Questions about fixing lower unit oil leak

I have a question about the oil seals under the water pump on my 1998 115 Merc.

I bought the engine new, and from the first oil change I was concerned about the appearance of the LU oil. It has always been a yellowish color.

This year I decided to replace the seals under the water pump impeller. In doing so I found some things that have me stumped.

The two seals that came out are marked 26-43036 and 26-43035. I looked up the specs on these seals, and they are designed for shaft sizes of 1.045 and 1.063 respectively.

I measured the sealing surface of the drive shaft and it is 1.025.

My questions are:

1) How could seals designed for 1.045 & 1.063 dia shafts seal effectively on a 1.025 shaft ?

2) Why would Merc specifiy seals for 2 different shaft sizes ??

Thanks for any help you can offer.

Re: Questions about fixing lower unit oil leak

And just where did you find those specs, as Merc doesn't post specs for their seals.

Re: Questions about fixing lower unit oil leak

Here is one link:

"http://www.eastmarine.ru/incp/katalog/sierra/outboard/MERCURYMARINER/oil_seals-web.pdf[/url]

(1st page, box on the left)

Here's a link that only lists the 26-43036:


"http://www.go2marine.com/product.do?no=82145F[/url]

Both specs quoted by Sierra.

Thanks !!

Re: Questions about fixing lower unit oil leak

If you post the serial number from your engine we will be able to determine if you have the right part number.

Chris......

Re: Questions about fixing lower unit oil leak

Basically your links show specs for Sierra parts. Not Merc. Sierra usually has parts that are close, but not exact. In my opinion (from experience) any of the gaskets, seals, and other rubber parts like bellows, seldom fit as good as the OEM parts.
So if you get your SN as Chris suggested, and make sure you have the right part numbers (which you do, they are common) then get the OEM part instead of some aftermarket part you should be fine.
Did you actually pressure and vacuum check the lower unit to find the leak or just assume it was those seals. They don't leak that often. Usually a prop shaft seal, or the worst problem, used drain and vent plug gaskets. They are paper and need replaced EVERY time they are removed.

Re: Questions about fixing lower unit oil leak

Serial Number is OG635204.

As you stated, I'm sure they are the correct parts, as my factory manual verifies the numbers.

I did a pressure check. It held 12 PSI for 9 minutes before I noticed a drop (manual spec's 5 minutes).

I assumed that the propshaft seal wasn't leaking, since there is no trace of oil leakage there. I've always replaced the gaskets for the drain & vent plugs.

At this point I suppose that I'll replace the seals I've disturbed with Merc parts, then do a pressure check with the LU under water to see if I can spot any air bubbles.

The seal surface on the drive shaft does have a groove where the seal runs, my impression is that the wear surface can be replace, but it looks like the drive shaft has to be removed from the LU to do so. Not sure what I'll do about that.

Any other ideas are welcome.

Thanks for the help so far...

Re: Questions about fixing lower unit oil leak

A pressure test will only tell you that oil won't leak out. You need to do a vacuum test to check that none of the seal will allow water in.

Seals (single lip) only stop a fluid passing in one direction, which is why there are two seal, back to back, on the prop shaft and under the water pump. One seal stops the oil getting out of the box and the other stops water getting in. And before you start saying that if oil can't get out, then how can water get in? It does... it just does....

Chris.............