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Author Topic:   Adjusting trim tab on a Merc 225HP two-stroke
Frank O posted 06-07-2008 01:23 PM ET (US)   Profile for Frank O   Send Email to Frank O  
I need to adjust the sacrificial trim tab on my 1994 Mercury Offshore 3.0L 225HP two-stroke outboard engine.

I assume this is done by fishing a socket on an extender through the access hole noted in this picture?:

Does anyone happen to know the size of the socket this calls for? I messed with it for a couple of minutes when I was down at my boat yesterday, but since it isn't possible to see what's going on inside, it could take a lot of experimentation to hit on the right size.

Tom W Clark posted 06-07-2008 02:36 PM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     

Yes, that is how it is done. There is supposed to be a black plastic plug you remove first but it doesn't make any practical difference if you have this plug or not.

Off the top of my head I would say it's a 9/16" socket you need with at least a 3" extension. But really, you don't need to know. It does not take a lot of experimentation. Like any nut or bolt you want to "wrench" you just take your socket set and see what size fits. It might require 10 or 20 seconds of your time if you don't hit it right the first time.

tmann45 posted 06-08-2008 07:56 AM ET (US)     Profile for tmann45  Send Email to tmann45     
Did anyone notice the insert added to the lower unit to increase length?
Tom W Clark posted 06-08-2008 11:02 AM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     
So use a three and a six inch extension. Doesn't change the size of the socket.
Frank O posted 06-08-2008 12:07 PM ET (US)     Profile for Frank O  Send Email to Frank O     
Thanks. Tom, the black plug is indeed around, I just removed it before taking the picture.

What's the story on the insert to increase the length? Does that mean this outboard was configured to be longer than usual? I think the engine is pretty much in the state it was when the boat was new in 1995. Did some hull configurations require a longer lower unit?

Tom W Clark posted 06-08-2008 12:16 PM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     
A 5" extension will convert a 20" midsection to a 25" and a 25" midsection into a 30". The rest of the motor is not in the photo so I cannot tell what you have.

Manufacturers could save costs by using one midsection for two shaft length motors. Mercury was notorious for doing this while most other manufacturers produced unique casting for all there different length motors. Yamaha still uses a lot of extensions on their big four strokes to make 30" shaft length motors.

I suspect yours is an original 30" shaft length Mercury motor made at the factory with the extension.

jimh posted 06-08-2008 11:23 PM ET (US)     Profile for jimh  Send Email to jimh     
A corollary question: would a motor originally manufactured as a 30-inch shaft have a one-piece drive shaft? Or would it have a 25-inch drive shaft and extension?
Frank O posted 06-08-2008 11:45 PM ET (US)     Profile for Frank O  Send Email to Frank O     
Thanks for the explanation, very interesting.

For what it's worth, here's the entire engine:

Tom W Clark posted 06-09-2008 12:48 AM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     

I cannot answer hat question comprehensively but my strong suspicion is the drive shaft is one piece. It is on the Mercurys.

Frank O posted 06-22-2008 09:55 PM ET (US)     Profile for Frank O  Send Email to Frank O     
In case it's of any use to other owners, 1/2" is the size of the socket to adjust the trim tab on this engine. That size of socket barely fits through the access hole.

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