Gas tanks, clutch issues and more in this installment of Military Vehicle's 'Tech Tips'

Holy Gas Tank, Batman

Q: The gas tank on my 1968 M715 is leaking badly and looks like it’s falling apart. Do you know of any company that sells replacement tanks for this truck?

— Luis Sanchez

A: Five-quarter fuel tanks are known for rusting and leaking, but you have several options. If you want to keep your truck stock, you could contact various advertisers in this magazine who offer M715 parts to see if they have an NOS or a good used tank. Depending upon the condition of your present tank, you could investigate various means of repair, including coatings touted as being able to seal leaking or rusted tanks. You could also search through truck wrecking yards to possibly find a tank that could be modified to fit. Or, if money is no object, you could have a tank custom built.

Bungle In The Jungle

Q: My grandfather is a Vietnam Vet who was drafted. He was not gung-ho about what he calls the “Bungle In The Jungle”, but he has always liked old military vehicles and I have too. We have two jeeps on our ranch, a Willys MB and a Willys M38. They are not restored but they are painted olive drab and mostly stock. I have a chance to buy a Kaiser Jeep M715 pickup. It has been re-powered with a Cummins JT 6-cylinder diesel engine and a Clark 5-speed transmission with overdrive 5th gear. It is an old conversion that was done in the 1970s and was by a professional truck mechanic. Except for this, the truck is stock and looks very nice. The truck seems to run and drive very well, but I’m worried because the engine is very loud and smokes a lot. The truck will run on the road at 60 mph in 5th gear, and has a lot of power off the road, but you need earmuffs to drive it. The guy says this is normal for this engine and the engine is in good shape. Have you had any experience with Cummins JT engines?

— Phil

A: As the cyber-kids say, “OMG!” When I think of all the diesel engines that could have been put in that truck, and someone chose a Cummins JT! In the trucking industry during the early 1950s, the JT was known for three things...noise, smoke, and lack of power. Like the GM 6-71, the Cummins JT could be heard and identified a mile away by its sound. In fact, if one wanted a diesel that “sounds like a diesel”, and also an engine ten times its size, the JT would be an excellent choice. While the power issue shouldn’t be a problem in an M715 - in fact I’d drive carefully in low range to avoid snapping axles or drive shafts — there isn’t much you can do to muffle a JT’s iron bellow, except maybe to pad the engine compartment and hood with fireproof sound-absorbing material. As to the smoke, assuming it’s not blue, which indicates that an engine is burning its lubricating oil, you would just have to live with it. Other than these faults, the JT isn’t a bad engine, and its bum rap in the power department was mostly due to its being installed in trucks that were simply too large and heavy for what it could put out. As I say to most folks who ask if they should buy a certain HMV: bottom line: If you like it, buy it!

A Fork In the Road

Q: I was driving my 1944 Willys MB last week. When I came to a stop sign and stepped on the clutch the pedal went to the floor and stayed there. No clutch. I crawled under and checked the linkage but nothing looked broken. But the clutch wouldn’t work and I had the jeep towed home. What’s wrong?

— Barry McFarlane

A: I had this happen on a CJ3A, but managed to drive it home without any clutch, though with considerable grinding of gears when shifting, and shutting off and starting the engine in first gear the few time I had to stop. I suspect your clutch release fork has failed, which is a fairly common occurrence on older model jeeps. The clutch fork is a steel stamping and pivots on a ball and socket point within the bell-housing. It’s common for the socket on the clutch fork to fatigue and wear through since no provision was made to grease it. While all one needs is a new clutch fork to fix the problem, it’s not a simple repair since one must usually pull the engine to replace the fork… though some folks have said they managed to do it through the access opening. You could also weld or braze your old fork, but you run the risk of another failure if the job isn’t done right. Put a healthy dab of sticky grease in the socket of your new or repaired fork.

Big Holes

Q: I have a 1942 Ford GPW that is my daily driver. The engine needed a complete rebuild, so I pulled it out and took it to a shop that does classic cars. The mechanic said they would have to bore the cylinders .080 oversize. My grandfather warned me that this is too much and the engine will overheat because the cylinder walls will be too thin. He had this happen with a 1946 Ford pickup. The engine in my jeep is original with the correct serial number, so I hate to replace the block. They can sleeve the cylinders but that is too much for my budget. Do you think it is safe to just have them bored to .080?

— Carl Weston

A: Your grandfather may indeed have had a bad experience with a Ford flathead V-8. Boring these engines over .060 did sometimes create overheating problems. However, it’s been my experience that L-head Jeep engines have sufficient cylinder wall thickness to tolerate being bored .080, so overheating shouldn’t be a problem. Of course, since your jeep has its original engine, you may want to pay the extra expense of having the, cylinders sleeved.

The Smell Of Rubber In The Morning

Q: My M151A1 has started having a strange problem. When I go out on the road in the morning the engine makes a squeaky noise and sometimes I can smell burning rubber. This happens when I step on the gas but it goes away when I back off. The engine temperature and oil pressure are normal, and the engine doesn’t misfire or lose any power. I have checked to find where the burning smell is coming from but I can’t find anything. Any suggestions?

