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Answers to All Your Jeep Questions

Posted in How To: Tech Qa on November 15, 2018
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Five-Speed Flattie

I want to swap a ’03 Chevy S-10 4.3L V-6 engine and transmission into my ’46 CJ-2A. Is there an adapter available to go from to the Spicer 18 to the NV3500, or would I have to swap the transfer case too? If so, what will bolt up that will give me offset front and rear outputs on the passenger side?
Via Instagram @cappaworks

Swapping pretty much any engine and transmission into a flatfender Jeep requires quite a bit of planning, measuring, and preparation. Unlike most Jeeps and other 4x4s, the flatfender Jeeps do not have very large engine bays. When combined with the short flatfender wheelbase, there isn’t a lot of room to work with. An overly long engine, transmission, and transfer case combo can chew into rear driveshaft length. Advance Adapters ( offers an adapter to mate the Spicer 18 to the Jeep NV3550 (part number 50-8602), which is similar to the GM NV3500. However, this adapter requires the transmission to have a 23-spline output. Unfortunately, your GM NV3500 has a 27-spline output, so the stub shaft in the adapter kit won’t fit. That doesn’t mean the swap is not possible. You may be able to swap the 27-spline GM NV3500 rear output shaft for a 23-spline Jeep NV3550 rear output shaft. This will require transmission disassembly if it is possible. Your best bet will be to call Advance Adapters to see if there is some combination of adapters that will work—before tearing into the transmission.

As for other transfer cases with both front and rear outputs to the passenger side like the Spicer 18, you don’t have a lot of adaptable options. Solid-axle Land Rovers and Toyota Land Cruisers have transfer cases with the outputs offset to the passenger side, but adapting them to the flatfender chassis and GM NV3500 transmission will be more difficult than using a Spicer 18.

If all else fails, you could locate a Jeep NV3550, which unlike the GM NV3500 has a removable bellhousing. From there, you can adapt the Jeep NV3550 to the GM 4.3L with Advance Adapters bellhousing part number 712591 for a 153-tooth flywheel, or part number 712591V for use with a 168-tooth flywheel. This would give you the ability to use Advance Adapters part number 50-8602 to mate the Spicer 18 to the Jeep NV3550. It’s kind of a roundabout way to get there, but it gets you sort of where you would want to be with your drivetrain.

Also, be careful. The overall transmission length could be too long for the stock wheelbase on a flattie, depending on where you locate the engine. The NV3550 is 16.7 inches long without the bellhousing. The 50-8602 adapter is about 1 inch thick. The combination is 17.7 inches before the transfer case is added. A typical swap using an SM420 is about 3 inches shorter. You may need to extend the wheelbase to allow for a long enough rear driveshaft to make a combination like this actually work. Engine location and suspension lift height will be critical.

Diesel Jeep Gearing

I have a ’89 YJ. I’ve swapped in a Mercedes OM617 diesel and mated it to a GM TH700R4 four-speed automatic transmission and an NP241 transfer case. The Jeep has 37-inch tires and the rear axle is an 8.8-inch Ford. I’m still running the same 4.10 axle gears from when the Jeep had a 5.0L V-8. With the current axle gearing and transmission the Jeep will not maintain speed in Fourth gear, but cruises fine in Third gear at 60 mph. I’ve purchased 5.13 gears to try and move Fourth down to where Third is in terms of speed verses rpm, but have not installed them yet. Is this gear swap going to make the Jeep more drivable on the highway again? Are my thoughts about the regear reducing load on the engine in every gear, thusly increasing the available power correct, or am I completely off base? Thanks for any input. With the old 5.0L gas engine I had an AX15 and 35-inch tires. Fifth gear was only for cruising. I needed to downshift for mild hills and I was certainly lugging the engine below peak torque. I know there is no comparison in terms of power, but the power loss was worth the gain in mpg. On the highway in Third gear at 60 mph I get 17 mpg.
Via Instagram @cappaworks

I believe you have the right idea. Lower gearing is needed to compensate for the increased diameter of the larger tires. Although, in most cases you can get by with slightly less gearing with a diesel engine than with a gas engine. This is because most diesels make more torque down lower than their gas counterparts. Unfortunately, the OM617 isn’t much of a powerhouse. It depends on the year of the 5.0L and OM617, but both make peak torque around 2,400 rpm and peak horsepower around 4,000 rpm, meaning you would likely want a similar axle ratio for both. However, the OM617 makes significantly less peak horsepower and peak torque than the 5.0L gas engine. You can make up for the lack of power with lower gears, but only up to a point. It’s kind of like putting a small V-6 in a 1-ton truck. The gearing can only compensate so much before more power is needed. You’ll also have to watch the rpm. The governed redline on the OM617 diesel will be lower than redline on the 5.0L gas engine. As long as you have realistic drivability and top-speed expectations, I think the 5.13 axle gears will serve you better on the highway than the 4.10 gears. Off-road, you’ll appreciate the deeper gearing with the diesel. It will chug along over the trails even better than before.

