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Where is the serial number?

K

km1125

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I have (what I think) is an early 80's 20HP Mercury outboard. Can't find a plate that has the serial number on it. Where would I find this? All I know about the motor so far is it has a "Thunderbolt" ignition and two-tone blue strip on the cowling. It is a two cylinder two stroke.

I'm actually looking for the impeller part number. Just poking around for various 20HP engines, it looks like it might be a Sierra 18-3057. Do most of these motors use that impeller regardless of year?

Thanks,
 
The number is usually on a plate riveted to the swivel bracket.Pic of the decals and powerhead will help ID it if the plate is missing
 
The serial is also on a welsh plug (silver disk about the size of a quarter). On the small twin cylinders it's typically on the starboard side of the motor (so right hand side if you are standing behind the motor) at/near the (heads) water jacket cover.

Here's a pic of a small twin. The motor is out of the cowl but will give you an idea where to look. The (disk) marked "here".
 

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    Outboard.jpg
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OK, here's some pics. Looked over it again and don't see any Welsh plugs with stamps (had one on the old Johnson 9.9). Don't see anything on the swivel bracket. After looking through a bunch of online parts catalogs, it looks like the impeller is a Sierra 18-3052, just by matching up pictures between the diagrams and the engine, but still not sure

Merc20_1.jpg
Merc20_2.jpg
Merc20_3.jpg
Merc20_4.jpg
 
Is there a way to cross a year (74-78) to a serial number or serial number range so I know I'm getting the right parts?

Just looking at the parts books, I think it's " Model 200, 20 HP > 4709593 & Up (USA)", but still not sure.
 
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