The Consumer Product Safety Commission filed a $1 million complaint yesterday against Sears, Roebuck & Co. and Roper Corp. for allegedly selling a product both firms knew could cause serious injuries.

The product is a Roto-spader roto-tiller, used to till gardens. According to the CPSC staff, the Sears Roto-spaders made by Roper between 1963 and 1979 can get locked in reverse and can mutilate the legs of its user.

The CPSC said that at least eight serious injuries have occurred as a result of this design defect, resulting in severe lacerations, fracture and/or amputation of legs.

The complaint, approved unanimously by the commission, charges that Sears and Roper knew about this potential hazard as early as 1975 but failed to report the potential problem to the commission--or to alert the public about the hazard in machines already sold.

The complaint seeks a $1 million fine, as well as a broad program in which the companies will have to notify the public of the potential risk of injury, and repair or replace the 220,000 machines that have been sold with model numbers beginning with the numbers 917.

Sears responded to the complaint by saying that the commission had the same information as Sears did in 1975. Further, a company spokesman said that the CPSC complaint was filed only because Sears and Roper had just informed the commission that it would no longer wait for commission approval to launch a recall program to repair the roto-tiller.