removing and reinstalling the 3 inch heat exchanger on the universal M25

December, 2011
Update Sept, 2012

Found that the Seakamp heat exchanger on Reality Check is Seakamp 302631 CN, aftermarket replacement for Westerbeke 302631, not the #299835 which is the upgrade part I thought I had. Seakamp makes all their heat exchangers to OEM specs, but without a welded-on bracket for #302631. They supply nifty Stainless T-Bolt clamps with this part number. What's the difference between 302631 and 299835? The 302631 has 45 degree angles and the salt water pump connection is also angled. The 299835 has 90 degree angles and the salt water pump connection is straight. Thank you to fellow Catalina 36 owner D.Ellens and Chad at Seakamp for pointing out the part differences!

I didn't do the original upgrade of the 2 inch heat exchanger, but it turns out it is really simple. This year I had to take the 3 inch heat exchanger off to remove the exhaust flange and replace the riser, so... might as well check it out and paint it. Along the way, replaced the 7/8" hose from the fresh water pump that was worn where rubbing in a spot you're not normally going to look.

Parts & Supplies

Seakamp replacement for Universl 299835
Seakamp replacment for Universal 299835
photo thanks to D. Ellens
  • Seakamp Cupro Nickel Heat Exchanger (CUNI or Copper Nickel alloy). Heat exchanger size: 3 x 13.5 inch - Copper Nickel alloy is an upgraded equivalent to Universal Part #299835 - between $357.00 - $400.00. Without straps $323.51 from Go2marine.
  • Pencil zinc with plug - 3" heat exchanger takes 1/2 x 1 1/2 with 3/8" NPT thread size (Torrenson P/N 301068) - West Marine has CME1, 1/2 x 2", 3/8" NPT. In the Seakamp CUNI I have, the 1/2" by 2" fits just fine - CME1 (Canada Metals) - $10.49, West Marine.
  • Thick rectangle sheet of gasket material for chafing under the heat exchanger. Home Depot used to sell this, or auto parts stores. Some people use cut up bicycle tire tubing.
  • Might as well replace some hoses...
    • 5/8" reinforced exhaust/water hose from salt water pump to heat exchanger.
    • 7/8" reinforced exhaust/water hose from heat exchanger to fresh water pump - 3 feet of 7/8" Exhaust hose #250 West Marine- $19.47, used 2 1/2 feet.
    • New stainless steel hose clamps.
  • If you want nifty stainless steel T-Bolt Band clamps like mine, the back of mine say Breeze 3 - You can find similar T-Bolt band clamps other places on the web.
  • Shop rags and paper towels - there's going to be water coming out of hoses.
  • Plastic cup to catch sea water that drains out of heat exchanger.
  • Plastic cups or bottles to catch what remains in hoses after draining.
  • 50/50 Antifreeze/water mix to refill coolant system (Prestone - green).

How to remove the heat exchanger

7/8 inch hose from fresh water pump to 3 inch heat exchanger
3 inch heat exchanger removal
  1. Shut off salt water inlet thru-hull valve.
  2. Drain the coolant from the manifold - how to do this is on the drain the coolant page.
  3. Unscrew the zinc plug from the bottom of the heat exchanger and drain into a container or big plastic cup or two.
    • Try not to spill salt water all over the bell housing, transmission and and engine mounts
    • but of course it will go everywhere, so have those shop towels on hand to mop it up
  4. Disconnect the hoses. Stick disconnected ends in cup or empty bottle to catch what's in the hoses. Have those towels on hand!
  5. Unscrew the 2 super T-Bolt clamps holding the heat exchanger on.

Inspect the heat exchanger

Thought I'd replace the end cap gasket and o-ring but the external part of the end caps looked OK. Next time. Ran fresh water through the heat exchanger and some zinc pieces came out. Ran some more until nothing. Taped up the connections and painted it. Put in a new pencil zinc.

