There seems to be some confusion over the functional nuance of the TJ OEM body mount. To clear it up, I've taken pics of the parts and then cut them in half to help understand what is going on.
We will use one of the 5 smaller ones that go under the grill, at the rear shock/gas tank crossmember, and rear torque box since that seems to be what is confusing folks. All 5 of those locations use a floating nut plate in a cage. This is an example of one since I didn't feel like cutting up a tub to show the ones in the TJ.
The other thing that needs to be paid a lot of attention to is the OEM bolt length. The reason for that is the grill and torque box over the rear bumper area have an excess of room over the top of the floating nut. Bolt length can easily vary more than 1/2" with no penalty or problem. The same is not true at the gas tank crossmember. There is just a bit more than 1" from the top of the body mount to the bottom of the sheet metal that forms the floor of the cargo area. The factory used 2 different length bolts to compensate although oddly, they only used the longer one under the grill. This is the comparison of the two OEM bolts and length.
This is the body mount we are talking about showing all the pieces.
It is a fairly innocuous looking bit of kit with a lot of function that doesn't seem obvious until it is pointed out. The mushroom stem and cap is domed. The cross section will better show why but the main reason for the dome is to control the compression of the top rubber element that the tub sits on. The hollow stem is to control the over all height when the mount is installed. You can NOT change the height of the mount past the design limit regardless of how much you tighten the bolt. The lower cup has three functions. The first is a large washer to bond the skirt to and give the stem a place to land. The second is to prevent upward movement of the tub relative to the frame, and the third is to act as a bit of a spring to pull down just a bit on the upper element to prevent it from moving around much in the hole in the frame. That is achieved by making the overall height of the body mount slightly shorter that the compressed condition so it will grip the frame some. The lower skirt also loses the compression battle so it does NOT compress the upper element very much at all when the bolt is tightened.
Here is the cut away where I sliced the whole mess in half so it is easier to understand.
Of note, pay attention to the very minimal gap between the upper and lower mount that will get pushed apart slightly by the thickness of the metal in the crossmember and frame mounts. That is the distance the lower skirt gets compressed to provide some lateral grip. Note how the stem cap is domed to capture the upper element to slow down how much it can compress. That also helps to locate the cap on the element to assist with a reduction in lateral movement. This is with the shorter correct bolt for the gas tank crossmember and there is only a scant 1"+ of room over the top of the mount. The end of the factory bolt is almost touching but it never gets there. Or at least I have never seen it under the carpet of more TJ's than I can count or even begin to remember lifting the carpet on.
This is what it looks like with the bolt under the grill.
That is why when you design a body mount lift kit, you measure those bolts, subtract the pilot depth, and then add the height of the lift puck to that length to get your supplied bolt lengths. If they are even a tiny bit close to too long, they will dimple the floor in the cargo area. I've seen way too many of them with the dimples including my own from when I did my first body lift. If you are running a variable height body lift with a compressible puck, I strongly suggest you compare the OEM bolt to the new bolt and make sure it is not longer by the OEM bolt length plus the height of the lift puck. Any extra will show up where you would least likely enjoy it.
If you want to see what the dimples look like with the OEM mounts in place, take the OEM grill bolt out, swap it with one of gas tank crossmember bolts and tighten it down. You may get lucky and only get a dimple instead of the hole we normally see when folks get confused and don't catch the longer bolt in the mix.
We will use one of the 5 smaller ones that go under the grill, at the rear shock/gas tank crossmember, and rear torque box since that seems to be what is confusing folks. All 5 of those locations use a floating nut plate in a cage. This is an example of one since I didn't feel like cutting up a tub to show the ones in the TJ.
The other thing that needs to be paid a lot of attention to is the OEM bolt length. The reason for that is the grill and torque box over the rear bumper area have an excess of room over the top of the floating nut. Bolt length can easily vary more than 1/2" with no penalty or problem. The same is not true at the gas tank crossmember. There is just a bit more than 1" from the top of the body mount to the bottom of the sheet metal that forms the floor of the cargo area. The factory used 2 different length bolts to compensate although oddly, they only used the longer one under the grill. This is the comparison of the two OEM bolts and length.
This is the body mount we are talking about showing all the pieces.
It is a fairly innocuous looking bit of kit with a lot of function that doesn't seem obvious until it is pointed out. The mushroom stem and cap is domed. The cross section will better show why but the main reason for the dome is to control the compression of the top rubber element that the tub sits on. The hollow stem is to control the over all height when the mount is installed. You can NOT change the height of the mount past the design limit regardless of how much you tighten the bolt. The lower cup has three functions. The first is a large washer to bond the skirt to and give the stem a place to land. The second is to prevent upward movement of the tub relative to the frame, and the third is to act as a bit of a spring to pull down just a bit on the upper element to prevent it from moving around much in the hole in the frame. That is achieved by making the overall height of the body mount slightly shorter that the compressed condition so it will grip the frame some. The lower skirt also loses the compression battle so it does NOT compress the upper element very much at all when the bolt is tightened.
Here is the cut away where I sliced the whole mess in half so it is easier to understand.
Of note, pay attention to the very minimal gap between the upper and lower mount that will get pushed apart slightly by the thickness of the metal in the crossmember and frame mounts. That is the distance the lower skirt gets compressed to provide some lateral grip. Note how the stem cap is domed to capture the upper element to slow down how much it can compress. That also helps to locate the cap on the element to assist with a reduction in lateral movement. This is with the shorter correct bolt for the gas tank crossmember and there is only a scant 1"+ of room over the top of the mount. The end of the factory bolt is almost touching but it never gets there. Or at least I have never seen it under the carpet of more TJ's than I can count or even begin to remember lifting the carpet on.
This is what it looks like with the bolt under the grill.
That is why when you design a body mount lift kit, you measure those bolts, subtract the pilot depth, and then add the height of the lift puck to that length to get your supplied bolt lengths. If they are even a tiny bit close to too long, they will dimple the floor in the cargo area. I've seen way too many of them with the dimples including my own from when I did my first body lift. If you are running a variable height body lift with a compressible puck, I strongly suggest you compare the OEM bolt to the new bolt and make sure it is not longer by the OEM bolt length plus the height of the lift puck. Any extra will show up where you would least likely enjoy it.
If you want to see what the dimples look like with the OEM mounts in place, take the OEM grill bolt out, swap it with one of gas tank crossmember bolts and tighten it down. You may get lucky and only get a dimple instead of the hole we normally see when folks get confused and don't catch the longer bolt in the mix.
