This upgrading of Armstrong lever shock damping rates for competition was originally directed to MGA Twin Cam, but it could apply as well to any MGA used for competition, as they all used the same shock absorbers. Original MGA shock absorber damping rates can be found in the tech book "Engineering Technical Data, MGA Twin Cam" in Section M for Suspension Dampers. Factory designated uprated damping rates and damping valve part numbers can be found in Tuning Information Memo T.I.M. No. 18.

Armstrong Lever Shock Special Tuning Valves BHA4136 - For MGA rear shocks

As Original:                - Blow Off -   - Time Setting -
Settings      Compression:     550 lb         350 lb
Front with    Rebound         1150 lb         725 lb
8" arm        Speed           180d/sec 18dC   20d/sec 18dC
                                  mean lever arm speed

Settings      Compression:     550 lb         350 lb
Rear with     Rebound         1150 lb         725 lb
5-3/16" arm   Speed           180d/sec 18dC   20d/sec 18dC
                                  mean lever arm speed

Uprated Valves:
Settings      Compression:     900 lb
Front with    Rebound         1240 lb
8" arm        Total Leak      1030

Settings      Compression:     600 lb
Rear with     Rebound         1800 lb
5-3/16" arm   Total Leak      420

I have some casual observations here. First you should notice that the blow-off force is different for bump and rebound directions. This allows the wheel to move upward relatively easily when you hit a bump, but will more greatly inhibit downward travel of the wheel on rebound. I don't want to get too deeply involved with suspension dynamics here, but one interesting effect is when you run on a washboard surface it will tend to make the car hunker down a little lower. Also notice that the standard settings for force and timing are the same for front and rear. However, the rear shock arms are shorter than the front ones, so the rotational torque on the shaft will be less at the rear, meaning that the rear valves will be set for lower blow-off pressure.

For the uprated (competition) valves, in front the compression force is much greater while rebound force is only a little higher. In the rear it is the other way around with compression force being only a little higher while rebound force is much greater. I expect that the factory engineers knew what they were doing here, and I'm not going to second guess them or make any other recommendations.

The correct uprated valves for MGA may not be available today, unless you might luck out and find some new old stock parts. Moss Motors sells uprated shock valves for the MGB, but these would definitely not be suitable for the MGA. As installed in the cars, the front shock bodies are oriented in opposite direction, inboard for MGA and outboard for MGB. This makes the shaft rotate opposite, thereby switching the oil flow direction for bump and rebound. So installing the MGB valve in the MGA shock would totally screw up the resulting forces.