Inspect and replace hoses

  • The 7/8" hose from the heat exchanger to the fresh water pump rubbed against the corner of the bell housing (behind the wiring harness). WOW, almost worn through. Dodged that one.
engine bracket for heat exchanger
engine bracket for heat exchanger, top left

How to reinstall the heat exchanger

  1. Get ready with your 3 arms. Hey! wasn't this a simple project?
  2. Position the gasket material on the bottom of the heat exchanger and rest it, holding all the while, on engine heat exchanger bracket.
  3. Get those super clamps around the heat exchanger and engine bracket. Tighten up a bit, but don't tighten all the way yet.
  4. Connect the hoses.
  5. Check the position of the heat exchanger.
    • Is the 7/8" hose from the heat exchanger to fresh water pump rubbing against the housing? If so, manuever the heat exchanger so it doesn't.
  6. Check the position of the gasket material and just try to adjust it.
  7. Tighten down those super clamps. This will take awhile.
repainted 3 inch heat exchanger
3 inch heat exchanger reinstalled on Universal M25

Fill the manifold and bleed the air from the system

  1. Check your dock lines, you'll be running the engine in gear to put a load on.
  2. Open the sea water inlet valve and check for leaks.
  3. Did you close the manifold drain petcock?
  4. Slowly put 50/50 antifreeze/water in the manifold reservoir to about an inch+ from the top. That's right, don't fill it all the way, not yet.
    • Leave the coolant cap OFF of the manifold. Excess air will escape through here and you'll be adding more antifreeze mix.
  5. Check for leaks.
  6. No leaks? Make sure the transmission is in neutral and the throttle in idle. Start the engine and run in idle for 2 minutes.
  7. With engine running, you'll notice the antifreeze has gone done some. Add some more antifreeze.
  8. Put in gear to put a load on and bring the RPM's up enough to warm up the engine and get the thermostat to kick in.
    • Watch the temperature! - when it gets past 165 degrees you should see it drop back down when the thermostat opens.
    • If it gets past 190, shut it down and go get your boat neighbor to help.
      • In this case I'd just repainted the engine and also the Temperature Sender wire connection on the thermostat housing that goes to the temperature gauge. Cleaning that connection solved the problem - the thermostat was working correctly and it wasn't really overheating.
      • If your thermostat doesn't open, you probably have air in the cooling system (air lock). More work to do.
  9. Shut engine off, fill coolant to below the coolant cap.
  10. Put coolant cap back on.
  11. Start it right up again, run it some more, turn off. Done.

Photos of this project

Heat Exchanger photos
Putting the Heat Exchanger back on the bracket photos

How do other people bleed the air from their engines?

Here's a link to a variety of ways other people use to bleed air from their engines - FAQ page on Engine Air Lock, Engine Heating Problem

How do you REALLY clean out the heat exchanger?

Here's a link to Maincruising's "Autopsy - A Westerbeke / Universal Marine Heat Exchanger" - at

From page 39 in the Universal M25 Owners Manual


  1. Coolant (fresh water with antifreeze) is introduced into the system from the coolant section of exhaust manifold by removing pressure cap and filling manifold tank to within one inch of lower rim of fill port.
  2. Fresh water engine driven circulating pump moves coolant from heat exchanger through engine block circulating pump and thermostat back through exhaust manifold into heat exchanger to be cooled.

    The manifold serves as the engine expansion tank to allow for expansion of coolant as it heated. You also fill the system at this point. In some cases when hot water heaters are installed an expansion or surge tank may be required to prevent air from entering the system.

    Should your engine overheat, first check coolant level to make sure it is not low which would cause air locks to develop. If air does enter the system, it may require a couple of fillings to remove the air pocket. A momentary increase of engine R.P.M. to approximately 2800 to 3000 R.P.M. may aid in removing the air from the system. In some cases it may require doing this 2 or 3 times. Do this just as you notice the temperature starting to rise above normal.

Related topics:
Heat exchanger hose connection diagram
Heat exchanger and overheating - upgrade to the 3 inch and you will not be sorry
Checking and changing heat exchanger zinc


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