ZJ Max Tire

I have a ’94 ZJ Grand Cherokee with a 5.2L V-8 and an automatic transmission. What’s the biggest tire size you would run on ZJ axles? And what would be a perfect daily driver/weekend warrior build for a ZJ in Colorado? I'm building an XJ for the really hard trails.
Via Instagram @cappaworks

Your V-8–powered ’94 ZJ should have a Dana 30 front axle and the aluminum Dana 44 rear axle. As in most Jeeps, both axles are fairly robust for their size, but I wouldn’t recommend more than a 32- to 33-inch tire for your application. The good news is that the power of the V-8 is cushioned through the torque converter and automatic transmission. This keeps shock loading of the axle gears and axleshafts to a minimum. Abusive driving can cause the ZJ Dana 30 front axlehousing to bend, so you’ll want to avoid airtime and high-speed jaunts down rough roads with lots of washouts and whoops. Other than the larger tires and the required lift to fit them, consider adding rocker protection. This is a very vulnerable area an all Jeeps, and it’s not cheap to repair damage here. Solid rocker guards are available from companies such as Kevin’s Off-Road (, Rock Hard 4x4 (, and others.

J-Truck Sleeper

I have a ’74 J10 truck with the AMC 401 V-8, TH350 automatic transmission, and a BorgWarner 1339 transfer case. In the long run, I want to keep the 401 V-8. I want to build it and make the Jeep a sleeper on 33- to 35-inch tires. What transmission/transfer case combo would you go with, or would you do a late-model 6.4L Hemi and drivetrain swap? Also, an overdrive would be a nice addition.
Via Instagram @cappaworks

If the drivetrain in your truck is original, it likely has the AMC version of the TH400 and not a TH350. I can’t imagine why someone would go through the expense of swapping out the venerable TH400 for the smaller TH350.

Anyway, I’d keep most of what you have since it runs. The BorgWarner 1339 transfer case is somewhat wear-prone, especially if not properly maintained with the correct lubricant. You might consider at least draining, inspecting, and refilling the fluid. If the oil appears to be in good shape, consider installing a Mile Marker ( part-time kit in the transfer case. Another option is to swap out the 1339 for something like a Dana 300 or NP208. Although, switching to the Dana 300, NP208, or any centered rear output transfer case may require a rear axle swap too. The differential on your factory rear axle is offset slightly to the passenger side to match the 1339, which could cause driveshaft vibrations as well as fuel tank clearance issues if coupled to a centered transfer case rear output. One simple solution to replace the offset Dana 44 rear axle is to swap in a Ford 9-inch from a ’74-’86 Ford F-150 or ’78-’86 Bronco. The only hiccup is that you’ll need to swap out the 5-on-5.5 lug axleshafts for custom 6-on-5.5 lug axleshafts and drill the brake drums to match the lug pattern of the Dana 44 front axle. Other than this, the 9-inch is a great swap candidate for the rear of Wide-Trac FSJs, especially if you are building a torque-monster, tire-spinning sleeper Jeep.

If you think you want an overdrive transmission, look into a GM 700R4 or 4L60E transmission swap. Down the road, these transmissions can be easily mated to a GM V-8, which is likely the most cost-effective sleeper engine swap, albeit boring to some. Of course, the 6.4L Hemi has tons of cool and wow factor going for it, but dollar for dollar the GM LS V-8 is a better swap for a sleeper. There are lots of aftermarket products available to juice and adapt the GM LS engine, giving your J-truck more sleeper potential overall.

Low-Pro 4.3L Swap

I’m building a Willys flatfender from the ground up using modern components, including a ’96 GM 4.3L V-6 out of a Chevy S10 4x4. It has a really deep cast-aluminum oil pan that is going to make my flat belly goal much harder. What are my options for a shorter pan? Aftermarket? Mod the existing pan? Compatible steel pans?
Via Instagram @cappaworks

There are several different OE oil pans available for the GM 4.3L. Each was used in a different application and some are significantly deeper than others. You’ll want to stick with the cast-aluminum pan design if you can. These pans add rigidity to the engine block and the engine and transmission mating surfaces. Simply put, the cast-aluminum pans are better at supporting the engine block and transmission than a stamped-steel oil pan. Take a look at the factory 4.3L oil pan for the ’96-’06 1/2-ton truck. Dorman ( part number 264-482 or GM part number 12597153 should help get you the ground clearance you are looking for. These pans are about 1.5-2 inches shorter than some of the deeper factory oil pans available for the 4.3L. The bottom is flush with the bellhousing. Don’t forget you’ll need the correct oil pickup and possibly a new dipstick and dipstick tube.

