Q: I’m changing the valve seals on my 1973 Camaro. The cylinder heads are on the engine, and the engine is in the car. I’m using compressed air to hold the valves closed. Also, the Camaro has an automatic transmission. How do I keep the engine from rotating?

A: This can be done by using an engine-rotating tool and a breaker bar. Attach the rotating tool to the harmonic balancer. Next, attach the breaker bar to the tool, and rest the bar’s handle against a rigid surface in the engine bay. Depending on the crankshaft position, the engine may rotate in either direction when you pressurize the cylinder. Apply compressed air slowly, making sure the engine is rotating in the desired direction. If not, re-position the breaker bar as necessary, keeping it clear of wiring, hoses, and delicate components.

This is another in a series of weekly Q&A Mailbag sessions with Summit Racing‘s tech department, in which there are hundreds more. Click here to see them all

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