The crack has no impact on the proper functioning of the engine. All cylinder heads crack between the two valve seats, more or less. On my 121cub engine, after engine overhaul, I noticed that an imperceptible crack only appeared after cleaning and polishing between the intake and exhaust ducts, which does not interfere with its proper functioning. After a certain mileage, it is good to rectify the valve seats as well as the valves which tend to deform and to do a manual lapping.
On cylinder heads with large valves this thing does not happen because after fitting the intake seat the cylinder head is machined again for the exhaust seat which ends up embedded in part of the intake seat.
