1969 Merc 200 20hp Lower Unit Seals

[gregorys]

Obvious leaking lower unit seal (foamy light colored gear oil). Pulled lower unit. Took off prop and unscrewed large ring/nut with spanner wrench. Behind it was a thin ring (almost looks like a shim but a somewhat wider) right against the bearing carrier assembly. Mercury service manual indicates that there should be an O ring oil seal there and another on the other end of bearing carrier right at the reverse gear. There was no O ring behind the spanner ring/nut. Kit I ordered has both O rings.....My question is: Does this large O ring go between the spanner ring/nut and the "shim".....or ......between the "shim" and the bearing carrier?

Before you ask I got the whole gasket seal kit from Vintage Outboards.com and received it in 3 days....and yes I'm replacing the water pump impeller!

Thanks ahead for any help....

 

[racerone]

The parts manual does not show an o-ring behind that washer ( it is not a shim ) and it only shows one o-ring further into the assembly.

 

[emckelvy]

Indeed! The large O-ring fits over the lip in the forward end of the reverse gear bearing carrier, and is compressed between the bearing carrier and that thick ring, which is called the "reverse gear thrust ring".

What that ring does, when you tighten up the gearcase spanner nut and push the bearing carrier into the gearcase, is to bottom-out against the machined internal ridge in the gearcase. The bearing carrier compresses the large O-ring in the process of bottoming against the thrust ring, squeezing the O-ring against the gearcase and making a seal.

On some gearcases, note that is is possible to find a very thin metal shim that will go against the gearcase machined ridge, to shim the bearing carrier rearwards to adjust gear free play. Sometimes the tolerances didn't stack up and this was a way for the factory to make an adjustment. In most cases the shim probably wasn't needed, so if there wasn't one in there, you probably won't need one to go back in. Needless to say, if you did find a shim there, it should be reinstalled!

If you look on the parts diagram, this is described as "Shim, Thrust Washer to Gear Housing" and came in a pack of various thicknesses, now an NLA part of course).

On the gearcase "nut", the only thing between that and the bearing carrier is a stainless washer, aka "Washer, Gear Housing Cover". It provides a slick thrust surface for the aluminum (or now maybe plastic) gearcase cover nut to tighten-up against.

It's a good idea to coat the large O-ring with either grease or Permatex #3, to prevent salt intrusion. Also on the threads of the gearcase nut. I've used Permatex #3 for years and it does a great job of keeping salt & other corrosion products at bay. You'll never have to "Heat and Beat" the gearcase apart once the bits & pieces have been coated with Permatex!

HTH & G'luck with your repair; enjoy your old 20hp, they are a strong-running motor and give a good turn of speed on a light boat............ed