Fitting a DC5 (RSX) Rear Diffuser to an 8th-Gen Civic Coupe (FG2)

Lift the rear of the vehicle with a jack and secure it on jack stands. Remove the heat shield above the muffler, which is retained by a handful of 10mm bolts. This shield occupies the space the top edge of the diffuser needs, so leaving it installed makes accurate test-fitting impossible.

Dry-fit the diffuser and note that its molded tow-hook opening does not align on the FG2. Open that cutout up by roughly 2-3 inches so the panel can seat correctly, positioning it as close as possible to the rear control arms (the area with the two large bolts).

With the tow-hook side set, the far side of the diffuser will project past the bumper. Measure that overhang and remove the same amount from the front edge (the side opposite the tow-hook cutout). This cut passes through both center fins. Trim up to the edge of the two holes meant for the small L-brackets. Because the cut also removes the factory holes that hold the ends of the fins in place, drill fresh holes and re-bolt those fin ends.

Check the outer wings against the rear springs; they will sit extremely close. Set two trim lines with the measuring tape and marker, then cut back about 1 1/2 to 2 inches on each wing. That should leave enough room for suspension movement.

Take off the rear bumper cover to reach the mounting points behind it. A separate tail-light and bumper removal writeup on the same forum covers the process if you have not done it before.

Find the two central holes below the trunk that normally accept the bumper's plastic tabs. Use them to mount the rear of the diffuser with the supplied metal brackets, which fit the hardware tightly for a strong, snug hold. If the bracket holes do not line up with the factory ones, mark and drill them with the 1/4-inch bit. Once bolts occupy these holes, the bumper tabs can no longer slide into place, so cut those tabs off the bumper so it fits again.

The supplied L-brackets are bent at an angle suited to the DC5 and are more aggressive than the Civic needs; bend them back slightly if you prefer. Enlarge the frame-side hole of each bracket so it clears the larger chassis bolts (roughly a 1/2-inch hole). These mount at the rear lower tie-bar location, which uses two 14mm bolts; an FG2 rear lower tie-bar writeup documents that hardware. To reseat each control-arm bolt, raise the control arm with a jack until the holes nearly overlap, align the arm to its housing with a pry bar or screwdriver, feed a smaller bolt through the rearward thread so it holds everything steady, then drive the main bolt home with a hammer or mallet. Do not fully tighten yet; the author torqued these to about 40 ft-lb only at the very end of the build.

Line each L-bracket up against the diffuser's front edge and mark where the holes fall using the marker. Reaching this area often means crawling deep beneath the vehicle unless your arms are unusually long. Drill the marked holes, bolt the front of the diffuser to the brackets, and rest the assembly on jack stands while you continue.

The kit provides one long and one short side bracket, and the long one needs trimming; cut along the bottom of its U-shaped section. Mock both brackets up on the panel and find the pinch welds, the thin metal seams running along the underside of the frame, which serve as the drilling anchor. Cut away sections of the lower bumper edge so the brackets have clearance. Test-fit the bumper to see where each bracket lands before drilling.

For the cleanest contour, bolt the side brackets to the outer face of the pinch welds, then twist the diffuser wings until their edges follow the upward slope of the bumper's lower edges. Expect several rounds of fitting the bumper on and off to judge the angle. With the bumper on, press each bracket against the pinch weld and scribe a line using the weld as a stencil; remove the bumper, rotate the bracket until the line meets the weld again, then mark, drill, and bolt. Apply the same technique to the trimmed long bracket. Only once every panel sits flush and even with the bumper's natural lines should you torque all fasteners, then reinstall the bumper.