## Facts

- Ronnie McKenzie, a truck mechanic, was removing cylinder-head bolts during an engine overhaul when his air impact wrench developed a low-pressure problem.
- McKenzie switched to a ¾-inch ratchet wrench manufactured by S K Hand Tool Corporation (SK) and used it to loosen and remove several bolts.
- For a bolt that had broken loose but remained difficult to turn, McKenzie attached an extension (“cheater”) to increase leverage.
- While pulling on the wrench from atop the truck, the wrench came apart; McKenzie fell to the concrete floor and injured his right side and shoulder.
- McKenzie sued SK in strict products liability, alleging a manufacturing defect: the wrench deviated from SK’s design specifications and was unreasonably dangerous.
- McKenzie presented expert testimony comparing measurements of the failed wrench’s component parts to SK’s internal specifications; coworkers testified the wrench had not been disassembled or altered after purchase.
- At trial, the court struck all evidence concerning SK’s specifications, the component measurements, and the expert’s comparison testimony.
- The court also admitted evidence that there had been no prior accidents involving similar SK wrenches, over McKenzie’s foundational objections.
- The jury returned a verdict for SK; the trial court denied McKenzie’s motion for a new trial and his motion to amend the complaint to conform to the proof.

## Issues

1. Whether the trial court erred by striking evidence of SK’s design specifications, measurements of the failed wrench, and expert testimony comparing the measurements to the specifications offered to prove a manufacturing defect.
2. Whether the trial court erred by admitting evidence of the absence of prior accidents involving SK wrenches without an adequate foundation.
3. Whether the trial court erred in denying a new trial and leave to amend the complaint to conform to the proof.

## Decision

- The appellate court reversed the defense verdict and remanded for a new trial.
- The court held it was reversible error to strike the specification-and-measurement evidence and related expert testimony supporting McKenzie’s manufacturing-defect theory.
- The court held it was reversible error to admit no-prior-accident evidence because SK failed to establish the required foundation.
- The court directed that, on remand, McKenzie should be allowed to amend his pleadings to conform to the proof consistent with Illinois’ liberal amendment practice where no unfair prejudice results.

## Legal Principles

- In strict products liability for a manufacturing defect, evidence that the specific product deviated from the manufacturer’s own specifications is probative of defect and may be shown through expert comparison of measurements to specifications.
- Disputes about an expert’s methodology or the strength of the comparison generally affect weight, not admissibility, when the evidence supports a central defect theory.
- Evidence of an absence of prior similar accidents is admissible only with a foundation showing substantially similar conditions of use, substantial exposure under those conditions, and a reliable system for receiving and recording accident reports.
- Improper exclusion of a plaintiff’s core defect proof, combined with improper admission of unsupported no-accident evidence, can be prejudicial and require a new trial.

## Conclusion

The appellate court ordered a new trial because the jury was prevented from considering McKenzie’s primary manufacturing-defect evidence—deviation from SK’s specifications—while SK was allowed to present no-prior-accident testimony without the foundational showing needed to make such evidence reliable and relevant.


McKenzie v. S K Hand Tool Corp., 650 N.E.2d 612 (Ill. App. Ct. 5th Dist. 1995)