You could. Chock the wheels and set the brake.
My old mentor taught me to hold the crank with a breaker bar at TDC. But he also warned me that leaning over the engine and listening at the intake, oil fill hole etc. risks getting the wrench launched into your teeth when the air pressure pushes the piston down.
It’s all in the physics. The crank has no torque at TDC. It’s like a balancing point. Once it slips past the piston will push down on the crank and generate increasingly larger amounts of torque. By the time the crank hits 90 degrees that wrench will be flying if you are using decent amounts of air pressure.
Just emphasizing this like my mentor did on me. It’s like being warned to use jackstands.
This is why I’d rather use a wrench than first gear. If you have any get lash in your transmission or differential the crank can move a little to an angle where it can create torque.
I’ve also noticed when rebuilding that bores in worn engines have the most wear at the tops where the piston tops out and the charge gets ignited. By holding the piston precisely at the top you get to check your piston to wall seal where it really counts.
Oh yeah and I found it was difficult to find TDC (compression) on our diesels because I cannot stick anything down a spark plug hole. I use a party balloon on the glow plug fitting to find TDC convenient. Relying on the balancer marks sometimes puts you at TDC exhaust where the valves are open.
I have a leak down tester someplace. It’s just a primitive flow meter (two pressure gauges with a constriction between them) and some fittings plus a regulator. It isn’t rocket science. After awhile I started ignoring the minimum and maximum flow and just listened to the hissing at various points on the engine. It’s helpful for isolating a leaking valve or head gasket leak. I just set upstream pressure with the regulator on the compressor and the leak becomes my flow restriction. Downstream pressure is atmospheric and upstream pressure is what you dial in on the compressor regulator. Easy.
This is what a professional one looks like.
