Sierra 18-1700 Lower Unit Seal Kit, Yamaha 60-70 HP

When your Yamaha 60 or 70 HP outboard starts leaving a milky film on the dipstick, or you spot gear lube weeping past the prop shaft, it's almost always the lower unit seals letting go. This Sierra 18-1700 lower unit seal kit gives you everything you need to reseal the gearcase in one go. Seals, bearings, gaskets, and O-rings, all matched to fit Yamaha 60-70 HP engines.

Here's the thing about lower unit seals. They're cheap. The damage they prevent isn't. A weeping prop shaft seal lets water sneak into the gearcase, and once saltwater mixes with your gear oil, the bearings and gears start to corrode. Catch it early with a fresh set of seals and you're looking at an afternoon of work. Ignore it and you're pricing out a whole lower unit. That's why we tell folks to pull the gearcase and pressure-test it every couple of seasons, especially if you run in salt.

Sierra builds this kit to meet or exceed the original equipment parts it replaces, so the fit and the sealing surfaces match what came off your engine. You get the seals that ride on the prop shaft and driveshaft, the bearings, the gaskets, and the O-rings that live in the housing grooves. One box, one part number, no hunting down individual seals by the piece.

A few things worth knowing before you dive in. Seal orientation matters. A lot. The lower prop shaft seal has to face the right direction or water walks straight past it into the crankcase, and you'll be back in there in a week. Take a photo before you press the old ones out. Clean the housing grooves until they're spotless before you seat the new O-rings, because even a little old gasket material or debris will keep a seal from sitting flat and it'll leak under pressure.

While you've got the lower unit off, look at the driveshaft splines. On older Yamahas the splines like to corrode, so wire-brush them clean and grease them before you slide the bearings and driveshaft back together. If the pump housing is scored or worn, replace it too, or the new seals won't have a clean surface to work against. And if you're this deep into the gearcase already, it's a smart time to drop in a fresh water pump impeller and key. Most folks running saltwater refresh the impeller every two to three seasons anyway, and doing it now saves you pulling the lower unit twice.

Fitment covers Yamaha 60 HP and 70 HP outboards. These engines ran across a lot of model years, and the bearing sizes on some of the 1990s blocks differ from the 2000s ones, so match the 18-1700 part number to your specific engine before you order. When in doubt, give us the year and model of your motor and we'll help you confirm it's the right kit before it ships, so you're not stuck mid-service with the wrong seals on the bench.

Reassembly is straightforward if you take your time. Use a proper seal driver so you don't cock a seal in its bore, seat everything square, and torque the housing bolts to the factory spec. That last part gets skipped more than it should. Under-torqued or uneven bolts warp the housing plate and you'll chase leaks all season. Do it right once and this reseal holds for years.

For a first-timer this is a very doable job with a manual, a seal driver, and a rainy Saturday. For a shop, it's routine. Either way, starting with a complete Sierra kit beats piecing seals together and hoping you got them all. Keep the old seals set aside until the job's buttoned up and the engine's run, just in case you want to double-check a size or an orientation against them.