it's the 1st gen 22r with double row timing chain. i went to the junk yard and got the rear head plate off of a newer 22r. i assume it is the same part for all 22r's... but i was curious, so i took another one off of a 20r and although they have the same shape and bolt pattern, the 20r plate has a semi-circular protrusion about 3/4 inch wide that sticks into the head. either one of these plates will bolt up to my motor. which one should i use? the 22r is flat on the back, no protruding parts... also the 20r plate is aluminum, the 22r is cast iron...