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Answers To All Your Jeep Questions

Posted in How To: Tech Qa on February 7, 2019
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SM420 Oil

What oil are you supposed to run in a GM SM420 four-speed manual transmission?
@switchlloyd
Via Instagram @Cappaworks

The GM SM420 manual transmission has been a popular swap in Jeeps for decades, thanks in part to its availability, strength, compact size, adaptability, and impressive 7.05:1 First gear ratio. The low First gear allows for easy crawling at slow speeds over incredibly technical terrain. The thing to remember is that the SM420 was designed in the ’40s. The lubricants that were available then are far different than what is readily available today. The biggest concern is with the bronze synchros inside of the SM420 transmission. Bronze is often referred to as a yellow metal. Many modern lubricants are not yellow-metal safe. The sulfur in them will attack and damage the bronze synchros. The SM420 calls for 90W mineral oil. You can also use a true GL-4 rated gear oil. This includes GL-4 75W85 and GL-4 75W90. However, not all GL-4 oils are suitable for the SM420. Avoid GL-4 gear oils with a dual rating that includes a GL-5 rating. The GL-5 gear oils contain 30-50 percent more antiwear/extreme pressure additives. The additives will protect the gear teeth, but over time will damage the synchros. When using 5W30, 10W40, 15W40, or 50W you will have to look at the additive package to make sure it is safe for yellow metals. Not all of them are safe for the SM420. Check the API Service that can be found on the front of the bottle. You can generally run high-performance engine oils that have higher levels of ZDDP in the SM420.

Unfortunately, the correct lubricant can be hard to find locally; you may need to order it online. Napa (napaonline.com) generally carries GL-1 90W mineral oil, especially at more rural locations. You’ll likely find it way in the back in dust-covered 1-gallon jugs or 5-gallon pails. It’s commonly used in the transmissions of antique tractors. Tractor Supply (tractorsupply.com) also carries GL-1 90W mineral oil. Another option is to go with synthetics. Synthetic oils are more expensive than mineral oil or a standard GL-4 oil, but Red Line Oil (redlineoil.com) MT-90 is safe for the SM420. Royal Purple (royalpurpleconsumer.com) Max Gear is also safe for all yellow metals and the SM420.

Swamper or Not

I love the Garage Project GPW! I think it’s awesome that you kept the red windshield frame. Do you like the TSL tires? They look cool, but I don’t know how well they actually hook. I had a set once and I wasn’t impressed. I can’t help but wonder if a different tire would grip better.
@todd_dunbar
Via Instagram @Cappaworks

Glad you have enjoyed the project! Just like almost every other modification you can make to a Jeep, the tire selection has a lot to do with personal preference. However, it’s also important to select the right tire tread and carcass for the terrain you plan to hit most. There are always trade-offs no matter what tire you choose. Of course, the bias-ply 33x15.50-15 Interco (intercotire.com) Super Swamper SX tires on my GPW aren’t very road friendly, the carcasses aren’t all that flexible, and they may not provide as much traction on the rocks as some of the radial tires out there today, but the Super Swamper SX tires have other advantages. The SX works great in the sandy and rocky desert terrain I find myself in most. The bombproof sidewalls take a beating without getting punctured. There is almost no need to carry a spare, so I don’t. I keep a plug kit and a small onboard air compressor packed away in case I need it. Thanks to the ATX (atxwheels.com) Slab beadlock wheels, I can air the Super Swamper SX tires down into the single digits. I typically run them at 3-5 psi in the dirt. This allows the tread and carcass to flex and conform to rocks and other trail obstacles. Some of the notches and small desert washes that I frequent have steep walls and abrupt ledges that are made from packed sand. Most of the time, the only way to get through them is to let the tires chew through the dirt. The massive lugs of the SX tires move a lot of material quickly, allowing the GPW to make forward progress. If I frequented boulders and rock slabs more often I’d probably make the switch to a sticky compound tire. I’d likely go for a BFG (bfgoodrichtires.com) Krawler or competition Maxxis (maxxis.com) Trepador, although neither is available in a diameter I can use or for a 15-inch wheel. The other problem is that the sticky compound tires don’t hold up well in winter conditions. The frigid temperatures can cause the rubber to crack. So I’ll probably be sticking with the Super Swamper SX tires a while longer.

Rear Disc TJ

I have a ’99 Jeep TJ Wrangler with a Dana 44 rear axle? What is the best rear disc conversion for it?
@rckennerson
Via Instagram @Cappaworks

Rear disc brakes at all four corners of a Jeep are a huge advantage over drum brakes in most cases. Where they really shine is with larger-than-stock tires and in the mud. Disc brakes don’t capture and hold the mud like drum brakes do, and their braking performance is less affected by water and mud than with drums. Because drum brakes retain the dirt, the shoes will wear out more quickly and the drum brake components rust more easily than disc brake parts. Fortunately, the Jeep TJ Dana 44 is one of the most popular axles to receive a disc brake swap. There are several different rear disc brake kits available for the TJ Wrangler Dana 44 rear axle. Take a look at the disc brake kit from TSM Manufacturing (tsmmfg.net). The kit is completely bolt-on and is available with or without a parking brake.

