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Bug-Out Blazer Part 7: Installing An Offroad Design Magnum Box

Posted in How To: Transmission Drivetrain on January 30, 2017
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We’re now back to making good progress on our ’88 Chevy K5 Blazer doomsday-survival rig, the Bug-Out Blazer (B.O.B.). If you haven’t been keeping tabs, we’ve knocked out some of the bigger line-items on our mechanical-survivability checklist, including the basic & reliable leaf-spring suspension system, modified 1-ton CUCV axle swap and beadlock wheels wearing 40-inch tires. The list still remains long, but this installment gives us a glimpse at the light at the end of the drivetrain-tunnel.

To draw from the library of redneck-analogies cataloged in our heads: the struggle of building a highly capable, highly reliable off-road rig is like floating a duck boat that’s been ventilated with OO buckshot—you’re forever plugging holes to keep the vessel afloat. To us, that’s all become part of the fun, but given that B.O.B. is being purpose-built as an ultra-capable life-sustainment vehicle, there’s no room for trial and error with this one. Any component that won’t hold up to the thrashing of a lifetime, any component that limits B.O.B.’s basic capabilities in any way, and in general, any component that could potentially demand you stop doing what you’re doing to deal with it immediately had to be given the heave-ho in the name of 100 percent (or as near as we could get) “you’re all on your own” reliability. Fact: Everything mechanical will give up the ghost eventually, but as with all of B.O.B.’s modifications, it’s all about stacking the deck in our favor with dirt-simple, robust and reliable components.

With the goal of our vintage 1/2-ton GM iron turning 40-inch tires (without the persistent fear of breaking drivetrain components), there were some pretty big holes that needed to be plugged in B.O.B.’s hull before it saw active duty. Upgrading the factory 1/2-ton axles (previously scrapped in favor of a modified 1-ton CUCV Dana 60 front and 14-bolt rear in Part 3) was a huge step in the right direction. Another potential ship-sinker that needed to be addressed before B.O.B. rolled bumper first into the apocalypse was its factory-original, chain-driven and aluminum-cased NP208 transfer case.

The NP208 is a reasonably strong transfer case but only to a point. The 208 can be counted on to operate reliably under a stock or mildly modified Blazer Suburban or K-series pickup, but start adding huge tires, more power and less throttle discretion, and all bets are off. Also, since it uses a slip-yoke output shaft, losing your rear driveshaft means there’s nothing to stop the 208 from puking its oil out the tailshaft housing. That won’t work for B.O.B.

Luckily, there’s a relatively inexpensive, easy-to-swap and positively bombproof alternative: the New Process 205 (NP205). This fixed-output-shaft (with most, anyway), gear-driven, cast-iron-anvil of a transfer case is about as durable and reliable as it gets, and even though most every truck sporting a 205 from the factory can now be considered an antique, that doesn’t stop it from being tossed into the category of God’s finest gifts to the off-road community. Every one of the “Big Three” U.S. auto manufacturers used them in various applications over the years, and just like the venerable 14-bolt rear axle, there’s still plenty of them out there to lay claim to. When I asked Stephen Watson, owner of Offroad Design and authority on all things K-series, here was his take on the 205: “I think one side effect of the lack of design sophistication is the 205’s strength and durability. When you don’t have a computer to tell you how small you can build something and still get away with it, you tend to over-build it so it doesn’t break. I think this is something we’re losing in today’s trucks. Today’s trucks are built great, but you can’t just bolt 40-inch tires on them and expect them to live like an ’80s K30. The OEs know exactly what they want the truck to do and can computer model everything to do it efficiently. And they simply don’t care about bolting 40s on their trucks.”

There was no question B.O.B. had to have a 205, and after a cursory 30-second Craigslist search, we found a clean early-’70s GM 1-ton example for a couple hundred bucks. However, for all of the folklore revolving around its superman-strengths, there is one glaring flaw to the NP205: its not-low-enough 1:96-ratio low range. While wheel speed and momentum are great friends to have when you need them, some situations require you to slow your roll to avoid component damage. In B.O.B.’s case with its manual-shift SM465 trans, using the “granny-gear” while slipping the clutch and doing the three-pedal boogie would be one way to compensate, but burning up a clutch is not something we want to risk—we’ll hazard a guess that Rock Auto will not be drop-shipping car parts amidst nuclear fallout.

