Nth Degree Mobility Tummy Tucker Install by Will Belknap

Vehicle:  '98 TJ 4.0 Auto with 3.5-4" of suspension lift
**Disclaimer: This write-up is not meant to replace Nth Degree’s installation instructions, it is only meant to document one particular installation experience.
For more information about this product, visit the website at http://www.nthdegreemobility.com/tt.htm

Purchased from www.northridge4x4.com, cost is $460 for just the skid plate.  If not already present, the cost of setting the jeep up with a slip yoke eliminator kit should also be factored in.  I went with this skid because it did not require a body lift and after reading through the installation instructions seemed a very straight forward installation (which it was).  It is however, the most expensive I’ve seen.

Other requirements:
This skid plate raises the transfer case output by 2" or more so the jeep will require a slip yoke eliminator be installed to lengthen the rear driveshaft and facilitate the installation of a double cardan style rear driveshaft.  I did not have a SYE on my TJ prior to this skid.  I have about 3.5-4" of lift on my jeep and had previously been using a 1" transfer case drop to correct the rear driveline angle.  I had been wanting a SYE anyways to get rid of my lowered stock skid.  All of the other items purchased for this install were related to the SYE install.

• Slip Yoke Eliminator Kit (I used Advance Adapters)
• Rear adjustable upper control arms (I used Nth's budget arms with stock bushings)
• Nth Degree Shock Shifter kit to provide shock clearance after rear axle is rotated for pinion angle (alternatives are to run shocks upside down or use other bolt-on shock relocation options)
• CV rear driveshaft (I got a Rubicon Express 17.5")

The install began by supporting the transmission, unbolting the 4 tranny mount nuts, 6 frame rail bolts, and removing the stock skid plate and installing the slip yoke eliminator kit.  An excellent writeup on an Advance Adapters SYE install is available at http://www.4x4xplor.com/SYE-CV.html 

I also took this opportunity to paint my rusting frame rails with por15 to inhibit further rusting.

Install Nth transmission mount
Once the SYE was installed it was time to begin the tummy tucker install.  With the stock skid plate still removed, the first step was to remove the stock transmission mount.  This is done by removing two 5/8" head bolts through access holes in the bottom of the mount.  For my 3 speed auto I had to first remove four 15mm nuts holding the rubber isolator to the stock mount.  You then remove the rubber exhaust hanger bushing from the stock tranny mount and transfer it to the new Nth transmission bracket supplied with the skid.  I used the recommended 3/4" socket and a large vice to press the bushing out.  The Nth bracket was then bolted to the transmission and torqued to the specified 50 ft/lbs.

Modify the transfer case linkage
The next step was to modify the transfer case linkage for the new transfer case height.  The TJ’s transfer case linkage pivots between two points, one being a bracket on the transfer case and the other being the body.  As the transfer case is raised with the Nth skid plate the shift lever would bind if it wasn’t corrected.  The installation instructions walk you through relocating the transfer case side down approximately 1" so that the linkage is close to level after the new skid plate is installed.  A new hole is drilled in the stock bracket and the pivot bushing is reused.  I followed the instructions and removed the bracket from the transfer case (have an extension for your ratchet and a pivot joint handy for the top nut).  The instructions recommend redrilling the new 15/32" diameter hole at least 1" below the original and 1/8" forward if possible.  My bracket had enough metal so I ended up with the following relocated bushing (in green).  After the skid was installed this location turned out perfect.

Getting the end of the linkage back into the pivot bushing can be a real challenge.  Greasing the grommet helps but be ready to exert some real muscle to get it in there.  I had to do this previously in the stock location and it wasn’t any easier so this wasn’t a result of the new hole, its just a tight fit.

Install the Nth Transmission mount isolators
The next step is to install Nth’s new transmission mount isolators.  Only attach the bolts finger tight, do not fully tighten the bolts on the isolators yet, you need them to be easily moved so you can line the holes up with the skid.

This is the one area of Nth’s installation instructions that had an error.  The instructions said to verify I have the right isolators by locating a paint dot on the underside.  Well I didn’t have this dot.  A quick call to Nth degree confirmed that the dot was no longer on the isolators and should be omitted from the instructions.  Install the isolators diagonally so they will be somewhat close to lining up with the holes in the skid plate.