Spending Your JK Money

My JK is finally paid off. Should I swap in a diesel engine, Atlas transfer case, Tom Wood’s Custom Driveshafts, and new axles, or should I buy a glossy new diesel JL, if they ever come out?
@30sauce
Via Instagram @Cappaworks

The Jeep JL Wrangler diesel is slated for 2020, so you’ll likely see them on the dealer lot in mid-2019. Unfortunately, performing a diesel engine swap with new axles, a transfer case, and driveshafts could never really be considered a financially sound decision. These are modifications based on passion and necessity. It’s kind of impossible for me to make the decision for you, but I’ll go ahead and try. If you have the idea that you want to swing 40-inch tires under your Jeep and hit the most difficult trails you can lay your eyes on, then your modification plans are completely sound. Regardless of what Jeep you start with, you’d be tossing pretty much everything to build up what you need to support 40-inch tires. There would be no point in starting with a brand-new Jeep only to throw away most of what you just paid for. However, if you simply want a comfortable and reliable daily driver that acts as a weekend warrior running a stock drivetrain on sensibly sized tires, the new diesel JL is probably a better choice, and a much less expensive proposition. Your used, currently-running JK has some value. The cost of a turnkey diesel swap performed by a reputable shop would likely be in the neighborhood of $30,000. Axles that would support 40-inch tires will set you back around $15,000. Just with the value of your current JK, plus the $45,000 mentioned here, you’ll likely be way into paying cash for a new ’20 Jeep Wrangler Rubicon Unlimited diesel rolling on 35-inch tires and a small lift. A full tilt ’19 Jeep Wrangler JL Rubicon Unlimited is tickling $55,000. I suspect the diesel option will bump it up to about $60,000, which would be less than modifying your old Jeep, but the end result is two entirely different vehicles. So which is right for you?

High-Mile JK Options

With all the older Jeep Wrangler JKs getting into the high-mileage range, what is the best reasonable option for engine replacement? Is a stock replacement the best route? Are there upgrade and performance engines and parts out there or is there a stroker kit available? There are so many used JKs coming up that have great off-road parts and very little off-road use, but high mileage. There are plenty of mall-crawl Wrangler Rubicons for $20,000-$30,000 just waiting for an engine swap, which just isn’t in the cards for everyone. We have a ’07 JK with 170,000 miles on it. She can’t go forever, so we’re looking at options. The Jeep is how I like it and it has good parts on it. I would love a solution to keep it going. Thanks in advance.
Dennis Lederer
Via facebook.com/JohnCappa4x4

You’re not alone. The ’07-’11 JK 3.8L V-6 engine swap dilemma is extremely common among early JK owners, and a regular reader question here at Jp. The simple answer is that if money matters more than power, stick with a stock replacement. An engine swap is just not a cost-effective option. Even punching out and boosting the performance of the 3.8L is pretty fruitless. Adding a supercharger or turbo kit will just about get you the same performance you can have with a completely stock and reliable ’12-’18 JK with the 3.6L, only the boosted 3.8L will require more expensive premium fuel and get worse fuel economy than the stock 3.6L. The JK 3.6L makes 85 more horsepower than the 3.8L. And before you get too excited, the 3.6L really isn’t a viable swap for the 3.8L. Ultimately, if you want 85 more horsepower, you’re better off selling your 3.8L-powered JK and purchasing a ’12-’18 JK with the 3.6L.

Now, if you want more power and are fully comfortable with throwing away most of your factory drivetrain parts, then a Hemi or LS V-8 swap can be performed. If you go with 35-inch tires or larger, the only factory drivetrain part you likely want to keep is the transfer case, so set aside some cash and time for a swap like this.

If you need a replacement engine, look for a good low-mileage take-out that hasn’t been in a bad front-end wreck or locate a remanufactured crate engine. Companies such as ATK (atkvege.com) and Golen (golenengineservice.com) offer complete 3.8L long-blocks with a warranty. All of your engine parts will bolt on, and the rebuilt engine will bolt between the framerails just like stock.

SEMA Showing

Why do they allow so much junk at the SEMA Show? There are so many vehicles there that were hastily put together with a lot of cut corners and then there are some that are above and beyond top notch. The spectrum is broad.
Daniel T. Noda
Via facebook.com/JohnCappa4x4

The SEMA (semashow.com) Show is always a bit of a gamble. There are so many builders putting vehicles together in a short amount of time that there will almost always be some that cut corners, although some are much worse than others. The thing to keep in mind is that the SEMA Show is not just an off-road show. That’s only a small part of what the show is about. It’s a celebration of all aftermarket automotive accessories. There is an unbelievable amount variation in vehicle tastes and aftermarket modifications made across the U.S. and the world. Many of the show vehicles were never intended to hit the dirt or even drive any real distances on the street, so it’s not uncommon to see missing front driveshafts and poor steering geometry. Of course, shoddy fabrication is inexcusable, but it’s fairly common at SEMA. New and up-and-coming builders literally get in line to display vehicles at the show. Unfortunately, just because someone has a welder, a tube bender, and a few fabrication tools does not mean they are a fabricator. But hey, everyone has to start somewhere. It might as well be the SEMA Show.

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