While maintaining your beloved Roadster, that nasty “squishing” sensation transmitted through your hand the moment you turn the wrench, followed by a dry “crack”… If you’ve been there, you know that spine-chilling moment. Yes, the fracture of the bolt securing the undercover.
Don’t be fooled by thinking “it’s just one bolt.” This bolt is fixed to a special nut inside the frame, and when it breaks, recovery is notoriously difficult worldwide.
This article clearly explains the cause of this trouble that plagues Roadster owners around the world, as well as what to do if the bolt breaks.
1. Why does this bolt keep breaking?
For NA/NB Roadster owners, this might be considered a kind of “rite of passage.” Actually, it’s not because your skills are poor—it’s a structural inevitability.
The “hidden trap” inside the frame
The bolt securing the undercover is threaded into a “captive nut” welded inside the frame (side member). The problem is that the original bolt is designed to be slightly long, so the tip of the bolt pierces through the nut and protrudes into the hollow cavity inside the frame.
The inside of the frame is a place where water, mud, and de-icing salts easily enter, and moisture tends to accumulate. As a result, the protruding bolt tip rusts intensely and swells, becoming enlarged into a “mushroom shape.” When you try to loosen the bolt in this state, the enlarged rust mass catches on the back of the nut, becoming a powerful stopper that shears the bolt.
This bolt is an M6 x 15, Grade 8 tapping bolt, and in Mazda genuine parts it is 9078-60-616 Screw & Washer, Tapping. It’s an FT screw from Morishita, with peaks sharpened into hexagonal shapes, adopted to remove paint film and spatter.
2. A tragedy shared worldwide: “Oh, it broke…”
This trouble is not limited to Japan. The same cry of anguish is heard on forums around the world.
- Japan: Reports of bolts breaking when trying to remove the cover for an oil change never cease.
- UK: In humid Britain, it’s said that “it will always break if you try to remove it,” with the breakage being so frequent that there are many reports of “co-rotation,” where the force of shearing the bolt actually strips the nut weld inside the frame.
- USA: Owners in snowy regions in particular call this bolt a “nightmare,” and discussions about drilling it out never end.
3. Prevention measures first: how not to break it
If the bolt is still intact, you’re lucky. Never force it with brute strength.
The magic liquid: “ATF + acetone”
Commercial penetrating lubricants (such as Laspene) are good, but in cases of severe seizure, a solution mixed 1:1 of ATF (automatic transmission fluid) and acetone is said to be the strongest. Inject this through the frame’s drain hole so it reaches directly to the back of the bolt (the threaded portion) and let it sit overnight.
Rocking Technique
Don’t immediately start turning in the loosening direction. Repeat small “loosening and tightening” motions to pulverize the rust on the threads while gradually expelling it. Patience is essential.
4. If it breaks anyway… recovery methods
Even if you’re unlucky and hear that “crack,” don’t give up yet. Here are three recovery methods depending on the situation.
Level 1: When the bolt head remains
If any part of the bolt shaft protrudes, there’s hope.
- Nut welding method: Fit a large nut over the remaining bolt, weld the inside, and turn it. The heat shock also loosens the rust, killing two birds with one stone.
- Vise grips: Clamp it firmly with Screw Extractors or vise grips and turn.
Level 2: Snapped off at the root (submerged)
The most common pattern. Drill out the bolt. In this state, extractors will break.
- Center punch: Make a small indentation in the middle. If this slips, you’ll damage the threads in the frame.
- Reverse rotation drill: Use a drill bit that rotates in reverse (counterclockwise) instead of a normal drill. The cutting force works in the “loosening direction,” so if you’re lucky, the bolt will pop right out as you drill.
- Gradual expansion: Don’t use a large drill right away; gradually expand the hole in stages: 2.0mm → 3.5mm → 4.8mm. Since the pilot hole diameter for M6 is approximately 5.0mm, stopping at 4.8mm leaves the bolt remains with just a thin skin of thread remaining.
- Pickup: Peel off the coil of the remaining threads inside with a sharp pick tool.
- Tapping: Finally, run an M6×1.0 tap through to clean up the threads.
Level 3: Worst case scenario: “Nut loss”
The rust is so severe that the moment force is applied, the nut weld inside the frame breaks loose and spins freely. Once this happens, there’s no way to remove the bolt from the outside. In this case, the final resort is a “nutter (rivet nut/blind nut)”. Enlarge the original hole with a drill (approximately 9mm), and crimp a new female thread (rivet nut) to fix it. This restores strong threads like the original. Even without specialized tools, there’s a method of crimping using a bolt and nut.
※ Some people use cable ties (zip ties) as a temporary fix, but they can break from vibration or cause the cover to flap, so use them only as a quick workaround.
Conclusion
The undercover bolt fracture on NA/NB Roadsters is an unavoidable passage. However, with the right knowledge and preparation, there’s nothing to fear. If it does break, see it as a good opportunity to deepen your bond with your car (and expand your tool collection).