1991 Mazda MX-5 1.6L Petrol · H425 BAU

Early 1.6-litre B6-ZE engines produced between 1989 and mid-1991 feature a 'short-nose' crankshaft design where the crankshaft snout is shorter and utilizes a narrow woodruff key to locate the timing belt gear and crank pulley. If the central crankshaft bolt is not tightened to the correct torque specification during a timing belt change, or if an old bolt is reused, the pulley begins to wobble. Over thousands of miles, the woodruff key chews into the keyway slot in the crankshaft snout, throwing off ignition timing, causing engine misfires, and eventually ruining the crankshaft snout completely. Later engines from 1991 onwards received a revised 'long-nose' crank that eliminated the vulnerability.

The hydraulic clutch slave cylinder on the MX-5 NA is located on the lower driver-side of the transmission bellhousing, where it is exposed to road grime, water spray, and heat soak from the exhaust downpipe. Over time, the internal rubber hydraulic piston seal degrades, allowing brake/clutch fluid to leak past the pushrod into the protective rubber dust boot. As hydraulic fluid is lost, the clutch pedal becomes spongy and gradually loses pressure until it drops flat to the floorboard, leaving the driver unable to disengage the clutch or select gears. Fortunately, the slave cylinder is cheap and straightforward to replace.

Corrosion in the rear sills and lower wheel arches is the single most common cause of structural failure on early Mk1 MX-5s in the UK. Rainwater flowing off the folding soft-top collects in a plastic rain rail channel that directs water into two internal drain tubes located behind the seatbelt towers, discharging through the lower sills. In the UK, pine needles, leaves, and road dirt clog these tubes. Trapped water overflows into the enclosed rear sill cavities, where it sits against bare internal steel, rotting the structural sills and rear wheel arch seams from the inside out. Decent-looking exterior paint often conceals crumbling structural rot.