The following article was published in the September 1972 issue of Cars & Car Conversions magazine, the absolute 'bible' for anyone wanting technical advice on tuning and maintaining the BMC A Series range, and especially the Mini.
I used this article to successfully build a straight cut close ratio gearbox for a rally Cooper S back in the mid-1970's, never having played with a gearbox before. All the greasy finger prints are mine.
Whilst the gearbox in the article is a 3-speed synchro there's details of the differences when rebuilding an all syncro box.
