AP automatic transmission

Last updated
AP Automatic Transmission
Overview
Manufacturer Automotive Products for British Motor Corporation
Production1965-2000
Body and chassis
Class 4-speed transverse automatic transmission

The AP automatic transmission is a 4-speed automatic transmission unit developed by Automotive Products and the British Motor Corporation specifically for use in the Mini.

Contents

It was unique in that not only was it one of the first transverse mounted automatic transmission units tailored for front wheel drive, it was one of the smallest in order to be used in the transmission-in-sump installation beneath the A-series engine used in the Mini, and later the 1100/1300 ranges. Another novel feature was that the transmission was designed to use the engine's lubricating oil, rather than conventional automatic transmission fluid (ATF). The units were manufactured in a BMC plant in Kings Norton, Birmingham which supplied complete powertrain packages (the unit required a unique engine variant) to the Longbridge and Cowley assembly plants.

Around the early 70s, the transmission received revisions with the new pre-oiler pump that prevented the 3-4 engine surge between shifts, and the quiet engagement into drive and reverse thanks to a revision valve that held the gears until it was fully engaged, eliminating a hard clunk heard on earlier versions of the transmission.

It was also later used on the Austin Allegro, and also in the original A-Series engined version of the Austin Metro, which remained in production until 1991 (outlasting the manual transmission version which ceased production the year before in favour of the heavily revised Rover Metro).

It also remained an option on the Japanese Domestic Market Mini until the demise of Rover Japan in 2000.

Known Issues

The best oil to use is a JASO spec MA2 oil used by motorcycles that a) share the oil with the engine, b) have wet clutch plates / packs c) a far greater performance than lower performance motorcycles.

There a few companies that can remanufacture these units but they are getting more and more scarce as time goes by.

The gear sets can fail if overloaded, this tends to happen after the unit has suffered a serious amount of wear or damage that causes the transmission to engage "come in" with a thud, this can cause the double bevel gears (an especially interesting part to Engineers) to break at the weld between each pair of laser welded gears.

Applications

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References

https://web.archive.org/web/20131018030057/http://www.austinmemories.com/page172A/page172.html