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Answers to your Jeep questions

Posted in How To: Tech Qa on February 7, 2017
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New Jeep Blues

My wife traded in her beautiful Audi, which required a two-hour one-way trip to get serviced, for a Jeep Trailhawk, which can get serviced about a mile away from our house. We love it, except for that damn engine stop. Plus, it has an infuriating habit of lurching from a stop or lurching and then pausing before accelerating and jerking your neck. I've almost been rear-ended in this thing. The dealer can't fix it because apparently there are no more old-school mechanics that you can discuss your problem with. The new guys hook it to a computer, which says everything is fine. So, jerking down the road we go.

Randall Sexton
Junction City, KS

Some late-model Jeep Cherokees and Renegades have had unpredictable and wonky transmission shifting issues. If I remember correctly, both vehicles have had TSBs released for transmission computer reflashing. If your vehicle doesn’t have the latest version, this could be the culprit.

Unfortunately, modern vehicle problems almost always have to be diagnosed using the OBD II port. For example, you generally can’t simply look at an electronic sensor, computer board, or other digital device to see if anything is wrong with it. The problem you are experiencing is more than likely software based, not something mechanical. So in all honesty, the mechanics are checking out your vehicle in the most effective way. However, I do understand the frustration of a problem that isn’t cured when you bring it to the dealer. I suggest you take the mechanic for a test drive with you. Take the vehicle through its paces and replicate the problem with the mechanic in the Jeep. Once they see the problem in person, the likelihood of them finding a solution is much greater.

Keep in mind that you can turn off the engine stop feature. It’s simply designed to save fuel when the vehicle is not moving.

Jeepster Compatibility

I am restoring a ’68 Jeepster convertible. I need both front fenders. I have not been able to find any good replacements. I am wondering if I could get two ’68 CJ front fenders and modify the lower part of the fender to include the bottom edge of the Commando trim? I think that the front fenders are the same except for the lower body trim where the fender meets the tub. Do you or anyone else know if the CJ fenders will bolt to the front grill and tub, with the exception of the lower trim piece? If the CJ fenders fit, than all I would have to do is remove the lower trim piece from the old fenders and weld them to the new CJ fenders. Do you think this will work?

Robert Dunkel
Westerville, OH

As you have found, it can be difficult to locate ’67-’71 C-101 Jeepster body parts. They are certainly not as plentiful as the CJ-5 components, which are still manufactured by companies like Omix-ADA (omix-ada.com). However, companies like Jeepster Dude (partsdude4x4.net) offer many new and used Jeepster specific parts. If you are looking to keep your restoration pure Jeepster, you can start here and see if any good used front fenders are available. You can also try eBay (ebay.com) and possibly Montana Overland (montanaoverland.com).

The Jeepster front fenders are very similar to the short-style early CJ-5 front fenders. If you are handy with body metal, you should be able to modify the CJ fenders to fit the Jeepster bodylines at the bottom. We have also seen people put the entire CJ front clip, including the grille on the early Jeepster. However, if you are restoring the Jeep to original, you likely will not want to go this route.

Tie Rod Tuning

What is the proper way to measure for new tie rods? I need to measure my CJ-7 for one and am not sure if I measure just the rod or the rod and ends?

Michael Donnel
Via facebook.com/jpmag

Your Jeep CJ-7 could have one of two different length tie rods. The ’76-’81.5 CJ-7s have narrow-track axles, while the ’81.5-’86 CJ-7s have wide-track axles. The wide-track CJ Dana 30 front axle is about 4 inches wider than the narrow-track CJ Dana 30 front axle, so therefore the tie rod tube will be about 4 inches longer on the wide-track Jeep. The narrow-track axle will have a stock tie rod tube measurement of slightly more than 40 inches; the wide-track tie rod tube will have a measurement of nearly 44 inches.

