Cessna

Assuming that this engine is installed in a plane, you should get this information from an official engine manual. Someone is going to trust their life to this powerplant - If someone was to answer this question - where did they get the number? did they guess? who knows?

If the rocker comes loose in flight, the engine would lose oil. It would eventually lose oil pressure and fail. Someone could die.

Don't guess.

There is a torque listed in the manual, i can't recall it just now. The screws are only 1/4", and just need to be fairly firm. Perhaps the most important, is to have two 1/4" flat washers under any P-clips supporting the HT leads. This ensures the screw is not bending the P-clip over the rocker cover flange, and likely to come loose. Personally, i use a drill/driver, set at torque number 3. Just whizz them all in, and final check by hand.

Remembered to check the Lycoming manual today, and the official torque for the rocker box screws is 50 inch/pounds. You need to be a bit careful here, the torque can probably be applied ok with the standard cork gaskets, but there are some rubber re-useable types around, that may squidge out before you get that tight. This will then cause a leak.