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Author Topic:   Yamaha - changing lower unit fluid
cohasset posted 03-17-2002 10:44 AM ET (US)   Profile for cohasset   Send Email to cohasset  
I have a '91 Yamaha outboard on my '73 Cohasset and I need to change the lower unit fluid. Everybody says that I should remove the bottom screw and the top screw to drain, then fill from the bottom screw. Well, I have two screws side by side. One is labeled "wash" the other "oil level". Anybody know the procedure for changing the fluid on this motor?
Outraged posted 03-17-2002 12:27 PM ET (US)     Profile for Outraged  Send Email to Outraged     
The labled wash is for flushing the outboard after use in saltwater. The one next to it labled oil level, is where the oil comes out when it's full. Remove both screws,(upper & lower) to drain, fill it from the bottom hole, until oil comes out the top hole. Replace the top plug first,this will help keep the oil in the lower unit while you remove the oil filler line from the lower hole, & quickly replace that plug.
cohasset posted 03-17-2002 01:18 PM ET (US)     Profile for cohasset  Send Email to cohasset     
Thanks for the quick response, but...The issue is that I don't have an upper and lower screw. Only the two side by side ("wash" and "oil level").

Also, I've always just used the cups and a water hose to flush my engine after use in salt water. Is that a problem?

John from Madison CT posted 03-17-2002 07:17 PM ET (US)     Profile for John from Madison CT  Send Email to John from Madison CT     

Sure you have the lower oil drain screw. You have to look real hard, it's located near the front of the lower "bulbous" part of the lower unit. It's there trust me.

Once you find it, do as you mentioned. Drain then fill from bottom and once you see it coming out the top put in the upper plug to creat a seal, then put in the lower plug. Tighten, but not too much.

Good Luck,


jimh posted 03-17-2002 08:12 PM ET (US)     Profile for jimh  Send Email to jimh     
The Yamaha drain screws use an odd-sized gasket washer. Be sure you have a pair of these on hand before beginning the job.

A common pitfall in lower unit oil change is to either put no washer or two washers. Examine the drain hole to be certain there is a washer in place and intact. If not, scrape the remains of the old washer out until you have a solid, smooth metal base around the drain hole. Then install a new washer when you replace the drain plug/screw.

Check the color of the oil you are draining. If it looks beige and milky you have had water contamination of the lower unit.

If the oil shows signs of water, check those drain screws and washers.

My experience was that there was some sign of water ingress, but I thought it might have been caused by a poor drain washer/gasket. After careful attention to that area, I found I still had a little milkiness to the oil at the end of the season. I am having the prop shaft and drive shaft seals replaced this winter, so that should seal it back up for another 14 years.


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