Re: 1996 Johnson 225 venom COMPRESSION
C&P from Mar-Fab:
"Approximately 70% of the 90 degree closed-deck loopers we receive for machine work need repair because of piston ring dowel pin migration.This typically occurs when cylinder/piston temperatures are sustained at slightly over their threshold for extended running periods. As a result, the aluminum piston lets go of the steel dowel pin in the top ring groove. The pin will usually leave a notch or groove above the corresponding fingerport in the sleeve. In several cases, exhaust port damage will result from the top ring trying to exit out the
exhaust port.This occurs when the dowel pin forces the top ring gap open while the pin is exiting the piston.The cause of dowel pin migration in these cases is not a defective piston, but a lackof water (coolant) in particular areas of the block, resulting in elevated temperatures in the cylinder/ piston. By modifying the cooling passages around the cylinders(fitch style), dowel pin migration has been significantly improved. MarFab has been doing cooling modifications since mid-summer of 2001.We have had no returns with dowel pin migration thus far." This is why impeller needs serviced year no matter how good its pumping,non modified blocks need to keep the internal water pressure and flow high to cool block internally. 90% of the time if a piston looses a pin ,inspection will show the others have shifted also thus the need to replace the other ones. Testing has shown the Mallory impeller the best choice as different compound and will have 2-4 psi improvement over OEM and Serria and works great in the Fitch engines.