Trial fit skid plate
At this point the skid is ready for a trial fit.  Use a jack to lift the skid into position as it is quite heavy and unless you braced the tranny really high you will need to lift the drivetrain as well.  As the skid begins to reach the frame rails be sure it is centered side to side, the vertical gussets running inside the skid can catch on the frame rails.  Make sure you don’t crunch anything above the drivetrain such as wiring harnesses as the tranny/tcase start to lift with the skid.  Start the provided skid plate allen head bolts by hand to line the skid up with the holes in the frame.  The skid comes drilled with many holes for different applications, for my ‘98 the front frame bolt needed to line up with the 2nd hole in the front of the skid.  Once the skid is bolted to the frame, line the transmission isolators up with the four holes in the skid plate.  The top bolts for the isolators can now be tightened, it is a tight fit but I was able to reach both bolt heads from the front of the skid plate.  You will not want to put the small allen head bolts attaching the isolators to the skid plate on yet as you will likely need to drop the skid again to make adjustments after checking your clearances.

Adjust the transfer case linkage
 Adjust the transfer case linkage at this point by loosening the locking nut, shifting the transfer case into 4low, and adjusting the linkage bar until you can shift from 2wd to 4low without interference.

Clearance checks
With the skid plate installed you can begin to check your clearances.  Here are some pictures of the impressively tight clearances this skid is designed with.  The instructions indicate a minimum of 1/4" clearance at all points to account for movement.  Be sure to remove your jack from beneath the skid before checking clearances as the plate will deflect slightly with a jack under it and skew your inspection.

The first place I checked was the lowest point of the transfer case which is directly below the front output shaft and visible from the rear of the skid plate.  I did not have the required 1/4" clearance, it was more about 1/8”.  The hardware bag included with the skid plate includes 4 washers which can be inserted between the transmission mount isolators and the Nth transmission mount bracket.  These washers space the drivetrain up slightly from the skid, mine required all four be used, 2 on each side to get the required 1/4" clearance below the transfer case.  Two of the washers are visible in the previous isolator installation picture.

The next clearance issue I found was the catalytic converter was being pushed into the heat shield.  This proved to require the most amount of work in the entire installation to correct and would probably not be an issue for somebody with a body lift of any size.  Simply put, the exhaust hanger that attaches the catalytic converter to the transmission mount needed to be bent to lower the catalytic converter slightly.  This is a very thick bar, maybe ½" round stock and I don’t have a torch to soften it up to bend.  What I ended up doing was:

• lower the skid plate
• use the angle grinder to cut through about 3/4 of the hanger right next to the pipe
• make an adjustment by bending at the cut (usually upwards to lower to the catalytic converter)
• welding the cut up to make the bar solid again and hold the new position
• reinstall the skid plate and retest clearances.

This ended up taking about 6 iterations to finally get the catalytic converter to have the required 1/4" clearance above and below.  I ended up making two slight bends to the hanger so that it could be bent the required amount to lower the cat without also contacting the skid plate.

This is where I did the cutting/bending.  I also made another cut/bend about an inch to the right to make a very mild S shape for clearance.

The last clearance issue I had, and one you do not want to forget to check before firing your engine, is the engine fan and radiator shroud.  With the drivetrain now angled forward slightly due to the lifted tranny mount, there is a chance the fan won’t have enough clearance.  When I checked mine it had zero clearance and was resting on the shroud.  The skid instructions suggest removing the shroud (which also requires removing the engine fan) and widening the holes so the shroud can be lowered.  I knew I needed more clearance than simply opening up the existing holes would allow so I drilled new holes 1" above the stock locations in the shroud.  This required cutting some fins on the shroud to allow clearance for the bolts.  Moving the shroud down this amount does make it rest on the lower radiator outlet but there isn't much pressure on it so I don't think it will be an issue.

With all of the clearance issues resolved it is ready to drive. 

Conclusion
The 1/4" clearance so far has been adequate and I have experienced no clunking or any other indications the drivetrain or exhaust are lacking in clearance.  The passenger side floor has not become heated from the catalytic converter.  The Nth transmission mount does transfer significantly more engine vibrations through the jeep at idle but I notice no difference off idle while moving.  I’d say the idle vibrations are maybe 50-75% more than the stock mount and skid.  Luckily my interior is pretty tight but I could imagine discovering a few more rattles here and there in some jeeps.  With my 3.5-4" of lift I am not experiencing any driveline vibrations.  The stock front driveshaft was left unmodified and no adjustment was made to the front pinion angle.  The rear pinion angle was set .5 degrees below the 17.5” long driveshaft.  Although the exhaust hanger modification was not fun, that was the only issue that took more than a small amount of effort to accommodate and all of the issues I found were predicted by the installation instructions complete with recommended remedies.  The skid has holes in strategic locations for transfer case draining & front driveshaft removal.

Before w/1” transfer case drop


After Nth 1.25” Drop TT & SYE

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