Water pump base problem on a 1969 merc 500 ss

AC5TF

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I recently purchased a water pump rebuild kit from our hosts. After a considerable amount of effort along with probably more than my share of colorful perjoratives, I was finally able to get the base off the lower unit housing. Much to my surprise, the new base (I've tried two different part numbers now), will not seat all the way onto the lower gasket. It would appear that the old base doesnt want to fit anymore, either. I honestly did not notice if the old base was actually seated on the gasket prior to removal, but - by the appearance of the gasket - I suspect it was. I can't see anything below the base that would obstruct it. Additionally, I have done extensive visual comparisons between the old and new bases and see no visible differences (other than the color). I have tried tapping it in place using long sockets around the mounting screws and can get it within about 1/8" from the gasket, but that's as far as it wants to go. My Mercury manual talks about shims, but never shows any in the accompanying diagrams and the text part of that discussion really doesn't make good sense to me. Does anyone have a clue what might be going on here?

Wishing I was Back on the Water...Fred
 

Moody Blue

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Re: Water pump base problem on a 1969 merc 500 ss

Did you replace the large o-ring seal on the base? You should. Lube the o-ring with oil/grease before trying to install the base back into the housing. It is a tight fit but it will go. You have to get it started square to the housing (not at an angle) then firmly press it straight down.
 

AC5TF

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Re: Water pump base problem on a 1969 merc 500 ss

Yes I did. Pressed it down as far as I could by hand, then tapped it down a bit further using a long socket. The further I tapped it down, the more pressure it exerts on the drive shaft, which has now become relatively hard to turn by hand.
 

AC5TF

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Re: Water pump base problem on a 1969 merc 500 ss ... ANOTHER ISSUE!

Re: Water pump base problem on a 1969 merc 500 ss ... ANOTHER ISSUE!

Okay, so my issue with the lower water pump body was with slight tolerance variations. The cylindrical part onto which the large compression ring fits was .005" too large in diameter at the top and .010" too large at the bottom...it would not fit! These measurements were made against the original Mercury pump body I removed. I also found interference with the dimensions of the body itself against the side-walls of the lower unit. Once I basically remanufactured this part, it fit perfectly. Now, however, I'm having another issue...I can't get the large compression ring started into the hole at all. I have tried pushing it down as uniformly as possible by hand, tapping it down with a hammer, even made a small jig that slides down over the driveshaft to allow me to apply more uniform force directly down toward the center. The ring is not even trying to start the path downward, as I can pull the pump base back out easily with my fingers after each attempt. I have lubed it beforehand. I noticed the original ring material is 0.13" in diameter and the new one is 0.14". Any suggestions? Could the slight difference in the ring material diameter be causing this???
 

Moody Blue

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Re: Water pump base problem on a 1969 merc 500 ss ... ANOTHER ISSUE!

Re: Water pump base problem on a 1969 merc 500 ss ... ANOTHER ISSUE!

Can you post any photos?

The pump base body that fits down into the lower unit housing is a relatively "loose" fit. Without the O-ring installed, it should easily slide down over the driveshaft and seat fully into the lower unit housing. It is the O-ring that compresses as it is squeezed into the cavity that seals the waterpump from the gear case. The variance in size of 0.005 and 0.010" is of no consequence in the fit of the pump base into the housing.

One thing I have seen before is that the wrong crankshaft seals are installed in the pump base. There are two different diameter driveshafts and if you have the small diameter seals trying to fit over the larger diameter driveshaft, well then that just won't work.
 

AC5TF

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Re: Water pump base problem on a 1969 merc 500 ss ... ANOTHER ISSUE!

Re: Water pump base problem on a 1969 merc 500 ss ... ANOTHER ISSUE!

Thanks for the response Moody. The inner seals appear to be the same size as those in the original base and offer about the same resistance when I slide the base down the driveshaft. I don't believe the shaft increases in size as it goes toward the bearing assy, but I'll certainly take a look at that. Without the o-ring, I can now seat the base to the lower unit with very little effort. WITH the o-ring...that's another story! That's why I'm wondering if I could have the wrong size o-ring, or if I'm just using the worn technique.
 
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