There has been some occasional confusion over what O-ring seal should be used in MGA rear axle hubs. This is due to a machining change in the hubs with introduction of the 1600 car. In pictures below, notice the narrow groove which holds the O-ring, just outboard of the steel bearing spacer ring. Click for larger pictures.
 

MGA 1500 disk wheel rear hub

MGA 1600 disk wheel rear hub

The 1500 hub on the left has a larger diameter 3-sided groove in the face of the hub, and a narrow dividing wall between the gland and the bearing spacer ring.

The 1600 hub on the right has the slightly smaller diameter gland composed as a simple step in the bore of the hub, and using the OD of the bearing spacer as the ID wall of the gland.

Wire wheel cars do not use the bearing spacer ring, as the wire wheel splined hub has a shoulder in that location which serves the same purpose.

The gland in both hubs is 0.088" deep and 0.131 to 0.133 wide.
Both use a 0.117 diameter cross section O-ring, but slightly different overall diameter.
These seal rings were originally rectangular in cross section. Aftermarket replacement parts were changed to round section O-rings around 1968. By coincidence, this is about the same time the MGB changed to use the later Salsbury (tube type) rear axle.

1500 gland OD=3.545 ID=3.283 width=0.131 depth=0.088
1600 gland OD=3.413 ID=3.147 width=0.133 depth=0.088

Some confusion may arise when rear axles may be swapped to change cars from disk wheels to wire wheels, or vice versa. A replacement hub may not be the same model as the original hub. Early MGB rear axles also can be installed in the MGA, where the MGB hubs are similar to the MGA 1600 hubs (except MGB uses 1/2-inch wheel studs where MGA uses 7/16-inch wheel studs). It is easy to end up with 1600 hubs on a 1500 car, or 1500 hubs on a 1600 car. The way to know which O-ring you need is to look at the style of machining for the gland. The larger diameter gland with the narrow inside wall should use the larger O-ring from the MGA 1500. The smaller diameter gland machined as a step in the housing bore should use the smaller O-ring from the MGA 1600 and early MGB.

If a person were to attempt to install the larger diameter O-ring from the 1500 car into a 1600 type hub, there would be some difficulty compressing it enough to fit. Just as the final bit is being pressed into the gland the O-ring will pucker up and leave a short loop exposed because or the excess length.

While the early and later type hubs are interchangeable, the appropriate size O-ring should be used for each. Additionally, some of the very early production MGA 1500 cars had no provision for using the O-ring. The O-ring gland was introduced starting with rear axle no. 3725 for wire wheel cars and 5225 for disk wheel cars. There was no change of part number for the hubs. DW and WW axles have different model number, so likely also different series of serial numbers. I do not know how this correlates with car numbers, and they may not be in exact numerical sequence (similar to engine numbers), so you would need to check the serial number stamped on the rear axle (assuming it still has the original hubs).

I keep in my personal inventory the O-rings currently specified by Moss for the 1500 car, part number 121-405. As it currently stands, this O-ring is a perfect drop in fit for the 1600 hub. Installing this in the 1500 hub requires a slight stretch. During installation I will roll the O-ring slightly inward as I stretch it into the gland. Then as the O-ring relaxes into place it will grip the inside wall of the gland and roll gently back into the gland to rest against the back wall, thus staying nicely in place without popping out.

Of course it's up to the vendors to supply the correct O-rings under the appropriate part numbers. At present (mid September 2003) Moss Motors Ltd, U.S., is investigating this item to determine if their O-ring sizes may be incorrect, or if a tech note such as this should be added to the parts catalog. Apparently Moss of Europe currently has only one part number listed for this O-ring for all MGA. Victoria British Ltd. also has only one part number listed for all MGA (except Twin Cam).

ADDENDUM (Oct 9, 2003):

Moss Motors report, conclusion, and fix: Item previously listed as 121-400 for the 1600 hub was too large diameter and too thick for either hub, and the inventory parts are therefore scrapped. Item previously listed as 121-405 for the 1500 hub is a perfect fit for the 1600 hub and will work for the 1500 hub with just a little stretch. In fact that is exactly what I have been doing for several years with my 1500. These parts are being moved into the 121-400 bin and are now specified for use with the 1600 hub under the 121-400 part number previously specified for this purpose. For the 1500 hub, customers may use the same part (with just a slight stretch), now stocked under the 121-400 part number. It is my impression that there will be only a single part stocked in the future for use in both applications, and that the part number in the catalog for the 1500 will be changed from 121-405 to 121-400, or the 121-405 part will refer to using part number 121-400. ADDENDUM (October 27 2024: Added factory part number pages for the O-rings ATB7229 and ATB7354 part numbers.