The horse power curve on the ’51 36 cu. in. powerhead peaked at 27 HP at 4,300 RPM. This guaranteed 25 horses from 3,900 to 4,600 RPM. The super quiet ’55 never got above 24 horses unless you opened the cover. So drilling that hole gains you an average of three horse power. A few owners realized this and drilled a series of quarter inch holes under the back of the upper motor cover where they wouldn’t be noticed. They boosted it to 30 HP in `56 by increasing the compression ratio, changing the port timing and putting needle bearings on the wrist pins. Some low speed torque was lost with the port timing change. This meant you often needed a lower pitched prop to pull water skis. So the next major change in ’57 was the bore out the cylinders. The first production run of 5000 35s revealed an engineering fault. After 3 or4 hundred hours of slow idling, the needles would ware through the retainer and drop out into the powerhead. Ralf Evinrude had 5,000 powerheads set aside as warrantee replacements. We were not able to contac all the owners of outboards we sold but eventually all suspect motors were replaced. Some were blown away when they brought the motor in and got a new powerhead five years after the warrantee had expired. To his credit, Ralf was a lot like his dad . . . 😀