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Answers to All Your Jeep Questions

Posted in How To: Tech Qa on March 2, 2018
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Metric Tire Converter
The tires on my Jeep are metric. Is there a mathematical solution to find out what size they are in inches?

Rick Philippi
Via email

True tire diameter has been a heated topic in the Jeep world for as long as most of us can remember. The actual tire diameter of a metric tire can be incredibly confusing to understand, especially since none of the numbers stamped on the sidewall identify an actual height. Even more unfortunate is that it’s not unusual to roll two tires of the same size from different manufacturers next to each other, only to find that one is bigger. The good news is that most tire manufacturers have websites with detailed specifications for each tire model and size, which provide all of the pertinent dimensions. If you don’t want to hunt down the manufacturer’s specifications or your tires are discontinued, you can calculate the diameter of your tires with some basic math. First, you need to understand what each of the metric sizing numbers and letters means. We’ll use a LT315/75R17 tire as an example. The letters in front of the numbers identify the tire type. An LT means light truck and a P means passenger car. As you can probably imagine, the LT tires are generally more heavy-duty than the P-series passenger car tires that come on many Jeeps and other 4x4s from the factory. The first number identifies the section width in millimeters. So our example tire is 315mm wide from sidewall to sidewall, which is 12.4 inches. The two-digit number after the slash mark on a metric tire size is the aspect ratio. It’s the ratio of the height of the tire’s cross-section to its width. In our case, the tire sidewall dimension is 75 percent of the tire width, which is 236mm or about 9.25 inches. This number is multiplied by two because there is a sidewall on either side of the wheel that makes up the total tire diameter. The R simply identifies the tire as a radial design. The final number is the wheel diameter. For this application we have a 17-inch wheel. Next, we need to add all of these numbers together to get the actual height in inches. An equation of sidewall height + sidewall height + wheel diameter = tire diameter. For us, that would be 9.25 + 9.25 + 17 = 35.5 inches in diameter. So our LT315/75R17 is actually a 35.5x12.40. Again, there is some tire size variance from manufacturer to manufacturer. Another, much faster method of converting the actual tire diameter of metric tires to inches is to use one of the many tire size calculators that can be found online, such as tiresize.com.

4BT Commitment
Has anyone has done a V-6 to 4BT Cummins engine swap? What would I need and what is recommended for the swap?

Ian Greenwald
Via email

Swapping a Cummins 4BT four-cylinder diesel engine in place of a modern gas engine isn’t all that difficult, mostly because the 4BT is an incredibly simple wood chipper of a diesel engine, but it does require a bit of commitment. If you are not familiar with the Cummins 4BT, you have to realize it is an industrial engine. It was never designed for passenger vehicles. The RPM and power band is very narrow and the vibration and noise alone will scare off most potential 4BT engine swappers, especially if the swap is planned for a daily driver. Now, if you can get past the narrow power range, noise, and vibration, the 4BT provides incredible low-end torque, which is great for towing and technical off-roading, but abusive to drivetrain components. Because of the output shaft twisting low-end torque, a 4BT will generally need to be backed up with a heavy-duty transmission, such as the GM TH400 three-speed automatic. The 4BT and TH400 engine and transmission combo can be found in some delivery trucks. It’s become a coveted pairing, so a bargain price isn’t likely unless you can pick up the whole truck from someone that is unaware of what they have. You’ll need an adapter to mate the TH400 to your transfer case, and you’ll likely need new driveshafts once you get the engine located in the chassis. Other components you need to locate and install include motor mounts, an intercooler, exhaust, a proper radiator, miscellaneous hoses and fittings, and so on. If you’re getting the idea that doing an engine swap is expensive, you’re right. A professional shop might charge in the neighborhood of $20,000 to $30,000 for a complete turnkey 4BT engine swap. A home swap will less, but the price will vary depending on options. Companies like Jeff Daniel’s Jeeps (jdjeeps.com) specialize in Cummins 4BT diesel swaps. The company offers swap components like 4BT motor mounts, adapters, and fuel tank conversions. Jeff Daniel’s Jeeps can also perform a turnkey 4BT diesel swap for you.

Fueling CJ-7
I have an old basic CJ-7 question. I'm rebuilding my ’77. It has a ’86 4.2L motor. I'm replacing the fuel lines with pre-bent lines (I'm lazy). There are a couple choices based on the production year. Do I buy the same year lines as my frame or do I match the motor?

