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March 2007 4x4 Tech Questions - Tech Line

Posted in How To: Tech Qa on March 1, 2007
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Four Wheeler
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Question: I have recently purchased an '86 Chevy D30 pickup. It is a military truck and it even has a tag stating "Property of the U.S.A." It has a 6.2L diesel, and due to its military heritage it has some unique features such as dual alternators, a plug-in fuel filter, oil cooler, 4.56:1 gearing, and a lot of unique wiring.

I would like to know where I can get more tech info on this truck, maybe even some tech manuals. Parts stores don't list D30 as a vehicle option. Do you know if it is the same as a K30 for the most part?
Rick Carroll
Chatham, MI

Answer: Yes, your truck is basically a K30 with the exceptions you have noted and a bunch of military-only items. And yes again, the wiring is quite unique with its dual electrical system that has two 12-volt alternators and dual 12-volt batteries. Troubleshooting that maze of wiring, should you have a malfunction, could be a real nightmare.

Tech manuals are available, though it takes (in typical military fashion) about 14 of them to cover the truck. It's referred to as an M1008 series cargo truck in military lingo. I think that your best bet would be to order a CD from

Question: I have a '79 Bronco running a 400M V-8 and C6 trans. It has a Detroit Locker in a Dana 44 up front, and a 9-inch with a posi in the rear. I'm currently running 3.55:1 gears in both. I'm putting a 6-inch lift with 15/38.5-16 Boggers and I want to change the gears to a 4.10:1 or so, but I've been told I can't without putting in a new Detroit. Is this true?
Name unavailable

Answer: Yep, you will need a new Detroit in your 44 to make use of those new lower gears. You're in luck with the Ford 9-inch, however, as it will take the lower gears. You may have to do some slight grinding around the pinion support for clearance but it's no big thing. Also, I sure would consider a 4.56:1 gear ratio with those big Boggers, and if you don't drive it much on the street, maybe something in the 4.89:1 range.

Question: I have a Jeep Cherokee with a rebuilt I-6 engine with 20,000 miles on it. For the first 10,000 miles, it did not leak oil, but then it started to leak at the rear main seal. My mechanic has replaced the rear main seal seven times already, but it still leaks! He also replaced the oil pan. He said the crankshaft is worn and it leaks because I used 10W-30 synthetic oil. The engine is still under warranty, but I really don't want to replace it again. I heard from another guy about an offset or double-lip rear main seal. If these exist, please tell me where I can buy them.

Answer: I doubt very much it's leaking because you're using a synthetic lubricant. Yes, I have seen some high-mileage engines leak oil out the rear main seal and other gasket areas that had been using a straight petroleum oil and then switched to synthetic. The detergent action of the synthetic loosens up all the crud that had kept it from leaking in the first place. In fact, the synthetic lube should prevent excessive wear to the crank seal area.

The problem as I see it is that the crank seal area was improperly ground from the remanufacture of the engine. I did some checking and couldn't find any double-lip seal or offset seal for your engine.

In reality, the only way you're going to solve the problem is to have the engine rebuilder replace the engine with one that the guy who did the crankshaft hopefully did a better job on.

Question: I have a '74 Chevy Nova with a full 'cage and fuel cell in the trunk (it was originally destined to be a circle track racer). But because I already have a circle track car, I want a somewhat unique 4x4. I plan to build it on a Blazer frame. I want to run it mostly on sandy/muddy trails. I want a 451 or 502 with multiport fuel injection.

I was wondering if it would be worth it to put a twin-turbo system on it or a big 8-71 blower. Which would be more cost-effective, and which is better for off-pavement use (strength and rigidity)? My concern is which setup will make better torque sooner. I know that I can change the pulleys on a blower, but what can I do to a turbo to change the rpm range? Also, I was reading a magazine and they mentioned a 10-71 blower-do you know anything about it and if it really exists?

My plan is to have about 40 inches of spinning rubber, and of course gears to match, but I am also wondering if I should get a TH400 or 700R4 tranny (unless you know of something better); either one would be built up, of course.

I am leaving for Fort Benning near Columbus, Georgia, for my Basic/AIT, Airborne, and Ranger school in January and won't be back until at least July. From then, I have to save money and find time to do this build. I plan on keeping you posted as to my progress (pics, notes, videos, and so on). This will be a trail-only rig, so legality is not a concern since it will be spending most of its time on tank trails and the like.
Gregory Ray

Answer: Before you go forward with this project, I figure that you must have a rich relative who is going to pay for all this-or that military pay has taken a big jump. An 8-71-style blower on a built big-block most likely will be in the neighborhood of $20,000 to $35,000. The Turbo 400s can be built up to about 600 hp but cost at least $2,500 with the torque converter. Axles will need to be custom-built Dana 60s, or better yet, Dana 70s or 80s, so count on another $15,000. It will take a lot of radiator, oil cooler, and transmission cooler to keep temperatures under control, so toss in a couple more thousand. Forty-inch tires on matching rims will cost about $600 to $1,000 each, depending on the tire/rim combination you use. You're going to need additional stopping power, so put several thousand away for brakes. Now we haven't even got to suspension and frame modifications yet, or body modifications needed for tire clearance. Want to rethink this project a bit and scale it down?