Now, if you are replacing the entire tie rod assembly with a custom-made heavy-duty tie rod with spherical rod ends or larger tie rod ends, you’ll need to take careful overall measurements. First, you’ll need to make sure the toe-in is properly set to 0-3/8-inch and then measure between the centers of the tie rod holes in the knuckles. Your complete replacement tie rod should match this measurement yet still offer some adjustment for proper alignment.

YJ Tank Replacement

Is there an aftermarket company that makes gas tanks or fuel cells that accept the stock YJ fuel sending unit? I’m looking for a tank with the rectangle unit, not round.

Be Double-u
Via facebook.com/jpmag

You have several fuel tank options if you wish to retain the factory square YJ fuel pump/sender assembly. If you are looking for a fuel cell to install in the back of your Jeep, Summit Racing (summitracing.com) offers many different sizes of aluminum fuel cells that could be easily modified to accept the square YJ flange. If you want to keep the fuel tank in the stock location, and extend the wheelbase too, GenRight (genright.com) offers an aluminum 23-gallon Crawler Comp tank. It is specifically designed to allow 7 inches more clearance between the rear axle and the gas tank, making it a great option for an extended wheelbase and larger rear axle. Other GenRight YJ tanks include aluminum units with a 17- or 20-gallon capacity that fit in the stock location. Both of these tanks feature provisions for the square YJ fuel pump/sender assembly. Another option is the GenRight 23-gallon universal fuel tank. It is also designed to work with the YJ in-tank fuel pump/sender module.

Three- to Four-Speed

I want to swap my T-86 three-speed transmission for a GM SM420 four-speed. Would I need an adapter for the SM420 to bolt to the bellhousing? I'm running the stock Buick 225 V-6 engine and a Spicer 18 single-lever transfer case. What other mods, if any, do I need to make?

Brian Hanzel
Via facebook.com/JohnCappa4x4

As you have probably found, the 2.88:1 First gear ratio of the factory T-86 three-speed transmission leaves a lot to be desired in the dirt, even behind the venerable and torquey Buick 225 V-6. The GM SM420 with a 7.05:1 First gear is a great replacement to swap in, especially if you enjoy slow technical crawling off-road. To complete the conversion, you should be able to retain your factory Jeep/Buick bellhousing. It was originally adapted to the T-86 via a 2 1/2-inch-thick cast iron adapter plate. Simply remove the transmission and adapter plate from the bellhousing. The SM420 will then bolt up to the Buick bellhousing. Conveniently, your existing pilot bearing, clutch, release bearing and clutch fork can be retained. The linkage may need to be modified for clearance. If so, consider the Advance Adapters (advanceadapters.com) sprocket and chain clutch linkage (PN 716640). A hydraulic assembly is also available, but it requires the use of the Advance Adapters swinging pedal kit, dual master cylinder, and a custom-built (by you) slave cylinder bracket. If your bellhousing is damaged and you can’t find a factory replacement, Advance Adapters offers a new full 360-degree aluminum Buick bellhousing that will mate to the SM420.

The more complex part of this conversion is mating the SM420 transmission to the Spicer 18 transfer case. Both Advance Adapters and Novak Conversions (novak-adapt.com) offer the adapter you need, however they each do it slightly differently. Advance Adapters offers both a spud-style and new mainshaft-style adapter to mate the GM SM420 to the Spicer 18. The spud shaft version is 4 inches long and will require some futzing with the front driveshaft for transmission case clearance. The version of the adapter with a new mainshaft requires transmission disassembly and is 4 1/2 inches long. This adapter should provide enough clearance for most 1310 Spicer-style front driveshafts. Front driveshaft clearance will be at a premium around the transmission and bellhousing, even if the Jeep is lifted.

Novak Conversions only offers a spud-style adapter to mate the SM420 to the Spicer 18. This adapter does not require transmission disassembly and is 4.8 inches long, which provides a combined transmission and adapter length of 15.2 inches. This offers plenty of room for most 1310 Spicer-style front driveshafts. Again, you’ll need to carefully inspect the front driveshaft for clearance around the transmission and bellhousing.

