a portion of the U.S. Humvee fleet with the new Joint Light Tactical Vehicle (JLTV). The U.S. Army wants to buy 49,000 JLTVs over the next 25 years. The Marine Corps hopes to acquire 5,500 and shrink its Humvee fleet to about 13,000.

Still, it’s expected that the U.S. Army and Marine Corps will operate more Humvees than JLTVs for the next quarter-century; and by the time the Army finishes its planned buy of JLTVs, the average age of its Humvees will be more than 35 years. This is also what the Marine Corps plans to do, having canceled its Humvee modernization program in 2015.

AM General has continually evolved the HMMWV’s design and capabilities as required by the military to adapt the vehicle for changing conditions and tactical situations. The engine and transmission have been upgraded repeatedly.

One of the most significant enhancements was the addition of armor. About half the current Army Humvee fleet is now up-armored, with the addition of improved shocks and suspension.

The new Expanded Capacity Vehicle, or ECV, Humvee has a 6.5-liter turbo-diesel engine, a microprocessor-controlled engine, and increased payload capability. The ECV can support virtually all the different Humvee configurations. Red River Army Depot and AM General have a multi-year partnership to recapitalize Army National Guard HMMWVs.

I know it is easy to spot the difference between a Ford GPW and a Willys MB by the front frame cross member, but can you tell me some other differences? — J. Monroe.

This question has been answered many times over the years by experts, though with varying degrees of accuracy. Simply said, there are hundreds of small nuances between a Ford GPW and a Willys MB, but whether or not a typical WWII jeep is all Ford or all Willys after all this time — unless it’s been sitting in a museum — is subject to interpretation.

Complicating the issue is that most GPWs and MBs still in the wild are likely to have a combination of Ford and Willys parts, because they were repaired and maintained at government motor pools while in service, rebuilt at government depots, and then repaired and maintained by various civilian owners after being released from military service; and most of these owners were not aware of Ford and Willys differences when buying replacement parts (much of those being war surplus) and/or didn’t care. That said, here are a few differences.

Engine block serial numbers: These should start with either MB or GPW (though a civilian Jeep engine could have been swapped in since the war). Many GPWs ended the war with a Willys engine and vice-versa for the Willys MB.

Ford Script Bolts: GPWs were assembled with what are known as “Ford Script Bolts.” These bolts have a cursive “F” stamped into the head.

This F script can also be found on just about any part that came off of a Ford GPW. Several on the frame (like on shock mounts and other brackets) would indicate a GPW frame, and there are other frame cues.

The problem is F-script bolts and parts could be removed and re-used on an MB or a later civilian jeep.

Body Differences: There are some subtle differences between the later GPW and Willys military jeeps, but the early Fords and Willys were stamped as such on the left side of the rear body panel. That was before the military censored both companies, about mid-1942… no more free advertising.

Also, the impression for the rear toolbox opening button on a GPW should be square; round on an MB.

Lastly, the toe board gussets (support the firewall below the base of the hood) are different between Willys and Ford bodies.

There can also be “composite bodies” with parts from both Ford and Willys since neither company actually manufactured all the jeep body tubs. (Many were supplied by American Central Manufacturing).

GPW axle shafts may have an F-script.

Original GPW steering knuckles may have an F-script cast into them.

Frame differences: GPW frames have a rectangular upside-down C-channel front cross member, while Willys MBs have a tubular and round front cross member. Also, as previously mentioned, shock mounts and other brackets from a GPW will have F-script cast or stamped into them. This ranges from body parts to radiators, even some shims in the transfer case or axles will have a cursive F-stamped in them.

Both MBs and GPWs have small toolboxes integrated into the rear fenders. The recess below is where the button to open the door is located can be either squared off in GPWs or rounded. The rounded indicates either an MB … or a GPW with a composite body.

About once a year, the automotive gods grant me a reprieve from my own laziness and stupidity.

Lately, I have been dealing with a starter that sometimes functions. When it doesn’t, I go through the process of unhooking the battery and tracking down the grounds to clean them. It seems I have been going through the process a lot.

Doing the same task over and over sometimes leads to dropping one’s guard and becoming complacent. That is what happened to me.

Luckily, I am here to tell about it and possibly serve as a reminder to other weekend wrenchers.

I have disconnected my battery post clamp so many times that the tightening bolt just spins when I put the air ratchet on it. Realizing this, I laid the ratchet down (still with the socket on the battery post clamp nut) to retrieve a wrench.

