It is not a widely known fact and the only place that I have seen it documented is in the official Triumph Dolomite 1500HL handbook Edition 6. I have not been able to identify at what point in time Triumph changed the rear axle ratio or the reason behind it. If you fit the wrong one your speedo will read incorrectly. The later models are fitted with units identified by 25/115868/**** and the earlier units 25/115867/****.

Then because later 1500cc cars have a different rear axle ratio to the earlier cars there has to be two specific build standards of O/D units. For our 1500cc Dolomites the Overdrive ratio is 25 so the first two digits are 25. The “61985” indicates it is built for a TR6 and finally the “0013296” indicates that it is the 13296 unit to be built to that specification. On the plate is stamped the identification of the O/D unit. For instant he points out that all O/D units have a serial number plate, which is secured to the front of the hydraulic casing. In this article I do not intend to repeat all the details that are really available in both the Official dolomite workshop repair manual and the Haynes manual but I will attempt to identify possible pit falls and list the items required to undertake the upgrade as far as I can remember as it is now a number of years since I undertook this task.īack in editions 107 to 110 of Dolly Mixture, David Anderson, who was formally the Chief Design Engineer at Laycock Engineering kindly wrote a series of articles on the J type overdrive and I have taken some of my details from those articles. I have copied below an article that I produced for Dolly mixtures way back.Ī question that comes up from time to time is what is involved in upgrading a 1500 car to have O/D.

Laycock j type overdrive for sale