7-15-2020

The .20 and .28 appear identical from the outside.
Many parts interchange, BUT NOT ALL.

To tell the difference between .20 & .28
remove the back cover and look at the crankshaft,
.28 is stamped .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.
.20 Part No. 5600
.28 Part No. 5680
20 -- .650 bore .640 stroke
28 -- .725 bore .700 stroke
Thread size is 4x40
Prop thread 1/4x28
SPORTSTER .20 SPECIFICATIONS
Size
.20
Bore
0.65
Stroke
0.64
HP
0.5
Max RPM
13,000
Weight (oz)
10
Propeller
9 x 4
SPORTSTER .28 SPECIFICATIONS
Size
.28
Bore
0.725
Stroke
0.68
HP
0.625
Max RPM
13,000
Weight (oz)
10.5
Propeller
9 x 6