Normal wear and tear usually. If you have repeated head gasket failure, there is a problem with the surface or the torquing procedure
Age, misalignment, wrong tension, prolonged lunatic clutch dropping.
Chain too tight
What makes the Harley-Davidson V twin motor sound the way it does is the basic layout of the motor. It's a 45 degree V and both connecting rods ride on the same crankpin which causes a unique stacato exhaust rhythm. The arrangement also enhances torque output.
The most common cause is a grounded stator. Lack of primary oil or a too-tight chain can also cause excessive chaincase heat.
A faulty gasket, overheating of the engine or old age. Or, a combination of any of these causes.
The most common is the head gasket. BUTDepending on the engine there can be several other causes.
You could have a sticking solenoid on the starter, you could have a defective starter button, or your solenoid wires may be set up in a way that causes the current to ark between points when it's not supposed to.
Usually a tear.
to much heat
Defective gasket, poor installation, warped head, heat. Heat is the major cause.
Head gasket
You have a blown head gasket.