So, what are the options? A lower crawl ratio can be had by combining the low-range box portion of a full-time NP203 transfer case with the NP205 via a Doubler kit from Offroad Design. The Doubler system remains an excellent gear-reduction option (especially if you’ve got the requisite transfer cases kicking around), but for not much more money, Offroad Design also offers a stronger, lighter, and more-compact solution: the Magnum Box. Taking B.O.B.’s modus operandi of reliability and brute-strength into consideration, the Magnum Box represented the ideal solution for outfitting B.O.B.’s new 205 with more-than-adequate levels of gear-reduction.

Outside of its inherent strengths and multi-speed gear reduction, one aspect of the Magnum Box we honestly appreciate is that it took a true off-road enthusiast to come up with a product of this caliber and capability. Again, that enthusiast would happen to be Stephen Watson, owner of Offroad Design, who’s dedicated his business to fullsize GM iron like B.O.B. and the generations that came before and after. This isn’t a chain-store part, there is no outward bling factor attached to it, and—consequently—it does nothing to enhance your street cred at the local Auto Zone. What it does do, however, is add bombproof strength and multiple crawl-ratio options (see chart) to your NP205 and it’s not-so-great factory ratio, and it does it in a compact, virtually-indestructible package. All told, it fits B.O.B.s requirements to a T.

In short, the Magnum Box is a six-gear planetary gear-reduction box that can be directly bolted or adapted to the back of virtually any domestic transmission and in front of any NP205 transfer case and effectively turns your two-speed transfer case into a true four-speed. It’s compact, 6 1/4-inch-long aluminum-housed construction adds only 34 pounds of weight to your 205, making it lighter, shorter, and stronger than any other gear-reduction option available. One of the highlights of the Magnum Box is its massive “Fat Shaft” 205 intermediate/input shaft, which doubles the strength of a factory shaft. Another highlight is its super-strong planetary gears that can handle an enormous 5,500 lb-ft of torque input. To sum it up, you’re gonna break a lot of other things before you’ll put a hurtin’ on the Magnum Box. Tolerances are tight, and the build- and component-quality exhibited throughout the Magnum Box is very impressive. It never left us wondering where all the money went.

Step one in this segment of B.O.B.’s driveline-strengthening program involved laying hands on a good, useable non-slip-yoke, figure-eight-pattern GM NP205 transfer case. In New England this isn’t an especially difficult treasure hunt, and needing to “know someone who knows someone” to locate one isn’t a prerequisite—most trucks equipped with the 205 have long rusted out around their drivetrain parts.

If you’re thinking about pulling the trigger on a Magnum Box and are confused to what you’ll need to sandwich it between the variation of 205 transfer case/transmission combo you have, a call to the folks at Offroad Design will quickly steer you in the right direction. Since B.O.B.’s SM465 four-speed manual transmission originally had a 208 transfer case behind it and the requisite 32-spline output shaft from the factory, the transmission required zero modification in order to bolt on the Magnum Box. If your SM465 originally had a 205 case behind it, you have a 10-spline output and you’ll need to also order ORD’s 32-spline SM465 output shaft and adapter in order to mate the Magnum Box.

On the transfer case end, the Magnum Box can bolt up to any GM, Dodge, or Ford 205 using the figure-eight bolt pattern. The 205 input-shaft bearing came in two diameters: 80mm and 90mm. A new and much stronger 205 input shaft and 90mm bearing are supplied with each Magnum Box, so if you have a small-bearing case (like we did) with the 80mm-diameter input bearing, you’ll need to have a machine shop carve out the bore to 90mm—an inexpensive task most any shop can handle.

Stay tuned for the next installment where we’ll be bolting on an abuse-proof clutch from SPEC, slinging the driveline back up underneath B.O.B., installing an Offroad Design Triple-Stick shifter for full control over the Magnum and 205, welding up a custom transfer case/transmission-support crossmember, and connecting the dots with a couple high-strength driveshafts to get B.O.B. rolling under his own power again.