Question: I have a '94 Wrangler YJ that has a spring-over front and rear. Custom modifications were performed on the front for the steering and other required mods. A 4:1 transfer-case kit was installed as was a slip-yoke eliminator.

I currently have the heavy-duty six-leaf spring packs installed. The springs are approximately flat when parked. The rear does not appear to be lower than the front. The ride is fine with the heavy-duty springs and nitro shocks.

The problem is a vibration in my driveline at about 1,500 to 2,000 rpm in Second and Third gears. The vibration goes away in higher gears, but returns on uphill highway climbs. The gears have been changed to 4.10:1 and I am running 33x12.50R15 tires on 15x10 rims. The mechanic who did the work put shims under the springs to get the correct angle from the pumpkin to the slip-yoke. He thinks the vibrations are coming from the springs relaxing too much. I am thinking of installing some Firestone soft-ride air springs to bring the rear up some to correct the flat-out on the springs.
Hugh Richards

Answer: What is happening? The rear springs are flexing and turning the pinion upward when you accelerate or put added power to the driveline, such as when you're going up a hill. This change in pinion angle results in the U-joints at each end of the driveshaft rotating at different rates. Actually, the front half of the spring is flexing upward and the rear half downward. This is a pretty common problem with flat springs on a spring-over conversion. I am surprised that you don't experience wheelhop when climbing or accelerating hard when off-pavement.

I really don't like the idea of airbags. For one, it's something else to go wrong. Second, unless you run a lot of air to push the leaf springs into a considerable arch, they won't do much to help your problem. With additional arch, the back end will be considerably higher than the front, and your Jeep will look like a "stink bug" and the pinion angle will be wrong.

Some people have been able to stop or control this problem by a custom set of leaf springs that have a special loop around the spring eye, along with offsetting the spring's locating pin. The offset pin results in the front half of the spring being longer than the rear, which helps counteract spring wrap. This also will require new spring mounts. I believe Alcan ( has made some of these type springs. Another solution is some type of a center-mounted traction or antiwrap bar like the one that Sam's Off Road ( sells.

Question: I have been reading and subscribing to your magazine on and off since I was 10 and when you're that little, you're always running to the mailbox hoping the next issue was waiting.

Now 26, I have acquired my dream rig. It's a '67 F-250HD Hi-Boy. It has a swapped-in 390 that I put chrome fenderwell headers on, factory 4.55:1 and 4.56:1 gears in the HD Dana 44 front and Dana 60 rear. It has the NP435 trans, which I was very happy to read is a good unit with a killer crawl ratio, and finally the Dana 24 transfer case. In stock suspension form with no lift it clears 35s with no problem.

My question is this: for purposes of a functional 4x4 that I can really enjoy while retaining the restoration look, which driveline component is my weak link? It has enormous knuckles and locking hub units. I do want to lift it for 40s since 38s just don't look big on these trucks. This probably isn't smart with a Dana 44, but would 38s work? Can I twin-stick the Dana 24 case? I'd love to build it as a nostalgic-bodied functional 4x4 prerunner, but for now I'll shoot for hearty trail rig with the bead-locked, rock-slider look.
Micah Radnich
Yucca Valley, CA

Answer: It sounds like you have built yourself up a pretty nice classic Ford truck. Not knowing just how hard you romp on this truck, and to be on the safe side, I am going to say that in all honesty that I think you should stick to your present 35-inch tires. OK, they may look small to you, and the 38s or 40s would make this truck look even cooler. While you have the big knuckles and hubs on the 44 and a full-floating rearend, you still have 1 5/16-inch, 30-spline axleshafts in the 44 and 1 5/16-inch, 16-spline axleshafts in the rear. If you were using these components in a lightweight rig, then I would say go for it, but in your fullsize truck, I think that you would be asking for, well, broken axles. I know, I know, you see in the pages of this magazine (and other truck mags) people running 38s on even smaller axles. That doesn't mean that they really use these vehicles hard or that they don't break axles all the time. I guess it comes down to how hard you want to use your truck and how often you want to replace axles-that is, if you can even find stock axles.