After watching a bunch of sparks, I grabbed the ratchet by the hose and yanked it free from the ground it was making. As I did that, I felt a fine mist spraying all over me.

“Hmmm,” I thought, “I couldn’t have melted a radiator hose.”

Taking one deep breath, I realized what had happened. I had placed the ratchet right on the fuel injector rails!

Sure, I know better, but I became complacent from doing the same task over and over.

Maybe this tip can serve as a reminder to everyone to take the time to put your jack stands in place, disconnect batteries for simple tasks, check the pressure on shop fire extinguishers (or go out and buy one or two!), and make sure life insurance is up-to-date. — John Adams-Graf

I own a 1951 GMC M135 that has been dormant for years.Rather than spending time on the original in-line six, I’m considering replacing it with a small block Chevrolet 350.

Do you feel this is a feasible endeavor? My main concern is the V-8 compatible with the original transmission? — Doug Kline

I’ve seen this swap done several times with varying degrees of success. Sometimes, it didn’t seem worth all work it took to do them.

In many cases, the swappers either expected too much in the way of power, and/or lacked the mechanical and fabricating skills to do the job right.

The first thing I would ask myself is, “Do I feel I have all the skills to make such a swap and do everything right?” This would include fabricating new engine mounts, plus all the plumbing and engineering needed to put a totally different engine into your M135.

Then, there is the fact that your truck would no longer be desirable to many HMV enthusiasts as “original,” should you want to sell it.

As far as power and performance, there isn’t going to be a fantastic difference between the stock 305 GMC six and a Chevy 350 V8. If I was going to do all that work, I would probably opt for a dedicated truck engine or some sort of diesel.

On the other hand, a Chevy 350 could be fairly easily fitted to the original automatic transmission, though this would still leave you with the task of making new front motor mounts, throttle linkages, and adapting the radiator to an engine that is considerably shorter than the original. Then you can worry about installing the air compressor.

As I say to most queries of this type: Bottom-line — it’s your truck.

I paid a shop a lot of money to rebuild the cylinder head on my M151.

My MUTT is a daily driver, and about a month later it developed what sounds like a rod knock. The shop guy said that rebuilding the head raised the compression and put more strain on the engine’s lower end. Therefore, he’s not responsible.

What is your opinion?

The shop may indeed be responsible for your MUTT’s engine knock, but maybe not for the reason they gave you.

What happens a lot these days is that mechanics remove cylinder heads without completely draining the coolant from the engine block. The coolant then spills into the bottom of the engine. Even an oil change may not get all glycol antifreeze out of the oil pan.

When even the smallest amount of glycol antifreeze and water mixes with motor oil, it produces an acid that attacks engine bearing material.

If a CSI team were to autopsy your MUTT’s engine, they might find dark spots on its rod and main bearings. These spots indicate that acid has softened and eaten away some of the bearing material.

If this were the case, it would be the reason your MUTT developed a rod knock shortly after the head was rebuilt.

But, it would probably cost you more time and money to prove it, than to simply take your knocks, chalk it up to experience, and replace your MUTT’’s rod and main bearings. My advice is to do this before driving your MUTT any more, or you might end up having to replace the entire engine if it throws a rod.

ALWAYS completely drain engine coolant before removing a cylinder head.

Quick tip: Wiper blade efficiency can often be improved by lightly and carefully wiping the blade’s rubber with fine sandpaper or emery cloth.

I have a mostly stock and unrestored 1942 Ford GPW. My grandfather bought it as Army surplus in 1948, and it has been on our ranch ever since.

The engine has been rebuilt three times. The transmission, transfer case, clutch, and other things have been rebuilt or repaired when needed.

My question is: Do distributors ever wear out? If they do, how can you tell? – Dean

Distributors do, indeed, wear out. The primary wear in distributors, such as used on your GPW, occurs on the shaft bearings and centrifugal advance mechanism. As the shaft bearings wear, the shaft starts to wobble as it rotates. Though this wobble may be slight, it’s often enough to cause an erratic dwell — meaning the point gap changes — which can result in a rough idle and/or engine misfiring that can’t be corrected by carburetor adjustment. Wear on the centrifugal advance mechanism can result in hesitation upon acceleration and sluggish or erratic engine performance.

Rebuilding a distributor is not rocket science, and just about anyone handy with tools can do it. Complete rebuild kits for many HMV distributors and most other vintage distributors and parts may be purchased from companies that advertise in this magazine. The parts of many vintage distributors made by the same companies are interchangeable, and good low-hour vintage distributors my be found at wrecking yards as well as online. 

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