This section covers chassis repair sections and structural fixings for the MGA's separate steel chassis, the fully-boxed perimeter frame that carries the engine, body, suspension, bumpers and running gear. MGA chassis repair work is a specialist undertaking, but the most commonly needed repair sections and the fixings required to mount the body hardware to the chassis fall within this section.
Chassis extension repair sections
The MGA chassis extension is a major structural repair section, the extension is the forward portion of the chassis beyond the main perimeter frame, carrying the engine mounting brackets, the front chassis crossmember, and the front bumper mounting points. The extension is one of the most commonly damaged chassis areas on restoration-project cars, whether from accident damage, long-term corrosion at the engine mount brackets, or rust at the interface with the main chassis rail. Chassis extensions are handed (left and right are not the same section), so the correct-hand item should be confirmed before ordering. Installation requires the removal of the front wings, valance and the body from the chassis, cutting back of the damaged original extension, welding in of the new section to factory specification, and re-alignment of engine mountings, bumper brackets and front suspension mounting points.
This is professional welded repair work and should not be attempted by anyone without the specific equipment and skill.
Bumper brackets, front and rear, left and right
MGA-specific bumper brackets are supplied in a full set of four, covering both front and rear bumper mounting on both sides of the car: front left, front right, rear left, and rear right. The brackets are handed, left and right are separate items, as the mounting geometry differs between sides, so each bracket should be ordered to match the position being replaced. The MGA factory uses four bumper mounting points extending from the chassis through the front valance panel, carrying black-painted bumper springs or brackets split symmetrically at the centreline of the car. The three-piece factory front bumper is supported by these brackets; the single-piece rear bumper is similarly mounted via the rear brackets.
Because the factory bumper is three-piece front (split behind each overrider) and single-piece rear, the four-bracket system is specific to the MGA chassis geometry and the brackets are not interchangeable with MGB or other BMC applications. At chassis 102381 (July 1961), the factory introduced two additional distance pieces to the rear bumper brackets to bring the bumper further from the body, a subtle production change.
For cars being restored before this chassis number, the standard brackets without distance pieces are correct; after this chassis number, the distance pieces are factory-original.
Gearbox tunnel access plug
A rubber plug for the gearbox cover / transmission tunnel access hole is a service item for the small aperture in the gearbox tunnel cover that allows access to the gearbox or driveshaft without full removal of the tunnel. The plug seals this hole against ingress of water and road dust, and is often lost or perished on long-used cars.
Structural fixings, UNF sizes
The MGA is an Imperial UNF (Unified National Fine)-thread car throughout, no metric threads anywhere in the factory specification. The main structural fixings are available in the relevant UNF sizes: 5/16 UNF hex-head screws (3/4" length), plain zinc nuts and locking spring washers for smaller bracket fixings; 3/8 UNF hex-head screws (3/4" length), plain zinc nuts and locking spring washers for medium-duty bracket and bumper fixings; 7/16 UNF hex-head bolts (4" long), plain washers, locking spring washers and zinc nuts, the long 4" bolt specifically for structural fixings where the bolt must pass through a substantial thickness of panel and bracket stack, typically bumper-to-chassis mounting; and 1/2 UNF plain zinc nuts for use with 1/2" UNF bolts specified elsewhere in the bodywork assembly. All fixings are zinc-plated for corrosion resistance, the factory finish for MGA structural hardware.
Ordering considerations
Chassis repair work is a major undertaking and requires professional assessment before ordering. For anything beyond simple fixing replacement, the complete cost-and-time scope of the repair should be understood before purchasing structural sections. Bumper brackets should be confirmed as original (MGA factory geometry) rather than a previous modification before ordering replacements. The rear bumper bracket distance pieces for chassis 102381 and later cars are a small but visible production-correct detail worth getting right on restorations to original specification.