The UR1 engine was developed in a one cylinder toolcarrier tractor T18 in the mid 50s. Then the first 3011 prototypes came in 1958, serial production from 1960 onwards. In 1962 the heavier 4011 followed and in 1963 the 2011 followed.
In 1966 the 5511 was introduced to international dealers, and in 1968 it went in production. Changes over the 4011 were bigger ring and pinion gears, a different camshaft profile and M16 head studs instead of M14, these heads also have a bigger intake channel.
The later 6701 engine, which was identical except for the bore increased from 95 to 100mm, and the camshaft changed back to the original 4011 camshaft, used untill the 2000's...
The 7 series also saw introduction of the very crude air operated hi/low splitter, which used mechanical dog clutch and freewheel instead of friction clutches. Therefor it wasnt sold in many countries in the West.
Then, the 9 series in 1978 saw a major change of engines: The 5501 engine was dropped and the 6001 engine shared the 100mm bore of the 6701 engine, and the 7001 engine got a slightly bigger bore of 102mm which only marginally increased the engine displacement from 3.455 to 3.595 liter.
More effectively, with all current engines having at least a 100mm bore it was possible to put a bigger intake valve in the head. The rockers are different too, because the intake valve stem is moved slightly further off cylinder center.
Now the final change came in 1987 with the 7745: its increase of stroke from 110mm to 120mm which increased displacement from 3.595 to 3.922 liter. In this final incarnation it was used untill 2006 in the Major, although updated to TIER 2 emissions.
As far as rear axles go, the 4011 portals had M14 bolts. On the 5511 (or 7 series) they were beefed up to M16 and 3150kg capacity, and i think on the 10 series they were increased to M18x1.5 which are the only fine thread fasteners on the rear end, except for the wheel studs. With 3.6 ton capacity.
In the most recent incarnation, the ProximaPlus of 2008 the weak link was finally erased, and both the portal and the trumpet were united in a single casting, the gear lube prescription increased from ordinary gear oil to 10w/30 which is more heat resistant.
Most probably the hardening of gears was improved because nowadays 120hp is put through the 1963 gearset of a 50hp tractor
In the last 7340 engines, the connecting rods, camshaft, crank and rod bearings, were still the same as the 4011 engine.
