Sierra 18-2645-1 Lower Unit Seal Kit for Mercury Bravo Drives

The Sierra 18-2645-1 is a lower unit seal kit built to reseal the gear housing on Mercury Bravo sterndrives. If you've pulled your lower unit and found gear oil weeping past the seals, or your oil reservoir keeps dropping between trips, this is the set that gets you back to a dry, sealed drive. It fits Bravo I, II, III, X, and XR drives, so one part number covers most of the Bravo family you'll run into out on the water.

Here's what you're getting: the seals, bearings, and O-rings needed to rebuild the lower unit sealing surfaces. Sierra builds these to meet or exceed the original equipment part they replace, so you're not giving up sealing quality to save money on the rebuild. For a lot of owners, a seal kit is the difference between a weekend job in the garage and an expensive trip to the shop.

When should you reach for this kit? Anytime you're doing a lower unit reseal. Milky gear oil, a slow drop in the oil bottle, or water at the prop shaft are the usual tells. A lot of guys do the seals as preventive maintenance whenever the drive is already off for a bearing job or a prop shaft inspection. While the housing is apart, fresh seals are cheap insurance against pulling it all again next season.

A few things worth knowing before you start. You'll want a proper bearing puller for this job. Trying to hammer the old bearings and seals out is how housings get scored, especially on the Bravo II. Take your time and pull them clean. When you get to the upper seal carrier, clean every trace of old Loctite out of the O-ring groove. Old sealant left in the groove is a common reason a fresh seal starts weeping again within the first few hours of running.

Torque matters here too. The bearing carrier nut needs to go to the service manual spec, not just snug. Over-tightening can crack the aluminum on Bravo I and III housings, and under-tightening lets the whole stack move. If you don't have the manual spec in front of you, get it before you button things up.

On the heavier drives, plan ahead. Bravo III and XR rebuilds sometimes need extra shims to set the gear lash correctly, and some 2005-and-up units can come up a few thousandths short on the stock stack. Have a shim assortment on hand so a small clearance issue doesn't turn into a second teardown. If you're working an XR drive, use the shift shaft alignment tool when you reassemble. Skip it and the shift cable can start binding after the first handful of hours.

Don't reuse the parts that aren't in this kit. The number one cause of a leak coming back after a reseal is old drive bellows and trim ram seals going back on. If they're already off the boat, replace them while you have the access. And once the drive is back together and filled, change the gear oil after the first short run. New bearings shed a little metal as they seat, and you don't want those particles living in your first real fill.

Bottom line: the Sierra 18-2645-1 gives you the seals, bearings, and O-rings to do a Mercury Bravo lower unit reseal right, at a price that makes doing the job yourself worth it. Pair it with fresh bellows, the correct gear lube, and a service manual for torque specs, and you'll have a drive that stays dry season after season. Do the job carefully once and you won't be pulling that lower unit again for a long time.