CJ-5 Axle Swap

What axles are easy to swap into a ’71 CJ-5? I need a rearend with the differential offset to the passenger side for the Spicer 18 transfer case. I’d like to run some 35-inch tires, but I think they are a little more than I need. I’ll probably run 33-inch tires.
Don Tafjen Jr.

The offset rear axle that matches the offset rear output of the Spicer 18 transfer case is typically the toughest part to get around. There are very few modern-day offset rear axles to choose from, and none of the newest axles really make sense to swap in. These would include Land Rover axles and the axles from a Toyota FJ80 Land Cruiser. The good news is that for 33-inch tires, you may not need to completely toss what you have. Your factory ’71 rear axle with flanged semi-floating axleshafts should be completely useable in this case. Earlier Dana 44 rear axles came with the undesirable tapered two-piece axleshafts. These should be replaced with either custom aftermarket flanged semi-floating axleshafts from companies like Dutchman Axles ( and Moser Engineering ( or a full-floating axleshaft kit from Herm the Overdrive Guy ( Herm the Overdrive Guy offers both 19- and 30-spline complete full-floating axleshaft kits for ’46-’71 Jeeps. The full-floater axleshafts can be a big plus for those that plan to flat-tow their Jeeps and want the convenience of having manual locking hubs on the rear axle.

Up front, you could make the stock closed-knuckle Dana 27 survive relatively easily; however, if you plan to throw more power at the Jeep and drive aggressively, it would probably be a sound decision to swap the 27 out for an open-knuckle Dana 30 from a ’72-’75 CJ. The open-knuckle Dana 30 is beefier, turns sharper, and enjoys much more aftermarket support than the 27 in the form of a variety of ring-and-pinion ratios, locking and limited-slip differential options, heavy-duty axleshafts, and more.

If you’re looking for slightly wider and more durable axles, the offset Dana 44 front and rear axles from a ’74-’79 FSJ could be adapted. These Dana 44s feature larger-diameter axletubes made with thicker material. They can easily support 35-inch tires should you choose to increase tire diameter in the future. Unfortunately, it’s not at all an easy bolt-in conversion. Custom fabrication is required, and many things need to be addressed for these axles to be swapped in under your Jeep. Most notable is that the front leaf springs will need to be mounted outboard of the frame to properly locate the front axle and match the spring perch locations. Companies such as Dave’s Customs Unlimited (, M.O.R.E. (, and Poison Spyder ( offer full-width axle kits for the ’76-’86 CJs, which can be modified to work on your Jeep. The FSJ Dana 44 axles also have a different lug pattern than your CJ. Your CJ has a 5-on-5.5 lug pattern, while the FSJ Dana 44 axles have a 6-on-5.5 lug pattern, so new wheels would be required.

JK 3.6L Conversion

What is the feasibility of swapping a 3.6L into a ’11 Wrangler? What issues am I likely to encounter?
Rick T.
Via email

There is no arguing that the ’07-’18 Jeep JK is the most popular Wrangler ever offered. The available four-door model, soft suspension, quieter interior, and creature comforts make it one of the most successful Jeeps built to this day. However, in most circles the ’12-’18 model is preferred over the ’07-’11 model. There are many improvements that have been made to the newer JK, but the most notable is the switch to the 3.6L Pentastar V-6.

This smaller displacement engine replaced the 3.8L V-6 found in ’07-’11 Wranglers. The 3.6L makes about 85 more horsepower than the 3.8L, while getting better fuel economy at the same time, so the engine swap you propose would seem to make sense. However, there is good and bad news. The good news is, and what most people won’t tell you, is that the conversion to a 3.6L Pentastar V-6 engine is probably one of the easiest conversions on the planet. The bad news is that the conversion involves selling your ’07-’11 Wrangler and buying a ’12-’18 JK. Ultimately, performing a 3.6L engine swap on your 3.8L JK just isn’t cost effective. You’ll never see the return in increased fuel economy or resale value, especially with an ’11 model.

Currently, Kelley Blue Book ( lists the private party price of a typical ’11 Wrangler Unlimited Sport in good condition with standard options at $18,000-$21,000. The same Jeep in a ’12 model is listed at $20,000-$23,000. You simply could not perform a reliable engine swap for the $2,000 difference in price, which includes all of the other updates you get with the newer ’12 model. Save your money and put it toward a newer JK with the 3.6L already under the hood. Not only will you save yourself a lot time and money, you’ll avoid the many headaches of performing a conversion that is readily available from the factory.

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