What are common gearbox issues on a Series 2 Mazda RX-7 FC?

Just to clear up the chassis codes before we dive in: "Series 2" generally refers to the first-generation RX-7 (FB chassis, 1981–1983), while the "FC" is the second-generation (Series 4 and 5, 1986–1992). Because swapping FC transmissions into earlier cars is incredibly common, I'll cover the known issues for the FC gearboxes and the general rotary transmission problems you'll run into.

Here are the most common gearbox issues on this platform and how to address them:

1. Shifter Slop and "Diagonal" 1st Gear If your shifter feels incredibly vague or you have to push it diagonally (up and to the far left) to find 1st gear, your shifter bushings are shot. This is a very common issue on FCs, but it's an Easy fix that just requires pulling the shifter out from inside the cabin and dropping in new bushings ⁶.

2. Grinding on Downshifts (Synchro Wear) Grinding when shifting—especially on the 4th-to-2nd downshift—is a classic sign of worn synchronizers. Before you pull the transmission for a Hard internal rebuild, the community consensus is to drain the old fluid and switch to Pennzoil Syncromesh, which frequently masks or resolves the grinding on these older boxes ⁶.

3. Hard Shifting When Hot (Hydraulic Failure) If the transmission shifts fine cold but refuses to go into gear after 20 minutes of stop-and-go traffic, or if the clutch pedal goes soft and the car tries to creep forward with the pedal depressed, you are losing hydraulic pressure. This is a classic symptom of a failing clutch master or slave cylinder. They are notorious for failing when the seasons and temperatures change. Replacing them is a Moderate job ¹.

4. Lockout in 1st, 2nd, or Reverse If you are having intermittent trouble getting the car into 1st, 2nd, or Reverse, the issue is often outside the gearbox itself. Owners consistently trace this back to a failing pilot bearing, a worn throwout bearing, or a clutch disc that is on its way out ^{3 4}. Dropping the transmission to replace the clutch assembly and pilot bearing is a Hard job.

5. Input Shaft Spline Mismatches (During Swaps) If you end up replacing the gearbox entirely, be highly aware of input shaft differences between naturally aspirated (N/A) and Turbo II models. Standard N/A 13B transmissions use a fine-spline input shaft, while Turbo II gearboxes use a coarse-spline shaft of the same diameter. You cannot mix and match clutches and flywheels across these without getting the spline count right ².

Where to start: If you're currently experiencing shifting issues, check your clutch master and slave cylinders for leaks first, bleed the hydraulic system, and swap the transmission fluid. That solves the majority of shifting complaints before you have to drop the box.