@si_vis_pacem_
Via Instagram @cappaworks

Corroded, dented, and damaged hard fuel and brake lines can cause all sorts of trouble. Fortunately, very few vehicles enjoy as much aftermarket support as the older Jeeps like the CJ-7. There are several companies that offer pre-bent individual lines as well as complete hard tubing kits for the older Jeeps. Companies such as Classic Tube (classictube.com) offer all of the pre-bent fuel lines, brake lines, transmission lines, and so on for many different Jeep models. Just as in your case, many older Jeeps have aftermarket modifications made to them, which could cause the pre-bent factory replacement lines not to fit. Suspension, engine, exhaust, and transmission modifications may require you to modify the new lines to fit properly. Given your situation, I would recommend that you start with the new pre-bent lines that match your chassis and fuel tank and then modify them as needed to feed fuel to the newer engine. Doing it the other way around would likely require much more undoing of bends and fitting replacement.

Fuel Spray Away
The hard plastic fuel lines on my ’93 YJ are splitting and have started spraying fuel all over. Can I just replace them with rubber hose and clamps or do I need to stick with the same type of hard plastic?

@jdamianc9
Via Instagram @cappaworks

The hard plastic fuel lines found on some Jeeps can be problematic after they have been out in the elements for several decades. They often require special tools to properly connect and disconnect them without damage. There is no reason you can’t replace the factory plastic lines with the proper rubber fuel line. You should use high-pressure fuel-injection hose on fuel-injected applications. Along the frame you should consider using stainless steel tubing or standard steel tubing and then attach the flexible fuel-injection hose where the connection points are. Carefully route the lines away from heat sources and areas where they might be chafed or crushed. You can protect the fuel lines from heat with Heatshield Products (heatshieldproducts.com) Lava Tube or other protective thermal sleeves. Be sure to use the proper barb fittings with your fuel-injection hose. With the proper fittings, the standard hose clamps should have no problem with the 40-90 psi that most high-pressure fuel injection-systems require.

Rust Prevention
When doing bodywork and finishing a patch panel, I know I should prime and paint it to keep it from rusting, but is there anything else I, as a shadetree mechanic literally working in my backyard, can put down as a base or first coat to protect the metal even more?

@mhack73
Via Instagram @cappaworks

Making at-home bodywork survive the elements can be tricky. As with any paintjob, it will only be as good as the prep work. Cheap materials and cutting important paint prep corners will be the reasons for the paint to fail. However, it’s more likely that what you can’t see will cause a problem. You can paint and coat the exterior of a welded-in patch panel to protect it, but the hidden part on the inside is probably bare steel. This is where your rust problem will begin, and given the right conditions (salty and wet), it will eventually eat through like an alien popping out of John Hurt. Some internal body panels are near impossible to reach, while others can be accessed by simply removing an interior panel. Laying down the proper coatings on the hidden backside will ensure that your repair panel lasts just as long as or longer than the factory body part. If you can’t access the inside of the repaired panel, don’t worry; most inaccessible body areas have drain holes. Eastwood (eastwood.com) offers Internal Frame Coating. It’s designed to coat the inside of boxed framerails, but I don’t see why you couldn’t use it for the inside of hidden body panels. The Internal Frame Coating comes with a special application hose. You can snake the hose into the hidden body panel area through a drain hole and coat the bare metal to help prevent future rust. Pay close attention to the body seams in your repaired area as well. Water, salt, and other debris can accumulate in unprotected seams, which will surely lead to rust.

Dana 50 Swap Possibilities
I have a ’71 Jeep J-truck. It has the 5-lug semi-float Dana 60 rear axle and the less desirable closed-knuckle Dana 44 up front. I have been contemplating making a Ford Dana 50 front axle work. I can narrow it and make it 5-lug. The things litter the earth. It has to be better than the older Dana 44 right? Plus it’s high-pinion. The Ford high-pinion Dana 44 front axles are hell to find. Every goon with a 12-inch-lifted XJ wants one. I have even thought about using the Ford Dana 50 as a high-pinion rear axle for a flatfender. In a lighter, low horsepower CJ it should survive fine and solve steep driveline angles.

Aaron Holdaway
Via facebook.com/JohnCappa4x4

The closed knuckle Dana 44 found in ’73-and-earlier Jeep J-trucks isn’t known for being incredibly strong. It shares many parts with the tiny Dana 25 found in the early CJ models. I kept one alive with 37-inch tires, a V-8, and a manual transmission, but it was an exercise in restraint and it had an open differential. I was walking on eggshells off-road most of the time. Another drawback of the closed-knuckle design is that it doesn’t turn very sharp. Newer open-knuckle Dana 44 axles steer much tighter. An axle swap makes a lot of sense.

Heavy-duty high-pinion junkyard 4x4 front axles have become increasingly difficult to find, but you are right about the Ford Super Duty Dana 50. It might be a good swap candidate for your applications. It’s kind of a 3/4-ton in-betweener axle that has both 1/2- and 1-ton features. The Dana 50 has a ring gear diameter of 9 inches compared to the traditional 1/2- and 3/4-ton Dana 44 with a ring gear of 8.50 inches and the 1-ton Dana 60 with a 9.75-inch ring gear. The Dana 50 pinion diameter and strength is similar to the Dana 44. Both the Super Duty Dana 50 and Dana 60 have 1.50-inch-diameter axleshafts, but the Dana 50 inner axleshafts are machined down to 1.30-inch on the differential side, which is the same as a Dana 44. The Dana 50 features 30-spline inner axles, where the Dana 60 gets 35-spline inner axles. Both the Dana 50 and Dana 60 have the same steering U-joints and 30-spline outer axles that can be converted to 35-spline with bolt-on parts.

