Drilling Pilot Holes for the Dusan Strutworks RACP Reinforcement Brace: What Actually Works

Drilling Pilot Holes for the Strutworks RACP Reinforcement Brace

The drill-from-below, chase-the-threads approach works well here. If you’re fitting a Strutworks RACP reinforcement brace and cutting the pilot holes yourself, that’s the process — and there are a few details that determine whether it goes smoothly or becomes a pain.

What the RACP Brace Is For

The rear axle carrier position (RACP) mounting area is a known weak point on Mazda MX-5 platforms. Repeated stress — especially from track use — can cause cracking at the factory mounting locations. The Strutworks Style 1 brace reinforces those points, spreading load across a larger area and heading off the kind of structural failure that tends to announce itself at the worst possible moment.

The Right Drill Bit

A 13/32″ diameter extended-length bit — 12 inches — is what you need. Standard-length bits simply won’t reach the correct angle when working from underneath the car. Extended bits are available from most major tool suppliers in high-speed steel or cobalt. Cobalt holds an edge better in steel, which matters here since the chassis at these mounting points is steel, not aluminum.

The thing about 12-inch bits: they flex. A lot more than a stubby bit would. That flex isn’t a disaster, but it means you cannot muscle the drill and expect the bit to track straight. Controlled, patient drilling is what keeps it on line.

Why Drill From Below

Drilling upward through the underside of the car keeps metal chips and swarf falling away from your work rather than down into holes you’ve already cut or onto threads you’re about to use. You also get a direct sightline down through the brace’s pilot holes, which acts as a natural guide for your bit angle. Access from above at these mounting locations is awkward at best.

Cutting Fluid — Use It

Apply cutting fluid before you start and reapply every time you back the bit out to clear chips. A 12-inch bit packs swarf in the flutes faster than a short bit does, and running it dry in steel dulls the edge fast. A dull bit generates more heat, more friction, and wanders more. Cutting fluid is cheap insurance on what is otherwise a difficult hole to correct if something goes wrong.

Step-by-Step Drilling

• Position the brace firmly and use its holes as your centerline guide before touching a drill to the car.
• If the surface is smooth and the bit skates, use a center punch to dimple your starting point.
• Start at low RPM to seat the pilot, then increase speed once the bit is cutting cleanly and tracking true.
• Back the bit out frequently — every inch or so — to clear chips and let the flutes breathe.
• Reapply cutting fluid each time you back out.
• Let the bit work at its own pace. Forcing it increases flex and wander.

Chasing the Threads

Running a tap through each hole after drilling is not optional. The drill bit leaves burrs, and a bolt fighting burrs can feel fully seated when it isn’t. That’s a problem at a load-bearing mount.

If the holes at your mounting location are partially blind — meaning the tap hits a dead end before threading the full depth — use a bottoming tap rather than a standard plug tap. A plug tap leaves the last few threads at the base of a blind hole uncut. That’s acceptable in many situations, but at a structural reinforcement point you want complete thread engagement. A bottoming tap finishes what the plug tap leaves undone.

After chasing, clear the hole of any debris before running a fastener in. Compressed air works well here.

Forward vs. Rear Mounting Points

The same process covers both positions. The rear mount is generally the more accessible of the two. The forward mount tends to sit in tighter quarters, and you may need to finesse your drill angle to get straight access. Confirm clearance and mark both centerlines carefully before drilling either hole — there is no clean way to fix a badly placed hole at this location.

Thread a bolt in by hand after chasing each hole. If it binds before it’s properly seated, run the tap through again. Do not force it. A stripped thread here is a difficult repair.

Torque Specification

Use the torque value from Strutworks’ documentation for the specified hardware. If a spec isn’t listed, go by the torque table for the bolt’s grade and diameter. A reinforcement brace with fasteners that are merely snug rather than properly torqued isn’t providing the clamping force the design depends on. Torque it correctly or the installation is incomplete.