Now this brings up another point. You could always contact one of the custom axle makers and have a set made out of quality material. While you're at it, you could change out the spider gears in the rear for a set with 30-splines to upgrade the axleshafts.

If you don't plan on really serious trails where you're going to be really working those axles hard, then go for the 38s and keep your fingers crossed.

Question: I have a '90 YJ that I'm dropping an AMC 304 in. I've already found my motor mounts. Could you please give me a list of other components that are required, and where to find them? Also I'm trying to find a set of headers for the 304 that will fit with my auto tranny.
Jamie Legacy
Waterbury, VT

Answer: I've never heard of anyone putting a 304 AMC motor in a YJ, but I don't see any reason why it can't be done. Wow, a list of everything that you will need and where to find them? That's tough. Well, Novak Conversions (877/602-1500, has an aluminum radiator (PN CR1166-2) that will bolt in. It's designed for Ford conversions, but the AMC motors have the hose fitting in the same location.

I would think that for throttle linkage, you could use the parts from a CJ V-8 Jeep. This should still be available from a Jeep dealer or most off-road shops. Crown Lokar (877/469-7440, has some really nice custom throttle linkages.

Now as to headers, I have no idea. Hedman (562/921-0404, offers at least four different configurations of headers for the AMC engine, so you might give them a call and see if you can get some measurements.

One thing that you really need to keep in mind before doing this swap: Will it pass an emissions test in your area?

Question: I'm new to four-wheeling, and I have an '00 Jeep Cherokee that I am going to lift 4 1/2 inches. I was wondering if I have to get a slip-yoke eliminator? I was also wondering if I have to change my steering column? Is a 4.10:1 gear ratio good enough to run 33x14 Boggers?
Robert Dodman

Answer: No, you won't have to do a thing to the steering column. The 4 1/2 inches of lift just may be marginal without the slip-yoke eliminator kit. I think that I would put the lift and tires on and see just how far out of the end of the transfer case the driveshaft rides. It will be out quite a ways from stock and just may not have enough engagement, so you may need to have a longer driveshaft made. The real advantage of the slip-yoke eliminator kit is that it gains you a driveshaft several inches longer, which in turn reduces the angle at which it operates and will hopefully prevent any binding of the U-joints.

I suggest that you jack up the rear of your Cherokee by the rear bumper until the wheels are hanging free. You did block the front wheels on both sides to prevent the Jeep from moving, right? OK, now crawl under it and turn the driveshaft by hand, paying close attention for any U-joint bind. If it does bind, then you're a candidate for the kit. It's going to require a new driveshaft with a slip-yoke built into it.

If there is no binding, still consider a longer driveshaft so that you'll have a good positive engagement with the present slip-yoke that slides inside the transfer case. It's important to make sure that it is not too long so that at suspension compression, the end of the yoke doesn't bottom out and put end pressure on the case.

Using a 4.10:1 ratio will put it back pretty close to what the overall ratio was when the vehicle was stock. You will feel some performance loss because of the added rolling resistance of the larger tires. If you're going to see a lot of off-pavement travel and not as much highway with the Jeep, then 4.56:1 would be a much better ratio. Perhaps this would be my choice.

Question: I have a '92 Wrangler YJ with a Skyjacker lift and 35x12.50-15 Baja Claws. It has the original differentials, and I recently acquired a set of Dana 44s from a '76 Dodge which was full-time four-wheel-drive. I did not, however, get the transfer case. I want to lock it up but have heard so many different things about which lockers to use and whether to lock the front solid. They are both posi units-is that good enough for the front?

I play in mud, snow, and hillclimbs, and would like an all-around good machine. I've heard that locking it up all the way around makes it hard to handle in the snow. I live in Oregon, so the snow is wet and heavy. I didn't want to spend the money for selectable lockers, but is there any other way? Right now, it has the stock 4.0L and manual tranny and transfer case. A small-block Chevy will be added as well as an automatic. Can you recommend a transfer case, and are the Dana 44s stout enough to handle a set of 39x18/15 Boggers? I am leaving the axles full width since the tires rub on the springs now.
Molalla, OR

Answer: There is no reason why you can't run your present NP231 transfer case, even when you make the Chevy engine swap, and leave it in the two-wheel-drive mode when four-wheel drive is not needed. Why don't you try it with the present limited-slip differentials and see if they offer you the necessary traction for what you plan to do with your Jeep?

For snow driving, my favorite combination is a Detroit Truetrac up front and some type of a selective locker in the rear like an ARB Air Locker or the Eaton E-Locker electronic differential. Another good choice would be the Detroit Electrac which offers the advantage of the Truetrac but can be totally locked with the flip of a switch.

The Dana 44 axles will be marginal with a set of 39-inch tires, but maybe with a light foot they will survive.

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