At 01:00 PM 9/5/03 -0400, Jan Johnson wrote:
>"Do you have any information on seat belt installation for my 1958 MGA?"

Yes. The most important point is, do not attach the seat belt anchor points to the plywood floorboards. They need to be intimately connected to the metal frame.

The MG Series MGA Workshop Manual has a Section S.21, way in the back, detailing installation of the "optional" factory supplied and dealer installed seat belt anchorage kit for the early MGA. The inside lower anchor point was a flanged steel cup plate with a heavy tapped weld nut. This was to be attached to the tunnel (after drilling) with six #10 machine screws and hex nuts. The outboard anchor point had an "L" shape nut plate which was to be welded to the frame, positioning the female threads on top. This was followed by a bolted bracket and a large bail-handled pin which could be pulled out to detach the belt from the anchor bracket.

As an aftermarket retrofit for a shoulder harness, an anchor point could be attached to the top of the steel body tonneau panel a few inches back from the cockpit trim rail, latterly about even with the joint between the separate sections of the rear trim rail. Two holes were drilled, and a chrome plated anchor plate was placed on top of the tonneau panel with two pan head screws and spring lock washers. A tapped reinforcing plate was placed under the panel to accept the screws. This is not recommended due to relatively weak single layer sheet metal on the body tonneau.

This is a picture of the factory installed (welded) mounting bracket for shoulder harness, beginning with the 1960 model year. This piece was available separately for field retrofit, so the aforementioned bolt-on aftermarket top bracket was short lived and not very popular.
The shoulder harness strap and longer lap belt section were all one piece with a latch

Lower connector for 3-point belts

tongue sliding along the strap. One end of the longer belt would have to be detachable in order to raise or lower the convertible top. That was accomplished by pulling out the bail handled pin at door sill bracket. (see drawings in shop manual). This was a decent solution for aftermarket installation, but in fact I have never seen an installation like this. If anyone should happen to know of one, I would appreciate pictures to post here.

Anchor point for shoulder harness on MK-II cars

Additionally there follows Section SS.2 detailing installation of seat belts in 1600-MK-II cars, starting with car number 100352 in March 1961, in which the seat belt mounting points were standard from the factory, and no drilling or welding was required (if you bought the factory kit). There is a picture in the shop manual showing the outboard lower anchor point (base nut plate welded to the top of the frame) with a quick release pin (to facilitate raising or lowering of the convertible top).

Addendum, July 21, 2016:
Here are pictures of original bolt-in L-brackets for attachment of the upper mounting of the shoulder harness (in the MK-II roadster). These appear to relocate the mounting point slightly outboard.
 

And also in Section SS.3 detailing attachment of the shoulder harness mounting point to the inner wheel arch for the Coupe.

Mind you I have yet to see any MGA which was fitted with these original type of seat belts. This is probably because they were not required, except for later production or possibly for a few select export applications. The U.S. standards required fitment of only the (front) lap belt mounting points in 1960 (belts optional), then fitment of the rear lap belt mounting points (in cars with rear seat) in 1963 along with fitment of the front lap belts. But by that time the MGA was out of production, so apparently no MGA was ever fitted with seat belts at the factory (at least not for the North American market), although the MK-II cars were fitted with the attaching points. Original stye anchoring hardware for MGA seat belts is available from Clarke Spares and Restorations.

Original style seat belt anchor brackets from Clarke Spares

Bob West makes beautiful reproductions of the original three point belts. They are the correct gray webbing, have chrome plated aircraft buckles and are of very high quality. They are more expensive than those through Moss but the price is reflected in the quality of the belts.

Addendum December 2008:
On 21 December 2008 Mike Ellsmore in Victoria, Australia, wrote:
"I then opted to purchase a set of belts from Bob West in the UK (mentioned on Barney's site as high quality but expensive). I eventually received these about 5 months after ordering at the great expense of $A400. While these are supposedly period replicas I wasn't over impressed as the fixing plate that goes to the quick release pin on the door side was not chromed , neither was the adjusting buckle (these were cadmium plated only). See photo attached. Bob apparently sub-contracts out the fabrication of these belts and doesn't have much control over quality.


Addendum April 2021:
This is what the factory style lower anchors look like when installed. Notice this is a retrofit, and the inboard anchor is attached to the tunnel with machine screws, where it would originally be spot welded from factory assembly. We must be careful not to allow the fasteners to protrude too far inboard where they might interfere with free motion of the propshaft.

Addendum September 2022:
Note related to the picture at right. This inboard belt mounting kit uses non-original aftermarket parts with flat plates, causing the large center bolts to be too close to the propshaft. This installation is shown in more detail in article INT-121A. Otherwise I would not recommend it. Better to use the factorey stule dished plates that do not intrude so much near the propshaft.

Picture at right shows the factory style plate with welded nut, domed to protrude into the passenger compartment, not to interfere with the propshaft space inside the tunnel. This is the factory retrofit kit part using six #10 screws (where late production plates were spot welded to ths tunnel). The installation shown here is slightly incorrect, as factory instructions call for the screw heads to be inside the tunnel for least protrusion, and the washers and nuts to be on the passenger side.

Addendum September 2022:
Here is a picture of the factory domed plate with captive thread spot welded in place as original. The second picture is a retrofit factory style plate either spot welded or plug welded through the six punched screw holes provided in the retrofit kit. Note the captive nut is inside the dome, not protruding into the tunnel space near the propshaft.

The outboard anchor is welded on top of the frame where it will not interfere with side of the seat. The outboard attachment point needs to be detachable for use with 3-point belts with the convertible top in the roadster (not so with the Coupe or for 2-point lap belts).