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13.5 Shock Absorbers

1. The make of shock absorber may be substituted providing that the number, type, (e.g., tube, lever, etc.) system of attachment and attachment points are not altered, except as noted below. The interchange of gas and hydraulic shock absorbers is permitted. The following restrictions apply:
1. No more than two separate external shock damping adjustment controls. Gas pressure adjustment is not considered a damping adjustment.
2. Suspension geometry and alignment capability, not including ride height, may not be altered by the substitution of alternate shock absorbers.
3. Adjustable spring perches are allowed, but the spring load-bearing surface must be in the same location relative to the shock mounting points as on the standard part. Shims may be used to achieve compliance.
4. The fully extended length must be within plus or minus one inch of the dimension of the standard part.
5. Electronically controlled shocks may not be used on vehicles not originally equipped with such units. Vehicles originally equipped with electronically controlled shocks may use the standard parts or non-electronically controlled alternative shocks subject to all requirements of 13.5. Non standard electronically controlled active shocks are not allowed.
2. The mounting hardware shall be of the original type. The use of any shock absorber bushing material, including metal, is permitted. This does not permit the use of an offset shock absorber bushing. The bushing attaching the end of the strut to the body or frame on a strut type suspension is a suspension bushing, not a shock absorber bushing. Suspension bushings, including but not limited to those which carry the weight of the car and determine ride height, may not be replaced with bushings of a different material or dimension.
3. To facilitate the installation of commonly available aftermarket shock absorbers, struts, or strut inserts whose shaft is larger than the center hole of an upper mount assembly, that hole may be enlarged by the minimum amount necessary to accommodate the shock shaft size, provided the following restrictions are met:

1. the enlarged hole must remain concentric with that of the original configuration;
2. the enlargement of the hole does not require modification of a bearing (as opposed to a washer, plate, or sleeve);
3. neither the hole enlargement nor the location of the shock shaft changes any alignment parameter.

4. A suspension bump stop is considered to be performing the function of a spring. Therefore, the compressed length of the shock at the initial point of contact with the bump stop may not be increased from the standard part, although the bump stop may be shortened for the purpose of installing non-standard shocks. Bump Stops installed externally and concentric with the shaft of a shock may be drilled out to fit a larger diameter shock shaft. Bump Stops may be substituted for the purpose of installing non-standard shock absorbers.

The body length has to be equal, but travel, like Wes posted, has to be within one inch of stock, which for adjustable perch shocks is hard to find. - AB

Based on my reading of the rule, it looks like the shock body can be longer or shorter, so long as the spring perch is in the right place and the total extended length is + or - 1".

Scott