To tell the difference between .20 & .28
remove the back cover and look at the crankshaft,
.28 is stamped on .28 and .20 is not stamped.

The .20 came out first then the .28
The only design change was the remote needle.
The .20 and .28 both have 12 head fins and a
diameter of 1.425"

The early .20 had a O-ring for a head gasket but this was discontinued early in the production. The .20 does not have a head gasket. The .28 uses a head shim.

Part No. 5600
 
28 .723 bore .700 stroke --
Thread size is 4x40  --
Prop thread 1/4x28

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