Synchrotech L15B Transmission Synchro Master Rebuild Kit

Synchrotech L15B Synchro Master Rebuild Kit - 2016-2020 Honda Civic EX-T & Si

The 10th gen Si is a genuinely good car, and it's now old enough and driven hard enough that transmissions are starting to need attention. If yours grinds going into a gear, this is the complete fix: synchros, springs, sleeves, bearings, and seals to rebuild it properly while the case is open. Available in brass for a factory-quality rebuild or carbon for a real performance upgrade.

Yes, These Wear Out Too

Worth addressing, because people don't expect synchro problems on a car this recent.

The 10th gen Si is a car people shift hard. It's a turbo four with a good gearbox and an owner base that uses it, which means these transmissions accumulate aggressive shifts faster than a commuter would. Add a tune, more boost, or track days and the loads go up further.

So a grinding L15B isn't a sign something went wrong. It's a sign the car got used the way it was built to be used, and now the synchros want replacing.

How To Tell Your Synchros Are Done

A few reliable signs, and one test worth running.

It grinds into one specific gear. If everything shifts clean except third, that's third gear's synchro. Isolated grinding almost always means an isolated problem.

Downshifts are worse than upshifts. The useful one. On an upshift, revs are falling naturally and the synchro gets help. On a downshift it has to drag the clutch disc and input shaft up to speed by itself. If the car upshifts quietly but grinds or refuses on a normal downshift, your synchros are worn.

It's getting worse. Wear starts as slight notchiness and progresses to metal-on-metal grinding you feel in your hand.

One honest note: grinding can also come from a clutch that isn't fully disengaging or from low or wrong transmission fluid. Rule those out first, they're far cheaper than pulling a transmission.

Carbon Or Brass?

Both fix a grinding transmission. They suit different cars.

Brass (SYN125B) is the OEM-style material and the right pick if the Si is your daily. It restores factory shift feel, it's proven, and it costs less. If you're refreshing the car and want it back to how it felt new, start here.

Carbon (SYN125) is Synchrotech's Pro Series, using a full carbon composite lining built on carbon-carbon technology originally developed for aerospace. The material acts as a braking element during engagement, stopping the gear faster, which means less grind on high-rpm shifts and less wear on your sleeves and engagement teeth over time. If you're tuned, running boost, or seeing track days, this is the one.

Straight talk on one point: carbon shifts better, but don't expect dramatically longer life than brass. You're paying for how it engages.

Check Your Hubs Before You Order

The most important advice on this page. Synchros work against your hubs and sleeves, and if those surfaces are worn, new synchros of any material won't fix your shifting. New carbon on a chewed-up hub is no better than new brass on one.

This kit includes new sleeves, so half of that equation is handled. But inspect your hubs and your engagement teeth before you close it up. Sharp and clean means this kit transforms the shift. Rounded off means you need hubs too, and finding out now is far better than after everything's back together.

What's In The Kit

• Synchros for every gear (carbon or brass, your choice)
• Synchro springs
• Slider sleeves
• Bearings
• Seals

That's a complete rebuild rather than a partial one. Everything that wears is covered, which is the point of a master kit versus replacing one synchro and going back in a year for the next.

Worth Doing Once

Getting into the transmission is the expensive part of this job, whether you're paying a shop or spending your own weekend. The parts are the cheap half by a wide margin.

So while it's open, inspect everything and replace what's genuinely past spec rather than what you assumed was bad. That's how you close it up once and get the shift you were after instead of doing this again next year.

Fitment

2016-2020 Honda Civic EX-T and Si (L15B)

Carbon: SYN125
Brass: SYN125B

Who Should Run This Kit

This is for the 10th gen owner whose transmission has started grinding and who wants it fixed properly, and for the builder refreshing a gearbox on a car that's making more than it did stock. Inspect your hubs, pick brass or carbon based on how you actually drive, and get your shifts back.

Fits: 2016-2020 Honda Civic EX-T and Si (L15B). Part numbers: SYN125 (carbon), SYN125B (brass). Materials: brass (OEM-style) or Synchrotech Pro Series carbon composite. Includes: synchros, springs, sleeves, bearings, and seals.