GMC Ride Height is measured from the TOP of the designated chassis openings.
The front reference notches are in the front clip, just forward of the joint with the side rails. It makes more sense to measure the distance from the top of the notch to the bottom of the side rail behind the joint: That distance is 3-1/4” So the specified ride height dimension should place the bottom of the side rail at 9-7/8” +- ¼” from the floor (disregarding tire size, pressure, wear, etc.).
The rear reference notches are specified as 40-3/8” aft of the mid-point between the center and rear wheels on a 23’ coach or 56-3/8” on a 26’ coach.
I found the distance from the rear-most edge of the side rail, where it joins the rear clip, to that notch to be 10-1/4”; I suspect it’s the same on the 26’, but someone should verify that. The distance from the top of the reference notch to the bottom of the side rail is 3-1/4” there also. So a block under the side rail at about 10” forward of the rear clip joint should be 8-7/16” +- ¼” to establish the correct ride height (disregarding tire size, pressure, wear, etc.).
Personally, I’d prefer to adjust the rear ride height to have the Zerks at the same height as the spindles. Then measure the floor-to-bottom-of-frame height just behind the rear wheels. Add 1-7/16” (1-1/2” is easier) and use that as the target for the front ride height at the bottom of the forward ends of the side rails. That compensates for the differences in tire sizes, thickness of new side rails, etc.
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