To complete the conversion you’ll need to lengthen the front driveshaft a couple inches and shorten the rear driveshaft a couple inches. You also need to move the transmission crossmember or at least modify the transmission mount. Keep in mind that the adapters are not complete conversion kits. There is a lot of detail work that you’ll have to figure out on your own, and unfortunately, every Jeep is different. Pay special attention to the rear driveshaft length and angles. Overly lifted applications will require more creative driveshaft engineering.

Having said all that, the SM420 is an extremely durable transmission, and a great swap, but it can be a lot of work to properly fit into a small Jeep. In some aspects, the Ford T-18 or short-input Jeep T-18 four-speed manual is a better swap. It can be installed with a significantly shorter transmission and adapter overall length of less than 13 inches. The extra 2 inches may not sound like a lot on paper, but when the entire assembly is slung under your Jeep the additional clearance offered becomes more apparent. The T-18 is also a narrower transmission that provides more room for the front driveshaft. The downside is that the T-18 will require more adapting than the SM420.

Ratio Right

I’m running a ’95 Wrangler with a ’00 4.3L Vortec engine, a built TH350 transmission, and a Ford NP205 transfer case. It has a Dana 60 front axle and a 14-bolt rearend. With 40-inch tires, what axle gear ratio would you recommend I run? I’m currently running 4.11. I mostly want wheel speed and torque, but I also need to really crawl at times.

dougy3790
Via Instagram @Cappaworks

Unfortunately, the 4.11 axle gears are a long way from where you want to be, unless your Jeep spends the majority of its time cruising down the highway at 80 mph. With your setup, I think your Jeep will perform best on- and off-road with a minimum of 5.13 axle gears. The deepest gears you can get for the 14-bolt is 5.38, so that’s really the only other option that makes sense. You won’t see a significant performance difference between 5.13 and 5.38 gears on- or off-road, so you can choose either, but if off-road prowess is most important, stick with the 5.38:1 ratio.

The 1.96:1 low range ratio of the NP205 and the 2.52:1 First gear ratio of the TH350 are really hurting your overall crawl ratio. With 5.38 axle gears you would have a somewhat pathetic crawl ratio of about 27:1. Swapping 3.0:1 JB Conversions (jbconversions.com) LoMax gears into the NP205 will provide a more admirable crawl ratio of about 41:1 when combined with 5.38 gears. You can get just a bit more crawlability with the installation of a TCI (tciauto.com) 2.75:1 First gear in your TH350. It’s not much more, but would get you close to 45:1 when added to the 3.0 LoMax NP205 gears and 5.38 axle gears. With a crawl ratio of 35:1 to 45:1 you should have no problem getting the wheel speed up in low range. Manually shifting from First to Second should help provide a violent launch from a dead stop to help get up over ledges.

Another option is to add an Offroad Design (offroaddesign.com) transfer case doubler kit. This would give you multiple low range options, but it will chew up a bit of rear driveshaft length and may require some reconfiguring of your Jeep, depending on how the exhaust, chassis, and suspension are set up. You’ll still want the 5.13 or 5.38 gears if you go this route, unless your Jeep spends the majority of its time in low range. With both gears of the doubler in low, the stock 2.52 First gear in the TH350, and 5.38 axle gears you’ll have a formidable 52:1 crawl ratio, which is pretty good with an automatic.

Whatever you decide, make sure the TH350 stays cool. A big-tire heavy-axle combo like this can cause it to overheat if you’re crawling, mud bogging, sand duning, or snow bashing regularly, especially in warmer ambient temperatures. There are many different sized aftermarket transmission coolers available from companies such as Flex-a-lite (flex-a-lite.com). You’ll want to use the biggest transmission cooler you can reasonably fit and maybe add a gauge. For longest transmission life, you’ll want to keep the oil temperature in the transmission oil pan less than 180 degrees.

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