Subject: Close Ratio gearbox. -- It is helpful if you have completed GT-101, as the close ratio gearbox is very similar to the standard ratio gearbox, the primary difference being the number of teeth on each gear.

The close ratio gearbox was a factory option for the MGA, available on Twin Cam from April 1959 and also on pushrod-engined cars thereafter, but always most common on Twin Cam and "Deluxe" models.

On cars which were originally fitted with the close-ratio gearbox, the code letter 'U' (for center-change gearbox) in the engine number prefix was replaced by the code letters 'Da' (for close-ratio, center-change gearbox). In other cases the 'Da' or 'DA' could be hand stamped above or below the original power unit number, as shown below. This implies that the decision to install a close ratio gearbox was made after the number tag was already on the engine. The close ratio gearbox was also a common retrofit after production of the car, in which case there may be no 'Da' marking on the engine plate, and the only way to know for sure is to check the gearbox.

While the 'close-ratio' gearbox was typically fitted to Twin Cams and Deluxes, it was available as a production option on all models after April 1959. And, since the first 1600 was produced in May of 1959, the 'close-ratio' gearbox could have been a production option on the 1500, although this might be considered a bit of 'stretch' by some.

The 'close-ratio conversion kit' consists of the first motion shaft with input gear, laygear, third speed mainshaft gear, second speed mainshaft gear, and a second speed baulk ring. Gear ratios were as follows:

         Gear  Close-ratio  Standard-ratio
	 1st     2.448:1     3.636:1
	 2nd     1.619:1     2.214:1
	 3rd     1.267:1     1.374:1
	 4th     1.000:1     1.000:1

No telling how many close-ratio gear sets were produced, but there was apparently never a shortage of them when the cars were new. And, it seems they were often coupled with the optional 4.55 differential set.

You may notice that the close ratio gear set apparently has no low gear, more like two second gears. This is often partially compensated for by the change in final drive ratio to 4.55:1. With a high ratio rear end, the top speed also diminishes (at red line). Good for short track work, but not so good for the open road.

If you have an interest in buying a close ratio gear set, check here:
Jack Knight - close ratio gear sets for your MG

The Jack Knight web site seems to be defunct after 2016, so this link is from the WayBack Machine web archives (not current issue).