Synchrotech B16A/B18B/B18C Pro Series Mainshaft

Synchrotech Pro Series Mainshaft - B16 & B18 (1994-2001)

The mainshaft carries first and second gear, and those are the two taking the worst of what a B-series does. Launches, hard 1-2 shifts, and the highest torque multiplication in the transmission all land here. If yours has been damaged, or you're building a gearbox properly from the case up, this is the Synchrotech Pro Series replacement at factory ratios.

Two Versions, Split By Second Gear

Both variants share the same 13-tooth 3.23 first gear. What separates them is second, and that's what determines which one you need.

The Type R and B16 cars run a shorter second gear than the GS-R and LS, which is part of why those transmissions feel more aggressive through the low gears. Match your engine and transmission rather than going by the car alone, since plenty of these have been swapped over the decades.

Factory Ratios, So Nothing Changes

Both versions run the same ratios your transmission had from the factory. That means the car drives exactly as it did before, same gearing, same shift points, same speeds in each gear.

You're replacing a damaged or worn component with a stronger version of the same part rather than 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 far more load than fourth and fifth, and they see it in the most abusive way, on launches and hard shifts where the driveline gets shock loaded rather than gradually loaded.

Add power to a B-series and that gets worse quickly. A transmission that was fine at stock output starts damaging teeth or shafts once you're making real torque and using it. On cars that are now thirty years old and have often been built, swapped, and driven hard, the mainshaft is a common casualty.

Buy The Assembly, Not Just Gears

This is a complete mainshaft with first and second, which 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 again. Starting with a fresh assembly removes that variable rather than gambling on a used shaft you can't fully inspect.

Pro Series Construction

Synchrotech has spent decades working specifically on Honda transmissions, developing materials and heat treat processes with metallurgists rather than adapting generic parts. Their Pro Series components are built to handle considerably more than the factory pieces they replace, which is why their name comes up whenever 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

1994-2001 Acura Integra GS-R and LS (SG-B190-MS)
1994-2001 Acura Integra Type R, 1999-2000 Honda Civic Si, 1994-1997 Honda Del Sol VTEC (SG-B210-MS)

Select your application at checkout.

Who Should Grab One

This is for the builder who's opened a B-series and found damage on the mainshaft, and for anyone assembling a transmission for a car making enough power to justify doing it right. Factory ratios mean nothing about how the car drives changes, you're just replacing a weak point with something stronger. Confirm your second gear ratio, inspect everything else while you can see it, and close it up once.

Fits: 1994-2001 Acura Integra GS-R and LS, 1994-2001 Integra Type R, 1999-2000 Honda Civic Si, 1994-1997 Honda Del Sol VTEC. Part numbers: SG-B190-MS (3.23 first / 1.90 second) and SG-B210-MS (3.23 first / 2.10 second). Ratios: matching factory. Type: complete Pro Series mainshaft assembly.