MARK 55 NO VACUUM. SHOULD I WORRY

[jrm80]

I’M RESEALING THE LOWER UNIT OF A MERCURY MARK 55 AND JUST FINISHED IT UP. I’VE PRESSURE TESTED IT UP TO 20 LBS AND IT HOLDS THAT PRESSURE WITHOUT DIFFICULTY. ALTHOUGH I REALIZE THAT THEY DIDN’T REALLY PRESSURE TEST OR VACUUM TEST IN THOSE DAYS, IT IS STILL A GOOD IDEA TO PRESSURE TEST. THE QUESTION THOUGH IS IN REGARD TO THE VACUUM TEST. IT WILL NOT HOLD A VACUUM AT ALL. ALL OF THE SEALS ARE ORIENTED TO KEEP OIL IN SO THE PRESSURE RESULTS TEST ARE NO SURPRISE. THE VACUUM TEST HOWEVER DOES SURPRISE ME. I WOULD EXPECT THAT IT WOULD HOLD A LITTLE VACUUM ALTHOUGH MAYBE NOT A LOT. IT IS HOLDING ALMOST NO VACUUM, MAYBE 2 OR 3 INCHES. IS THIS NORMAL OR SHOULD I BE TAKING IT APART AGAIN TO FIND OUT WHY IT WON’T HOLD A VACUUM?
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ONE OTHER QUESTION IF I MAY. I AVOIDED TAKING THE DRIVE SHAFT OUT AS THERE WAS REALLY NO NEED SO BOTH THE DRIVE SHAFT AND FORWARD GEAR CLEARANCES WERE UNDISTURBED. WHEN I REMOVED THE REVERSE GEAR AND PROPELLER SHAFT, I NOTICED NO SHIMS BETWEEN THE BEARING CARRIER AND THE HOUSING AND I DID LOOK FOR THEM. THERE WEREN’T ANY. ON REASSEMBLY, THERE IS A DISTINCT GEAR RATTLE, EVEN BEFORE THE REVERSE GEAR AND PROPELLER SHAFT IS REINSTALLED. IF I LIFT THE DRIVE SHAFT TO IT’S UPWARD LIMITS ( 2-3 THOUSANDTHS) THE RATTLE DISAPPEARS COMPLETELY AND IT SPINS EASILY. I DON’T KNOW THE HISTORY OF THIS ENGINE BUT IT APPEARS SOMEONE HAS BEEN INTO THIS GEARBOX BEFORE JUDGING FROM THE BEAT UP APPEARANCE OF THE GEAR HOUSING COVER. SOMEONE USED A PUNCH TO REMOVE IT INSTEAD OF THE PROPER TOOL. ANYHOW, I DON’T KNOW IF THE ABSENCE OF SHIMS BEHIND THE REVERSE GEAR IS MEANINGFUL OR NOT. MAYBE THEY JUST GOT LUCKY IN THE ORIGINAL MANUFACTURE.

IS IT NORMAL TO HEAR GEAR NOISE LIKE THIS, OR SHOULD I WORRY? I WOULD IMAGINE THAT THE GEAR NOISE WILL IMPROVE SIGNIFICANTLY WHEN GEAR OIL IS PLACED IN THE UNIT, BUT IF I NEED TO GO FURTHER WITH THIS GEARBOX, NOW IS THE TIME TO DO IT.

ANY THOUGHTS ANYONE?

[Buccaneer]

No expert here, but if you only have a tiny bit of backlash between
the gears, and the noise goes away when you lift up on the drive shaft,
won’t the gears, being the way they’re designed, separate, as if you
were lifting the drive shaft, and hence be “quiet” while running.
Like you say, gear lube will make a big difference as well.

Prepare to be boarded!

[jrm80]

THANKS FOR THE REPLY. I THINK YOU’RE RIGHT AS IT MAKES SENSE THAT THE ROTATION OF THE DRIVE SHAFT BY THE CONICAL GEARS WILL CAUSE THE DRIVE SHAFT TO LIFT UP TO IT’S UPPER LIMITS WHERE IT ROTATES SMOOTHLY. THAT’S ALSO WITHIN THE ENDPLAY LIMITS CALLED FOR IN THE SERVICE MANUAL.

I’LL QUIT WORRYING ABOUT THAT PROBLEM, BUT I STILL HAVE THE VACUUM ISSUE TO RESOLVE. ANY TAKERS ON THAT QUESTION?

[dave-bernard]

first 20 lb pressure is very high. 7-8 is all you should be doing . seals can be ruined with to much pressure. second that unit is designed for grease not oil. I use john deer corn head grease. y0ou say you did not take the drive shaft out how did you get to the water pump seal ? that is the one that goes bad most of the time.

[Author]

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