— Mark Donaldson

A :My first suggestion would be to check the fan belt or belts. Loose or glazed belts will make a squealing or squeaking sound at higher RPMs and when the engine is revved. Loose belts will also give off a burning-rubber smell. Look for a lot of black dust around the front of the engine. This indicates that belts have been slipping. If the belts are lightly glazed on their inner surfaces, you can roughen them with sandpaper, but badly glazed or worn belts should be replaced. Also check to see if the water pump and/or the generator bearings aren’t binding up and about to seize, or if they feel rough. The pulleys should spin smoothly and freely when turned by hand. If they seem okay, you still may want to sand the pulleys if they’re also glazed. Hopefully, your MUTT’s fan belts simply need to be tightened.

How Hot Is It?

Q: My 1958 M37B1’s exhaust manifold seems to get very hot. It turns a light gray overall with a pinkish tan where the two exhaust ports are next to each other. The motor has been completely rebuilt with hardened seats and valves. It has about 500 miles on it. The distributor is advancing about 20 degrees. I set the timing, plugged the vacuum line going to the fuel pump for the wipers, and checked the intake manifold for vacuum leaks. I tested the fuel pressure, 3.5 PSI at idle and 4.25 PSI at high RPM going down the road. I adjusted the idle screw and installed a new muffler and exhaust system. I drove down the road at 35-40 mph for 2 miles, shut the motor off and coasted into the driveway. I pulled #1 and #6 spark plugs to read them: outer rings were black, porcelain clean, electrodes light gray to tan. I have built many motors and am 99 percent sure this one is together properly including valve clearance. Do you think valve clearance too tight would cause this problem? What other things could I check?

— Dan

A: So, what makes you think your M37’s exhaust manifold is getting too hot? If you painted it after rebuilding the engine, it’s normal for the paint to burn off and usually leave the colors you described. Does it seem to be getting hotter now after the engine rebuild than before? This might be due to the engine having more power than before. Gasoline engine exhaust can get hot enough to melt lead, but this usually means that the engine itself is running hot, which would be indicated by the temperature gauge, and/or a boiling radiator. Assuming the cooling system is in good condition, the usual things that can cause an engine to run hotter than normal are lugging, hauling a heavy load or climbing a steep grade, restrictions in the exhaust system such as a crushed pipe or the wrong muffler, a hot environment, too lean a fuel mixture at high speeds (sometimes caused by a vacuum leak) and/or improper engine timing. Retarded timing, for example, can make an engine run hot. So can timing that’s too far advanced, though you would usually hear an ignition knock. Improper valve clearance shouldn’t make the engine run hot, but if the valves are set too tight they may burn. Another thing to check is if the butterfly valve in the exhaust manifold — the one for warm-up — is opening all the way after warm-up.

Cracking Wise

Q: You helped me diagnose an engine problem with my 1945 jeep that turned out to be a cracked block allowing coolant to leak into the oil and cylinder. The crack was possibly caused by having the cylinder sleeved a few years ago, causing stresses that the cast iron could not stand. Since then, I have checked out my two spare old engines. One seems to be able to be bored and rebuilt (waiting for word from the shop) and the other had enough corrosion to need to sleeve one cylinder. When I had sorted though my options I talked with a so-called expert about repairing the first engine and he told me that these old blocks have problems with being sleeved, as the forces tend to crack the block, as I experienced. He suggested that these old cast-iron blocks need to have sleeves installed with less-than-normal clearance, and that they needed to be glued somehow. Since I don’t want to see otherwise usable blocks go to waste, what do you know about the best techniques to sleeve a cylinder without causing a crack?

—David Voit

A: This is like finding the right specialist for a major operation: you should have long conversations with people at different engine shops who have done this kind of work, especially on vintage engines. Get a feel for their experience and if they sound like they know what they’re talking about, and if they’re willing to stand behind their work.

New Daddy

Q: As a new and proud owner of a 1967 M35A2, there are a couple of issues with which I need help. First, the front axle will not engage or operate in an off-road situation. Is this operator error (or ignorance), or can you suggest a troubleshoot procedure? Second, this truck is multi-fuel: can you suggest a source to inform me of limitations for fuel used?

A : Congratulations on your new M35A2! You should get manuals for your truck. Most M35s of that era had automatic engagement of the front axle via a sprag-type clutch in the front of the transfer case, though many were later retrofitted with manual air shifts. I assume you’ve checked for obvious things, such as a missing front drive shaft, missing or stripped drive flanges on the front wheels, or a broken front differential? Assuming your truck has the automatic all-wheel-drive engagement, it’s possible that the sprag unit has failed. To check, jack the truck’s front end off the ground so one of the wheels can be turned by hand. With the engine off, put the transmission in reverse and try to turn the front wheel. It should turn backward easily but should not turn forward. Then shift to first gear: the wheel should then turn forward but not backward. If your truck fails this test, you may have transfer case problems and the truck should not be driven until the trouble is diagnosed and fixed. If your truck does have an air shift, you should check to make sure the shift mechanism is working. As for fuel, #2 diesel is the preferred fuel to use in the multi-fuel engine.