Development of the M-60 series tank....
The M-48:  precursor of the M-60
The M-60 series... development and modifications
The M-48 series was the descendent of the M-26 Pershings and M-47s.  Sometimes referred to as the Patton, the M-48 went through a sequence of upgrades and improvements.  One of the more common versions was the a3 model, armed with a 90mm main gun and a new 750hp engine.
The final development of the M-48 was the M-48a5.  This version had a number of the key components from the M-60 series added on, including the 105mm main gun, and an improved AVDS-1790-2 750hp engine.  A unique feature was the "Israeli-style" TC cupola.  The hatch was attached to a ring with a sky-mount 7.62mm machine gun.  The hatch itself could be partially closed to provide protection overhead, but still allow excellent all-round visibility.  Lighter than the M-60s, the M-48 had an excellent horsepower to weight ratio and ground pressure, making it responsive and maneuverable.  It could also acclerate quicker than the M-60s, and go about 5mph faster.   One odd quirk of the M-48a5 was the retention of the older stearing wheel, instead of the M-60 "T-bar."  The wheel was odd, because when reversing, one turned the wheel opposite of the desired direction.  This was difficult for crews, familiar with the M-60 stearing system, to get accustomed to.
The M-60 series began to be deployed in the early 1960s.  Sometimes referred to in modern literature as the "Super Patton," I have never once saw this reference while on active duty using such vehicles, and to my knowledge this moniker was never officially used.  The first M-60s retained a turret similar to the M-48, but had a revised hull with better ballistic protection.  It was also armed with the new 105mm M-68 main gun (also used on European tanks).  Although an "uninspired" design, the M-60 series was based on a reliable design concept.

A humorous note about the searchlight.  It was supposed to be a two-man type, but was far too heavy, thus being dubbed the "six-man" searchlight.
The second in the M-60 series was the M-60a1.  The a1 had a redesigned wedge-shaped turret with better ballistic protection, and a new mount for the M68 105mm main gun.  Early vehicles had no gun stabilization system, but later this was retrofitted, and by the mid-70s most were so equipped.

This vehicle became the mainstay of the US Army's tank force through the 1960s and into the early 70s.
Since it was US policy to be on the defensive in Europe, it was decided that a new tank with a small turret aspect needed to be fielded.  The type was developed in the 1960s, but was not contracted until 1971, when the Army agreed to purchase 526 rebuilt vehicles with the new turret.  This became the M-60a2 (the "A-deuce").  By the mid-1970s it was dubbed the "Star Wars" tank (or "Starship" before the Star Wars movie came out), and for good reason.  Armed with a revolutionary 152mm gun-launcher system, the a2 was also equipped with one of the first laser rangefinders ever fielded.  The gun-launcher could fire conventional ammunition with a fully combustible charge, or the Shillelagh laser guided missile. 

The conventional rounds were stowed with a neoprene wrap around the cartridge.  When the loader put the round in the chamber, he pulled back on the rubber wrap, exposing the cartridge.  The breach was a rotating screw-lock type, similar to what are used on ship-borne guns.  The gunner used the laser range finder to "lase" the target.  He often received three readings, and if different, he chose the middle reading.  Pressing a button selected the range and provided the proper superelevation to the main gun. 

The Shillelagh missle was actually a dangerous system to work with, and there were incidents in which the missiles, after being fired, actually came back towards the vehicles on the range!

The "A-deuce" was essentially a failure, but provided valuable technical research in preparation for the M-1s.
The ultimate development of the M-60a1 was the M-60a1 RISE Passive (RISE= Reliability Improved Selected Equipment).  These were rebuilt M-60a1s, with added passive "starlight" imagers for the driver, gunner and commander.  Other improvements included the AVDS-1790-2C RISE engine which helped to boost engine power. 

Also added were a battery of smoke dischargers on either side of the turret, much in the fashion of the Chieftain tank.  Some models were retrofitted locally, and the control cable ran up the side of the turret, protected by small strips of thin armor plate, and then entered the turret near the searchlight mount.  Those rebuilt at higher echelon depots had holes for the cables bored directly through the armor on the side.

These vehicles were quickly supplied to frontline units overseas, such as in Germany, being deployed by early 1979.  One batch of vehicles were shipped with a significant flaw in the bolts holding the torsion bar housing to the hull.  These vehicles went to 3rd AD and needed additional work afterwards to repair.
The final development in the M-60 series was the M-60a3.  This vehicle took the interim developments in the Rise Passive version, and added a laser rangefinder, new ballistic computer, a tank thermal sight (TTS), and a thermal sleeve to the main gun (to help prevent "gun droop").  The first a3s began to deploy in Germany in mid-1979, one of the first units to receive them being the 3/33 Armored Battalion ("The Pickles") of the 3rd Armored Division.

The laser rangefinder added significant capability to the M-60, and many of these are still in service, with many in foreign service.  The Tank Thermal Sight was a significant advance, and tankers who have operated a3s and M1a1s almost universally state that the TTS on the M-60a3 was the best thermal imager ever fielded.  It was not used on the M1 series due to cost and its large size.  Some US Army NG units used the M-60a3s well into the late 1990s, but all have now been replaced by M1A1s.

This diagram is incorrect in one respect, in that it has the older track system T97, instead of the newer T142, which had removable track pads.  This newer track was also standard on the Rise Passive models, and was installed on some late production a1s.
Schematics by Russ Rodgers