Step two was a call to Offroad Design to get our hands on their planetary underdrive Magnum Box system. The Magnum Box effectively remedies the 205’s not-low-enough 1.96:1 low range with a much more suitable 5.33:1 range, when both Magnum Box and transfer case are in low gear. Assuming the internals of your 205 are in good working order, the Magnum Box kit comes with all the gaskets and seals needed to tear into your case and reassemble it again.
In order to hog out the input-bearing bore to the requisite big-bearing-205 diameter of 90mm, the case needed to be completely disassembled. Tearing down a 205 is not rocket science, but a good repair manual would certainly come in handy if it’s your first go-around. After making sure all parts are bagged ’n’ tagged for reassembly, it was off to the machine shop.
Opening up the input bearing bore diameter of our small-bearing case from 80 mm to 90 mm is a task most any machine shop can easily (and inexpensively) tackle. We had ours carved out at Rod’s Machine Shop in Redstone, New Hampshire. If you have a GM 205 that was attached to a TH400 transmission, an SM465 trans in an ’85-’91 K30, a Dodge 205 from a five-speed ’89-’93 Cummins-powered truck, or a married version of a Ford 205, this is a step you can skip as the bearing bore is already 90mm.
Since the case-boring procedure filled the interior of our 205 with a metal-chip-infused cutting-oil soup, we took our case down to the local carwash and blasted it clean with hot, pressurized water. For safety’s sake, we also gave the inside a good once-over with a magnet to grab any metal chips still hiding in there.
The massive “Fat Shaft” intermediate/input 205 shaft (right) and 90mm input bearing included with the Magnum Box doubles the strength of the factory connection, creating a safe working-strength to run in any of the four available gear ranges. A break here would leave B.O.B. dead in the water and result in a major trailside wrenching session, which is exactly the type of thing we’re looking to avoid.
Machined in-house and made from heat-treated chromoly steel, we also installed Offroad Design’s modified NP205 shift rails during reassembly. When used with the Offroad Design Triple Stick Shifter that B.O.B.’s getting fit with in the next installment, these direct-replacement rails allow you to send power to the front wheels only to assist in pulling you around tight and tricky obstacles in a pinch—just another tool in B.O.B.’s toolbox.
With the case completely disassembled, we took the opportunity to replace the factory-style vent with a 1/8-inch pipe-thread hose fitting in order to run the vent tubing up and out of water and mud. We also replaced the two small sheetmetal plugs concealing the shift rail roll-pins with brass 1/8-inch pipe-thread plugs to make disassembly that much easier, should we ever need to tear into the case again. Arrows note the location of these mods.
Here’s a good look at the heart of the Magnum Box and its planetary-gear system. The first step in mating the Magnum to the 205 necessitated separating its two case sections by unbolting the shifter plate and unscrewing the six long bolts holding the front and back sections together. Fitment is super-precise, requiring a few “tunks” with a rubber mallet to separate and reinstall.
Next, we ran a small bead of silicon gasket-maker along the figure-eight mounting face of the 205 (where a paper gasket would normally reside) and bolted on the Magnum’s back half, using red Loctite on the bolt threads and torqueing them to 40 lb-ft. Then, the front portion of the Magnum’s case was reattached to the rear (again, with silicon and 40 lb-ft of torque) and the shifter plate bolted back into place (you got it: with a little silicon).
Before we mated the Magnum Box/205 combination to the back of the transmission, we first needed to verify that our transmission’s tailshaft did not stick out past the housing any more than 1/8 inch. If it did, we’d have to trim the shaft accordingly to avoid contact with the input gear and preloading of the Magnum’s bearings. Ours was good to go as-is.
We ran out of time before we could paint everything, but we mocked up our new Magnum Box’d driveline to show you what the finished assembly looks like. It’s now worlds stronger, and we have the ability drop B.O.B. down into some seriously-low crawl ratios. The Magnum uses its own separate oil supply (with the option of running either ATF or 80-90W gear oil) and is fully clockable to accommodate driveshaft angles or a flat underside.
Said and done, the Magnum Box/NP205 combination is almost 3 inches shorter overall than the greasy, aluminum-cased, slip-yoke NP208 we removed from B.O.B. With the Blazer’s height and relatively short wheelbase, we’ll take anything we can get to help alleviate driveshaft angles.

The Offroad Design Magnum Box gave B.O.B. multiple low range ratio options. This chart details ratios and settings.



Offroad Design

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