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Replacing an Exhaust Riser on a Budget

By Edited Mar 30, 2014 0 0

Catalina 36 yacht

The boattail transom series with a universal M-25XP motor

How do you know it’s your riser?  Usually you’ll have one or both of two problems.  You’ll have exhaust coming into your cabin, or seawater pouring in through a crack in the exhaust riser seawater cooling nipple.  The seawater will just be flowing into the bilge and you may not notice it.  The exhaust hopefully will smell of exhaust and you’ll become aware of increasing exhaust smell as the crack worsens.  If you run a motor with a cracked riser in a closed cabin you could kill yourself.  Well, you will kill yourself.  At some point the exhaust leak will become so bad that you won’t be able to stay in the cabin even when the hatches are open.  Ideally you’ll fix this before it gets to this point.

Get prepared before you start; you’ll be tearing up the cabin of your boat and this will take at least a week.  Close the through-hole for seawater coolant.  If you forget to do this, your cold seawater could continue to run through your raw water impeller pump and flood your cabin.  Have towels and other cleaning cloths handy.  Our goal was to do this as cheaply as possible, so we used old t shirts as rags.

Remove the ladder and engine cowling to expose the motor. To get to the rest of the motor you need to remove the aft cabin shelf and firewall.  To remove the firewall, you need to expose the screws.  They’re covered with plugs so you’ll need to find them.  Depending on the model there will be five or six.

Drain the fresh water coolant from the system by pulling the plug and draining it into a bucket.  Dispose of this antifreeze properly – IT DOES NOT GO INTO THE WATER.

Using hand tools (a half inch wrench and a six inch flathead screwdriver) remove the exhaust flange and riser assembly from the motor.  Use the screwdriver to remove the clamps that hold the riser onto the flex hose that’s connected to the top of the exhaust box.  There are four clamps holding these two pieces together.  Remove the three flange bolts using the wrench. The riser will come off as one piece with the hose still attached to the riser.  Remove the hose and inspect it.  it’s a good idea to replace this now, but you might get a couple more years out of the old one if you’re really strapped.  Now you can take the piece on deck, or to any more convenient work space.

25XP universal flange
Although Catalina will advise you differently, the exhaust flange can be reused if it’s in good shape.  You may need a torch and a vise to separate the flange from the riser.  You can order a new riser from Catalina, but they will tell you that it needs to be custom made, you’ll need to send them your old riser, and you’ll need to pay the shipping for sending these big chunks of metal back and forth.  Plan on shelling out about $500 and waiting several weeks to months for your riser to be made. We, instead, did something a little different.

Riser for 25xp
You need to match the shape of the riser so it fits back into the space allotted. We did this by taking the riser to our local home improvement store and spending a couple hours sitting on the floor with the plumbing department specialist.  We were able to recreate the riser using galvanized pipe and fittings.  The pipe can be cut to specific lengths to get the right shape.  The riser fittings will be sealed during assembly using non-hardening form-a-gasket.  To create the raw water coolant nipple we found a local shop guy with a tig welder.  This was a hobby welder and was quite a bit less expensive than a welding shop.  He also removed the manifold from the old broken riser for just a couple bucks more.  Total cost for riser and welding: less than $100, and it took two days.

Building exhaust riser from pipes

You’ll need a new gasket that goes between the manifold and the flange.  You can make this yourself by cutting it out of gasket material using the exhaust flange as a template or you can buy one from Catalina for $4.

Now you put it all back together in reverse of the way you took it apart.  Use the non-hardening sealant on both sides of the gasket between the engine manifold and the flange.  This prevents coolant and exhaust from leaking at this connection.  Assemble the new riser (which is about three 90 degree elbows and four pipes, including the one with the nipple you had welded on).  Do this as you go and seal each joint with the form-a-gasket.  Use a pipe wrench to get the pieces threaded together tightly AND make the angles line up correctly.  The liquid gasket gives you a little flexibility in terms of how tight the joints need to be so you get the angles as correct as possible.  Make sure you have proper clearance between the bottom of the shelving unit and the top of the riser.  If you sized your riser correctly this shouldn’t be a problem.

After you have it all assembled, open the through-hole valve that you closed at the beginning of the operation and fire up the engine.   Don’t forget to replace the coolant before you do this.  You want to heat the pipes to burn off any oil and inspect carefully for leaks.  Be aware that it will smell, the galvanized pipes will stink at first.  Look for water leaks or anything that seems unusual.  You can put a bit of dish soap around all your new joints and check for leaks by looking for bubbles.  Your riser will be heating up, so do the soap check as soon as you start the engine.  Run the engine for a couple hours, and then check the flange bolts for tightness.  When everything is tight and not leaking, and all the oil and other crud has burned off the new riser (it stops smelling so bad), turn everything off and let the engine cool. 

Now you need a thermal jacket for your riser.  You can get it from Catalina for $200, or you can get a roll of exhaust tape for about $20.  Make sure you get the high temp exhaust tape and create your own thermal jacket by wrapping it round your riser after it’s installed.

Wrap the exhaust tape carefully around the new riser.  We used about 15 feet of tape to wrap the riser starting at the flange and wrapping to the nipple and back to the flange.  Secure the tape to the riser using safety wire as the tape is not self sticking.  Run the engine again to break in the new tape, yes, it will smell too.  Turn on the fans. 

That’s engine repair part of the fun.  Clean the grease off your hands and put your cabin back together the same way you took it apart.  Be prepared for some odors for a while, but they do go away.  You might also want to consider a carbon monoxide monitor for the future.  If you ever smell exhaust in your cabin, it’s a bad thing.  Check your riser.



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