It is believed Tangye type "AA" Oil engines followed on from the previous type "O" engines.
As with most Tangye dating records when the first "AA" engine was built is unknown, but from catalogues available the Type "AA" engines were first listed in 1908.
As can be seen from the tables below type "AA" oil engines were given a two letter Reference designation the first starting with the letter A onwards indicated engine HP and the second letter always A indicated type "AA".
Type "AA" Oil engines carried a Tangye Patents plate that can be seen on Tangye Plates Page.
This information was taken from "Oil Engines Type AA" catalogue No 177 dated Sep 1908.
The serial number of Type "AA" engines originally used a suffix B after the number that carried over from the type "O" engines, at this stage the exact meaning of the B is unknown but thought to involve design modifications to previous engines.
Around 1908 major changes to incorporate improvements referred to by Tangye as Revision were released, these carried a serial number suffix of BR (the R stands for Revision) after the number.
The BR revisions included different HP ratings, which may include flywheel and bore/stroke differences depending on size, one easy recognised difference was that the BR engines included a oil grove formed all round the bedplate on all BR engines that I have seen, EXCEPT some small HP engines with suffix BR had the oil grove.
Note that suffix B (non revision) engines were also supplied well after the introduction of the BR.
A lot of confusion is caused by the designation letters of these engines, "P", "O" and "AA" are different engine types where as the B and BR after the serial number are not engine types. they are serial number suffix letters that designate the design modifications by Tangye.
As additional data is obtained the details on this page will be updated.