CAPA said, when it tested 1,907 car company-brand replacement parts, that it found 50 percent of them (954) did not meet its standards for fit, finish and appearance. In testing Ford parts, CAPA said that 60 percent of the 440 items it tested failed to meet its standards.

Mr. Gilbert said that a news release from CAPA concerning the findings had not given any specifics as to why parts had failed, beyond mention of fit, finish and appearance.

He noted that the correct tolerances for an original equipment or replacement part that Ford uses in a vehicle are not publicly available. "Our standards are intellectual property and we don't share them," Mr. Gilbert said.

"How do they establish their standard?" he asked. He also suggested that their research may not be a statistically valid sample size. Mr. Gilbert wondered how CAPA established dimensions for a fitting gauge and how they compared with original manufacturer tolerances.

In Mr. Gilbert's view the "overwhelming evidence" is that original equipment manufacturer parts fit better.

He noted that the dispute over whether generic secondary manufacturer parts are better than original equipment manufacturer parts has been going on for well over a decade. "The consumer should have the right to decide," Mr. Gilbert said.

Insurers have generally backed the use of less expensive generic aftermarket parts for repairs. The issue gained prominence within the industry in 1999 when State Farm was hit with a $1 billion judgment for failing to tell customers it was repairing cars with generic parts, which an Illinois jury found to be inferior.

CAPA said that, in checking manufacturers' parts, technicians mounted each part?both company brand and CAPA?and adjusted it on the vehicle to get the best fit. Fit evaluation points were said to include fit to adjacent parts, horizontal and vertical spaces and the position size and construction of attachment points.

CAPA, in rating other company parts, said General Motors had a 65 percent failure rate; Chrysler, 47 percent; Nissan, 41 percent; Toyota 39 percent and Honda 27 percent.