The Ford Super Duty Dana 50 can be swapped into a ’73-and-older J-truck, but there are a few hurdles to get over. Perhaps the biggest problem is that the Ford Dana 50 has the differential on the driver side and your J-truck has a passenger-side drop transfer case. It’s not a totally insurmountable problem, but the two possible solutions add cost and complexity to the swap, potentially negating any savings from using the junkyard Dana 50 axle to begin with. You could swap your transfer case out, which will likely require driveshaft modifications, an adapter, and exhaust modifications, or you could knock the tubes out of the Dana 50 and reassemble it as a passenger-drop axle. If you are planning to swap out the AMC engine, transmission, and transfer case anyway, then fitting the Ford axle unmolested is not as much of an issue.

Converting the Super Duty Dana 50 to 35-spline inner axleshafts isn’t as easy as bolting in new parts. Of course, 35-spline axles in the correct length can be machined, but I don’t know of a 35-spline locker or differential for the Dana 50. Although, Tri-County Gear (tricountygear.com) used to offer a 35-spline Dana 50 ARB Air Locker that was made up of the Dana 50 carrier stuffed with Dana 60 35-spline ARB side gears. Ultimately, converting the Dana 50 to 35-spline axleshafts throughout comes at a cost, possibly negating any savings of using the Dana 50 to begin with. As for the outer ends, Currie Enterprises (currieenterprises.com) offers 35-spline 5-lug unit bearings and Dynatrac (dynatrac.com) offers a 35-spline Free-Spin kit that can be used to convert the unit bearing design to a traditional fixed spindle with serviceable wheel bearings. Several lug patterns are available.

Converting a high-pinion Ford Super Duty Dana 50 front axle into a rear axle to be swapped into a small, lightweight Jeep sounds like an intriguing idea. Of course, it requires that you retube at least the short side of the axle and install the proper axle ends to accept flanged or full-floating axleshafts. The interesting part is that the Dana 50 high-pinion gear set would be stronger than a high-pinion Dana 44 gear set, which could be used to increase driveshaft ground clearance and decrease driveshaft angles. A high-pinion gear set is about 30 percent weaker when driven on the coast side in a rearend application. A high-pinion Dana 44 rear axle is probably good for 35s in a light, moderately powered 4x4. The Dana 50 has a ring gear that is about 12 percent larger in diameter than the Dana 44. If you trust the mathematics, you could argue that the Dana 50 high-pinion ring gear is 12 percent stronger than a high-pinion Dana 44 ring gear. This would still make the high-pinion Dana 50 ring gear about 18 percent weaker than a traditional low-pinion Dana 44 in a rear application, but this is much more tolerable than the 30 percent loss of strength when using a high-pinion Dana 44 in a rear application. Backing the axle up with an automatic transmission would help cushion some of the abuse. The Dana 50 has plenty of 30-spline traction-adding differentials available as well as ring-and-pinion gearsets with ratios down to 5.38:1.

Big Tires Low Lift
How exactly do highline fenders work on a flatfender?

@jwilson_55
Via Instagram @cappaworks

When the low center of gravity fad hit several years ago, many off-road enthusiasts were looking for ways to fit bigger tires without lifting their Jeep any higher. Some people simply limited the suspension compression with taller bumpstops, while others cut out, removed, or replaced OE fenders with aftermarket parts so the tires wouldn’t rub. The highline fender idea took hold first on newer Jeeps. The front fenders were raised and the hood was trimmed to accommodate the raised fenders. Companies such as AEV (aev-conversions.com) went so far as to offer a factory-looking conversion kit for the TJ Wrangler that was completely bolt-on. The kit included new front fenders, front and rear fender flares, and a host of other parts that were needed to complete the AEV Highline conversion. Creative professional and garage fabricators went to work creating their own highline kits for other Jeep models. The flatfender Jeep seemed like a natural for this conversion. The front fenders are modified and raised several inches on the grille and body tub. With the fenders raised, the hood has to be trimmed for clearance. In the rear, the factory wheel tubs are modified and the wheel openings are carved out to fit larger tires. It isn’t an easy conversion on a flatfender and requires good fabrication and body metal skills. It’s definitely not for those who are squeamish about cutting up vintage metal. In most cases, a flatfender with a well-done highline fender conversion can better fit 35-inch tires than a flatfender with the factory fender openings.

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