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Body and motor mount lifts....a pretty popular combination in the TJ arena. I put both on when I was getting my first locker installed. I spent part of a day over at my friend's house and we worked our way through both with no real problems. It doesn't take any exotic tools or special skills to install either one. I did find that about 18" of ratchet extension was pretty darn handy when doing the motor mount bolts on my TJ.
TJ owners on some on-line forums have somehow, over time, turned this pair of lifts into the magic cure all for rear driveline vibes. I chuckle every time I read the parroted comments that keep being offered when advice for a 3.5" spring lift w/ rear drive shaft vibrations is sought. How it ever came to be like that is beyond me....must be some kind of urban legend thing! So, let me see if I can outline some of the issues that must be evlauted when considering either or both of these lifts. I hope I can shed some light on this and answer any questions you might have.
My comments for the body and motor mount lifts assume you are doing one or the other. After covering both lifts, I'll comment on the issues encountered when doing them together.
Body lifts have been around for quite some time. They are pretty common on Jeeps, from what I have seen. It is a straight forward method by which to gain wheel well clearance while still keeping the center of gravity low (compared to a suspension lift of comparable height). Most folks don't like the gap that grows between the frame and body as one goes up in lift sizes. A 1" gap can usually be tolerated by most folks and is often not noticed by many (unless they have a dislike for body lifts).
The body lift is simple to install, assuming your existing body mount bolts are not rusted in place (something that the older Jeeps sometimes have problems with, especially if they live in areas that see wet roads and salty winters). In the simplest terms, you loosen the body mounting bolts on one side of the vehicle, jack it up just enough to slip the new spacers in, and install the longer mounting bolts. Repeat the procedure on the other side of the vehicle and you are done. On a TJ, there are 11 of them, with the 11th one being in the center of the front grill area.
With a 1" body lift, you shouldn't have to worry about your filler hose (for the gas tank) becoming to short. However, your transfer case shifter and manual tranny shifter may not be in the best position. Most of the body lifts come with a bracket that helps the t-case linkage regain its proper alignment. The manual tranny stick, however, is attached to the tranny which is still sitting in its original position (bolted to the cross member). Some folks have discovered that shifting into one or more gears, such as 2nd and Reverse, becomes difficult. When this happens, you need to carefully remove your center console and get down to the tranny stick and the body and check them out. You'll probably have to trim a little metal from the tub so that it does not interfere with the movement of the stick shift.
At this point, the other main issue you need to deal with is the improper positioning of the radiator fan in relationship to the fan shroud. Again, remember that the body moved up, and with it, the radiator and fan shroud (yes, these are attached to the body and not the frame). While these moved up, the radiator fan, which is attached to the front of the engine and thus the frame, stayed in its original position. The usual fix for this is to slot the mounting holes on the fan shroud and lower it back down a bit so that the fan is again centered in the shroud and there is no clearance issues.
The motor mount lift, as the name implies, lifts the vehicle's engine a prescribed amount, which is usually 1". I run a set of 1" taller Currie mounts on my TJ. The install of these are also pretty straight forward. Loosen both factory mounts (but don't pull the bolts out) and then support the engine with a floor jack. Remove an existing motor mount, raise the jack a bit, and install the new one. Repeat the process on the other side. While doing this, it is a good idea to loosen up the 4 tranny mounting bolts that are accessible through the bottom of the skid plate. This will allow the engine/tranny combo to move just a bit as you are installing the new engine mounts. Don't forget to tighten the 4 bolts back up when you are done.
When you finish the motor mount, the engine will be sitting 1" higher than it was when you started and your radiator fan is going to be crunched up against the top of the radiator shroud. As with the body lift, slotting the holes on the shroud is the most common method to get things back into alignment. You may also find that your t-case shifter linkage may need some adjustment since your tranny/t-case has moved up closer towards the body, albeit not nearly the full inch felt directly at the motor mount (remember that the tranny is bolted to the cross member, courtesy of the tranny mount, so this point acts as a pivot point.) As the front of the engine was raised 1", the back of the t-case dropped a small portion of that inch because it is located much closer to the pivot point, the tranny mount. This will be an important point to remember when we talk about drive line vibrations.
Aside from the change in radiator fan position, the tall motor mounts also moved the oil pan 1" up and further away from the rocks. This is one of the major benefits of doing the motor mount lift. Keeping the oil pan out of harms way goes a long way towards ensuring a timely return trip from the trail.
As I previously stated, I am running both of these lifts and have now for several years. I've noticed no adverse affects and can't remember reading about anyone else's problems that stemmed from these lifts.
I did my body lift to gain an extra inch of clearance for some new tires I was getting. At that time, I was running a 2" suspension lift and with the tire backspacing I had, the 32" BFG MTs were not going to miss the flares. I did the motor mount lift for two reasons. First, I didn't want to mess around with the fan shroud. Second, I had discovered early on that the rocks here in Arizona were looking to have a love affair with my oil pan and getting it an inch further away from them was OK in my book. Also, I found (after installing both of them) that my t-case linkage and manual tranny shifter were both just fine and I did not need to make any changes in those areas. In fact, I did not even have to install the t-case linkage bracket that came with the body lift kit.
When I first installed my Tera 2" spring lift, I got some rear drive shaft vibrations. Some folks do and others do not (or they don't notice them). I installed a regular 1" t-case drop kit and the drive shaft vibes were gone....easy enough to do. After putting on both of above lifts, I pulled the t-case lowering spacers. I went for a ride and found that I had some drive shaft vibrations. It was NOT nearly as bad as when I first put my 2" lift on, but it was NOT as good as when I had the t-case drop kit in place either.
So...yes, that little bit of lowering at the t-case rear output shaft that comes from the motor mount lift can help get rid of your rear drive shaft vibrations....IF.....they are not too bad to start with. But let me tell you, don't count on it very much. It does not come close to changing the output shaft position like a t-case drop does.
I don't like t-case drops because it really kills your break over clearance. Besides that, you just spent good money to lift your Jeep with a spring lift and now you lower the t-case skid....doesn't seem right, eah? Granted, if you are on a tight budget, it can get you buy until you can afford to do a SYE and adjustable control arms.
That is about it. I've covered, to the best of my knowledge, the pros and cons of both the motor mount and body lifts. I've installed both of them and have experienced both of them on my vehicle with different spring suspension lifts. I don't really see any negatives to putting them on the vehicle. Some purest that don't like body lifts will argue about it and that is OK. Everyone is free to do as they choose.
I hope this has shed some light on the subject and you now have a better understanding of what both of these lifts bring to the table.
Good trails and remember to TREAD Lightly!