When and why were pieces 3651 and 3652 discontinued?
3652 Technic Engine Piston Square 2 x 2



This part was included in sets from 1977 until 1995, but it began to be replaced by 2851 starting in 1990:



The main advantage of the newer piston is that it allows more pistons to fit in a smaller area of space. The piston is able to fit into the 2850 2x2 cylinder:



The old piston required a brick-built 4x4 cylinder, so the new part allows for either smaller engines, or engines with more pistons in the same amount of space.

Imagine how much space it would take to build this V8 using the old style of piston:



3651 Technic Axle and Pin Connector



This part was technically available in sets from 1977 until 2003, but it was generally replaced by 32013 starting in 1997:



This was part of a general overhaul of Technic axle connectors that replaced 4273 with the angled family of connectors (32013, 32034, 32016, 32192, 32015, and 32014).