This is a little info of bad luck, I found out on my 270 GMC during the motor rebuild.
1. Head has a crack in #4 cylinder, machinest did not find it until the head was rebuilt, lucky me.
2. Bad News, Cylinder #4 was Hydro Lock. Sounds Expensive, The cylinder block stretched to allow water to pass by the piston. THis required required not one but 2 sleeves, we should sleeve # 3 cylinder because of the groove caused by the broken Rings.
3. Last week I found out from another reader when the block is getting a new Thermostat, the air needs to be purged from the head by cracking the Temp gage. This bleads the air out fo the block, drill a 3/16 hole in the Thermostat housing allowing water to pass by the thermostat collar preventing an air lock in the head.
Somthing to check out.
