Replacing circulating pump on 8.1 w/v-drives


Well-Known Member
Oct 3, 2006
Orange, CT
Boat Info
2006 340 Sundancer
Twin 8.1 Horizons with V-Drives
I noticed on one motor a drop of antifreeze in the water pump weep hole so while I have the mufflers out I’m going to replace them.

For those who have changed these, do I have to remove the crossover tube or can I lift the water pump into the special seal on the bottom of the crossover tube while it’s still on the block?

Any other words of wisdom while doing this?

Thanks in advance.

Also - are Sierra pumps inferior to the Mercruiser circulator pumps?
IMHO... If it was my job, I would do the crossover, too. That whole assy looks to be a job for a contortionist. Make it as easy as possible to get everything aligned properly and sealed to avoid problems later.

On the quality, I would say any major named marine circ pump is GTG. Stainless internals and back cover are main concern. I stay away from the no name Chicom knockoffs.
I am lucky right now to have a lot of room as my mufflers and mounting plates are off. I figure I’ll change the pumps now while access won’t get any better:)
I have already cleaned the rust stains ;).... An will be changing the raw water impellers while in there.

I've never changed one on an 8.1, but did on my 4.3 and 5.0's. I used a Sierra on my 4.3, it was still working fine 10yrs later when I sold it. One other tip, loosent the bolts on the pulley before you take the belt off.
It's a closed cooled system so doesn't need to be SS. GM would probably be half the price and the pump looks like the one on my PU truck with the 8.1.
Boy, does a pic change the landscape. That change out will be easy. And the circ pumps are std everyday ones with black paint, since it is FWC. I dream of work space like that


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It's a closed cooled system so doesn't need to be SS. GM would probably be half the price and the pump looks like the one on my PU truck with the 8.1.
This is what has me stumped. The merc pumps are ~$300 +, the sierras are ~$130 and I did not price out the auto ones yet.
I wonder what would cause the merc ones to be so much more?

I figure while I’m in here I’ll change the thermostat and need all the special seals on the crossover pipe. It just doesn’t stop adding up….

I’m glad I’m doing all of this myself.
Merc uses the gm pump and marks it up a bit for profit.:eek:. Stay away from Chinese knock offs.
I thought about your note and pulled the pump off today on one side. It has the GM cast number on it and decided to check places that sold GM factory parts. Everyone I looked up shows this parts “discontinued”. Hmmm so is the Sierra part a Chinese knock off?

This is crazy - no one has these (Sierra) in stock and no delivery expected dates…

I even called the Chevy dealers and they state only re-mains available for these for gm vehicles.

I may end up paying twice the amount to get the Mercruiser ones :(
you could just replace the bearing and seal.

Autozone $130 lifetime warrantee

01-07 GMC 2500 8.1
you could just replace the bearing and seal.

Autozone $130 lifetime warrantee

01-07 GMC 2500 8.1
Scoflaw - the fine print excludes marine use on the warranty.

I just pulled the original apart and the impeller is in fact brass or bronze and the back plate is stainless. None of the auto ones offer this option. I did not find a bearing or seal kit for this yet, and I’m not sure how it all presses together anyway (front to back or back to front). Now I don’t disagree that since it’s fresh water cooled sitting in antifreeze probably won’t kill it, but I wonder why did merc engineers this it to be important? Maybe because the environment is salt (in my case)?

I talked to one vendor and he assures me the Sierra one is made in China, but really don’t know if that is a fact.

I ordered all the seals for now and pulled the heat exchanger to see if there were any barnacles - but it was clean as a whistle to my surprise.

Anyone else replace one of these pumps on a 8.1 yet and what brand did you use?

The marine pumps are always bronze / Stainless, as they are intended for both raw water and closed cooling. Yours is closed cooling (antifreeze coolant - not raw water), just like a truck, so the ordinary truck one will work.

Automotive ones say not for marine use because raw water (salt or brackish) would eat them up in short order. Your pump won't see raw water.
I bit the bullet today and ordered two Sierra pumps after finding someone who had two in stock finally.

Hopefully they are made well.

Also repaired the single point air escape barb that broke off the back of that device on the back of the raw water pump. It appears to be only an air escape for when you pump up the drain system and I’m guessing that the hose that “goes nowhere” is there to keep dirt out of the chamber where the piston rides during draining. I tapped and insert a barb fitting to attach the hose. What the heck, it’s winter and I have time ;)
This project did not go as planned but this should not have shocked me.
As mentioned above I purchased everything I was touching pulling the pumps which included all seals, short hoses and thermostats. The only thing I decided to leave alone was the crossover tube. I decided to just replace the tube seal and install the water pump “up and in” to it. We that did not work out. The bolts that hold the seal won’t budge so I ended up removing the crossover pipe and bring it home where I immediately snapped the bolts:mad:.
Looks like the pipe is coming to work with me to get drilled out…

What a pisser!
Well today I was lucky enough to have one of our R&D technicians mount it in a Bridgeport and drill it out. It was so corroded that he had to drill bigger and Heli-coil it. I'm back in business now but waiting on those darn expensive gaskets from the Crossover to the block.
Oh, what it must be like to have direct (free) access to and R&D lab with all it's nifty toys......

Can only fantasize.

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