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Techline: Your Top 4x4 And Off-Road Tech Questions Answered Here

Posted in How To: Tech Qa on June 19, 2017
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Best Transmission Ever

What is the best all-around granny four-speed manual transmission?
@gaybluejeep
Via Instagram @cappaworks

There are several great options when it comes to swapping an off-road-savvy manual four-speed transmission into your 4x4. Although, some people might think that the availability of modern five- and six-speed manual transmissions negates the need for the older heavy-duty truck four-speed manual transmissions. They would be wrong. The SM420, SM465, NP435, and T-18 are among the most swapped-in manual transmissions used by 4x4 enthusiasts and are still popular to this day. Of course they are comparatively noisy, clunky, and have awkward long-throw shift patterns when matched up to the modern-shifting NV4500 five-speed and NV5600 six-speed. However, the older rock-crushing manual transmissions can often be found used for less than 1/4 the price of their newer manual counterparts. Plus, the NV4500 and NV5600 may not even fit some applications. If you don’t need an Overdrive gear, the SM420, SM465, NP435, and T-18 are all great options. Which is best for you depends on your application and shifting expectations.

The GM SM420 is an excellent compact truck transmission with the lowest First gear of any common manual transmission. The 7.02:1 First gear makes it great for 4x4s that see a lot of technical crawling or rocks. It’s also good for pulling heavy loads through dense woods or other areas that require slow controlled speed. The SM420 is best suited in older short-wheelbase 4x4s that don’t have a lot of real estate to work with. Companies such as Advance Adapters (advanceadapters.com) and Novak Conversions (novak-adapt.com) offer plenty of adapters to mate the SM420 to popular engines and transfer cases. The shift pattern of the SM420 leaves a lot to be desired. Because of the up and over to the left Reverse location, it can be difficult to rock a 4x4 out of a stuck situation when shifting from forward to Reverse quickly. Keep in mind that the SM420 is an old transmission. The last one rolled off the assembly line back in 1967, so it’s not uncommon to find them with completely shot-out synchros. Even still, a worn out SM420 will keep going if you are good at double clutching. Most rebuild parts are readily available, although SM420 gears and input shafts have been out of production for decades and can be difficult to find. Completely rebuilt turnkey SM420 transmissions are available from Herm the Overdrive Guy (hermtheoverdriveguy.com) and Novak Conversions.

The GM SM465 is essentially just a larger, more-modern and heavy-duty version of the SM420. It’s best suited in larger trucks that have the appropriate real estate available between the framerails. High-mileage SM465 transmissions have a tendency to pop out of Third gear. This is a good sign your transmission is due for a rebuild. SM465 adapters are available from companies like Advance Adapters, Novak Conversions, and Offroad Design (offroaddesign.com). Rebuild parts are common, and complete rebuilt SM465 transmissions are available from Advance Adapters, Herm the Overdrive Guy, and Novak Conversions.

The NP435 is arguably the most modern and smoothest-shifting of all the common heavy-duty four-speed manual truck transmissions. It’s the only granny four-speed manual that can be found in Chevy, Dodge, and Ford applications from the factory. Rebuild parts are common and adapters are available from companies like Advance Adapters and Novak Conversions. Completely rebuilt NP435 transmissions are available from Herm the Overdrive Guy, and Novak Conversions.

The T-18 is extremely adaptable and versatile. However, the Jeep T-18 came in so many different versions that finding the exact model you need could be a fruitless exercise. There are more than 14 different models of the T-18 found in Jeeps, which includes varying input shaft lengths and either a 4:1 First gear or the more desirable 6.32:1 First gear. Some of the longer factory input shafts and bellhousing spacers are totally unusable in a short-wheelbase 4x4. The overall compact size of the T-18 when combined with a short input shaft makes it a great swap for smaller Jeeps. The end result is that some versions of the T-18 are more adaptable than others. Advance Adapters and Novak Conversions offer adapters for the T-18 and Herm the Overdrive Guy and Novak Conversions offer completely rebuilt T-18 transmissions.

Doubling Down

What's the best doubler to pair with an NV231 transfer case? Should I use another NV231, a Dana300, or something else?
@ddodd10
Via Instagram @cappaworks

Increasing your low range crawl ratio with a doubler is a great way to slow your 4x4 down off-road and give you more control on tight technical trails. There are several ways to increase the low range gear ratio of an NV231 transfer case. Perhaps the easiest route is to remove the NV231 and install a Rubicon NV241OR 4:1 transfer case. Of course this has some disadvantages, the most significant of which is that you give up the factory 2.72:1 low range, which can be useful in the mud and sand where the 4.0:1 may be too low and slow. The NV241OR has a single low range of 4.0:1, so if you want more off-road versatility, a doubler is probably a much better option.

The Advance Adapters (advanceadapters.com) RubiCrawler makes great use of the space between the ’03 to ’11 Jeep Wrangler NV231 or NV241OR and the 42RLE automatic overdrive transmission. In front of the NV231 transfer case the RubiCrawler provides a 1:1, 2.72:1, or a compounded 7.40:1 low range ratio. In front of an NV241OR, the RubiCrawler allows for a 1:1, 2.72:1, 4.0:1, or a compounded 10.88:1 low range ratio. The RubiCrawler can only be fit behind the 42RLE Wrangler transmission. It cannot be used behind any other transmissions.

NorthWest Fabworks (northwestfab.com) offers the Eco Box and Black Box underdrive planetary gearboxes, which can be adapted to your NV231. The Eco Box and Black Box provide a 2.72:1 low range as well as a 1:1 ratio. When combined with the factory NV231 low range ratio you get a compounded low range of 7.40:1.

Offroad Design (offroaddesign.com) offers the Magnum Box, which is similar to the Black Box. It provides the same 1:1 high ratio, 2.72:1 low range, and a compounded 7.40:1 low range when both the Magnum Box and NV231 transfer case are shifted to low range.

