Beta engines -

There should be the works order number on the engine - its a WOC K 1234 number and the speak to Beta Marine, they are very helpful. Tel no 01452 723492
 
They are different base engines. Both 3 cylinder, but the 20 is 719cc and the 28 is 1001cc. The 28 is 779mm long and the 20 is 656mm. On the 20 the seawater pump is to the left of the engine viewed from the front and on the 28 is just above and to the right of the crankshaft pulley. Lots of other detailed differences, but you should be able to easily tell the difference. However there is a 25hp which is similar to the 20 but everything is a bit bigger.
 
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