PRODUCT DETAILS
Synchrotech K20 6-Speed Mainshaft (SG-K213-MS)
The mainshaft carries first and second gear, and on a K20 6-speed those are the two gears taking the worst of everything. Launches, hard 1-2 shifts, and the highest torque multiplication in the box all land here. If yours has been damaged, or you're building a transmission properly from the case up, this is the Synchrotech replacement, built to OEM ratios with materials and tolerances meant to outlast the factory part.
Same Ratios As Stock
Worth saying plainly, because it's the first question people have. This runs a 15-tooth 3.266 first and a 23-tooth 2.13 second, which are the factory ratios.
That means the car drives exactly as it did before. Same gearing, same shift points, same speeds in each gear, no tuning adjustments and no relearning the car. You're replacing a damaged or worn component with a stronger version of the same thing, not changing how the transmission behaves.
If you wanted a ratio change, that's a different product. This is the one you buy when the gearing was never the problem.
Why First And Second Fail First
Worth understanding, because it explains why this is the part people end up needing.
The lower the gear, the higher the torque multiplication through it. First and second see dramatically more load than fifth and sixth, and they see it in the most abusive way, on launches and on hard shifts where the driveline is shock loaded rather than gradually loaded.
Add power to a K20 and that gets worse fast. A transmission that was fine at stock output starts damaging teeth or shafts once you're making real torque and using it. That's why mainshafts and gearsets exist for this gearbox, and why they're what serious builds go through.
Buy The Assembly, Not Just Gears
This is a complete mainshaft with first and second, and that matters more than it sounds.
The shaft itself is a wear item. Splines and bearing surfaces take load every time you shift, and a shaft that's been through a gear failure often carries damage you won't spot until it costs you a second time. Starting with a fresh assembly removes that variable entirely rather than gambling on a used shaft you can't fully inspect.
Built Stronger Than Factory
Synchrotech manufactures these to meet or exceed OEM specification, using quality materials and tight tolerances so the shaft holds up where the factory part gave out.
They've spent decades working specifically on Honda transmissions, developing materials and heat treat alongside metallurgists, which is why their name comes up constantly when someone's gearbox lets go on a car making real power.
While You're In There
Replacing a mainshaft means the transmission is fully apart, which is nearly all the labor on this job. Use the access.
Inspect your synchros, sleeves, hubs, and bearings while everything is exposed, and replace what's past spec. Anything that ran alongside a damaged gear has likely taken debris or abuse, and closing the case up with a fresh mainshaft next to worn synchros means doing this again sooner than you'd like.
If you're finding wear throughout, a full rebuild kit is better value than replacing pieces individually.
Fitment
2001-2011 Honda and Acura K20 6-speed transmissions, including:
2002-2006 Acura RSX and RSX Type-S
2006-2011 Honda Civic Si
Acura Integra Type R (DC5)
Honda Civic Type R
Part number SG-K213-MS. 6-speed transmissions only.
Who Should Grab One
This is for the builder who's opened a K20 6-speed and found damage on the mainshaft, and for anyone assembling a transmission for a car making enough power to justify doing it properly. Factory ratios mean nothing about how the car drives changes, you're just replacing a weak point with something stronger. Inspect everything else while you can see it, and close it up once.
Fits: 2001-2011 Honda and Acura K20 6-speed transmissions (RSX, RSX Type-S, Civic Si, DC5 Integra Type R, Civic Type R). Part number: SG-K213-MS. Ratios: 3.266 first (15 tooth), 2.13 second (23 tooth), matching factory. Type: complete mainshaft assembly, direct OEM replacement.
2002-2006 Acura RSX Type S
2006-2011 Honda Civic Si