Some at-home fabricators have built their own NV231 doubler kits. These kits require that you cut another NV231 in half and then weld in a cover plate to the low range planetary portion of the hacked aluminum transfer case housing. You then create an adapter plate and output shaft, which are used to attach the chopped T-case to your NV231. This combination provides a 1:1, a 2.72:1 low range, and a compounded 7.40:1 low range.

Yet one more solution is to replace your existing NV231 with an Advance Adapters Atlas 4SP four-speed transfer case. This option gives you the factory 1:1 high range ratio, a 2.72:1 low range, your choice of a 1.50:1, 2.00:1, 3.00:1, or 3.80:1 low range, and up to a 10.34:1 compounded low range.

I would avoid putting a doubler in front of a Dana 300 unless you are working with a lightweight low-powered 4x4. There are too many other upgrades that would be required to keep the Dana 300 alive behind the compounded gearing of a NV231 planetary doubler. Also, an application that originally came with an NV231 will have the differential on the driver side. The Dana 300 is a passenger-side-drop transfer case, so you would need to fabricate or purchase a flip kit as well, further increasing the complexity and expense of your conversion.

Free-Spin AAM

This question is in regards to an AAM 9.25 front axle, but I'm sure it applies to others as well. What is the big advantage to running a Dynatrac Free-Spin kit? I'd say the most common failure I see on the Ultimate Adventure is manual locking hub failure. I get that unit bearings can fail causing a wheel to fall off, but they usually provide some warning, unless you're jumping it or something. The selectable hubs just seem like something else to break.
@shanesaw5_9
Via Instagram @cappaworks

Installing a Dynatrac (dynatrac.com) Free-Spin kit on your AAM 9.25 certainly increases the number of components on your front axle. However, it is a significant upgrade over the stock unit bearing assemblies. There are several advantages to the Free-Spin kit. As you mentioned, the factory unit bearings are indeed failure prone, even more so when combined with larger aftermarket tires and wheels. The roller bearings themselves are caged in plastic, so if contaminates get past the seals, the bearing cages can melt much quicker than you might think, causing the rollers to fall out of place and the wheel to potentially free itself from your 4x4. The Free-Spin kit replaces the non-serviceable unit bearings with traditional spindles and serviceable wheel bearings with steel roller cages. If properly maintained, these serviceable bearings and spindles will survive many times longer than the factory disposable unit bearings, which can save you money over a long period of time.

Another advantage of the Dynatrac Free-Spin kit is that it comes with chromoly 35-spline outer stub axles. These 1 1/2-inch-diamter axles replace the smaller and weaker 30-spline stock stub axles.

Keep in mind that not all locking hubs are created equal. The optional Dynatrac DynaLoc manual locking hubs that are available with the Free-Spin kit are three times stronger than a typical manual locking hub for the same application.

Lastly, with the Dynatrac Free-Spin kit installed on your AAM 9.25, there is less rotating weight when the hubs are unlocked. This will increase fuel economy slightly and decrease front axle and front driveshaft wear, noise, and vibration.

Ford Sterling vs. GM 14-Bolt

What is your honest opinion of using the Sterling 10.25/10.5-inch rear axle from a Ford Super Duty versus a GM 10.5-inch 14-bolt? Or is it worth picking up the axles from a newer Ford F-450? I’m planning on running 40-inch tires.
Lee Smircic
Via facebook.com/JohnCappa4x4

In a stock application or for street-only use, I think both the Sterling and GM 14-bolt will hold up just fine. However, if this is on a heavy 4x4 with big tires making decent power and you plan to hit some formidable trails, I'd take a 10.5-inch 14-bolt rear axle over the Sterling all day long.

It’s not that the Sterling 10.25/10.5 is a complete pile or anything, it’s just that it has one major flaw in my mind. It's not at all uncommon for the axletubes in the Sterling to spin inside the cast housing when combined with heavy torque loads and large diameter tires. This generally destroys the Sterling axlehousing and rear driveshaft. Some people believe that it’s perfectly acceptable to weld the tubes directly to the centersection casting to prevent this. Welding the steel tubes to the cast material requires proper welding technique and experience. If not done properly, the weld will crack before the assembly cools. However, even if done properly, it’s a great field fix, but simply not an ideal long-term solution. The reason being is that the steel tubes and cast centersection flex differently under load. Eventually, the cast center portion will crack and fail if the tubes are welded in place. The factory plug welds allow for the necessary housing flex where a perimeter weld around the tube does not.

Now, all of this doesn’t mean you can’t make use of the easy to find Super Duty Dana 60 frontend with the oddball 8-on-170mm wheel bolt pattern. You can fit the Ford frontend with aftermarket unit bearings from Currie Enterprises (currieenterprises.com) that have been redrilled to match the 8-on-6.5 lug nut bolt pattern of the GM 14-bolt. This would make for a pretty solid axle combo.

As an added bonus, the GM 14-bolt is extremely easy to set up gears in. The only real bummer on the 14-bolt is that the low-slung pinion decreases ground clearance, but with 40-inch or bigger tires it’s usually not an issue, and certainly not as much of a liability as a Sterling that hits the trails on 40-inch tires.

The Dana 60 frontend and Dana 80 rearend found in some Ford F-450s are a great find, but the Dana S110, Dana S125, and Dana S135 rearends found in other F-450 trucks are very heavy-duty and are overkill for all but the most extreme heavy-hauling 4x4 applications. Ring-and-pinion ratios are limited for these axles and the gears are incredibly expensive. The ring gear itself is actually part of the differential carrier assembly on the Dana S110, S125, and S135 axles, which surely increases the complexity and cost of manufacturing the ring gear.

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