There are two different front suspension steering knuckles for the MGA. They are hard to tell apart at first glance, but have some dimensional differences. Note that the near vertical bore for the swivel pin is inclined with the top end closer to the centerline of the car. The horizontal bore for the steering lever is tapered with the larger end of the bore forward and a key slot at the bottom. These two bores overlap slightly, such that the tapered root of the steering lever mates with an indentation in the side of the swivel pin to lock it into place. Also notice a slight counterbore at the top of the vertical hole which accepts a small flange collar on the swivel pin.

The knuckle shown here is from an early model MGA 1500. You are looking at the smaller end of the horizontal bore (back end), which makes this a right side part (steering lever extends forward). This part is the same as on late production TD and TF cars. Sometime during TD production the thread on the end of the bearing spindle changed from 5/8 inch to 3/4 inch (and may not have had a change in part number). If you find one of these with 5/8 inch thread, it is NOT an MGA part. All MGA spindles have 3/4 inch thread. The early MGA parts are ATB4206 (RH) & ATB4207 (LH).

At MGA 1500 car number 54247 (July 1958) the knuckle and the steering arm (next page) and the nut securing the arm to the knuckle all changed at the same time. The thread for the nut on the steering arm changed from 1/2-inch to 5/8-inch thread. The tapered interface between the knuckle and the arm also got fatter, such that the early arm will not fit the later knuckle, and vice versa. Swivel pins and bearing hubs are otherwise interchangeable. The mounting plate for the disc brake caliper on the 1600 cars also fits on the same knuckle, so the 1500 cars can be converted to disk brakes without changing the knuckle. But you may have to change to the 1600 type steering arm, and the 1600 arm may not work with the early 1500 type knuckle. The late MGA 1500 and 1600 parts are ATB4268 (RH) & ATB4269 (LH).

The third knuckle type is easier to spot. For the Twin Cam car the knuckle has an asymetrical 2-bolt flange for mounting the Dunlop brake caliper. Part numbers are ATB4232 (RH) & ATB4233 (LH).