A new exhaust riser for the boat

A new riser for the Universal M25 on a 1984 Catalina 36 MKI

You know you need a new exhaust riser and mixing elbow, but you don't want to wait 6 weeks or more for Catalina Direct to have it built - and that's AFTER you send them your old riser. That's a long delay, especially when you're spending around $400 for the part and you want it NOW!

What can you do? Build it yourself, that's what! Building it isn't too difficult, but putting the pieces together can be a little tricky. It's really helpful if you know what parts you need. The most time consuming part of the project are all the activities leading up to building the exhaust riser.

How to build a new exhaust riser

Part 1: Get access to the exhaust system and drain the coolant

  1. Take off the teak front panel and back shelf - how to do this is on the checking the exhaust riser page.
  2. Drain the coolant from the manifold - how to do this is on the drain the coolant page.
Heat exchanger in the way of removing the exhaust flange
The heat exchanger is in the way of removing the exhaust flange

Part 2: Remove the old riser

My riser was already broken in two pieces which made removing it easy.

  1. Disconnect the 5/8" hose going to the riser injection nipple.
  2. If you have the 3" inch heat exchanger, it is probably in the way of removing the exhaust flange. How to remove it is on the removing and reinstalling the heat exchanger page.
  3. Remove the 3 nuts from the bolts holding the exhaust flange on
    • 9/16" socket wrench
    • If the nuts don't come off, spray some sort of penetrant to loosen seized parts (I use a product called Corrosion Block).
  4. Disconnect the riser from the aqua lift muffler
    • Unscrew the clamps holding that short length of stiff black exhaust hose on the aqua lift muffler.
  5. After the nuts are off and riser connection to muffler freed, a gentle whack on the riser above the exhaust flange was all that was necessary to release the flange from the manifold
  6. Take it off and home to your real workbench.

Part 3: Remove the flange from the riser

This may take awhile. A vise would help. I got the riser off the flange with liberal application of Corrosion Block penetrant and a hammer. Perhaps not the recommended method. If I screwed it up or couldn't get it off, I'd have to buy another exhaust flange.

mating surfaces have to be perfectly clean and smooth
The mating surfaces at the manifold and the exhaust flange have to be perfectly clean and smooth

Part 4: Remove the old gasket material from flange and manifold

Make sure the exhaust flange and the back of the manifold where it's going back on are PERFECTLY clean and smooth. Careful use of a razor blade (or chisel) to remove most of old material, a green scotch brite pad to remove any remnants. Acetone to clean up the surface. When you put it all back together this can't leak. Be careful not to scratch or nick the mating surfaces.

exhaust riser before wrap
Exhaust riser pipes. This photo is after unwrapping the pipe after 2 hours of motoring

Part 5: Put the new Exhaust Riser together

What you'll be doing - screwing the pipes together to get the orientation correct to make the connection to the flange and the muffler, all the while making sure the riser assembly will not touch the back shelf when it's back on.

Assembly Tips
  • Have a buddy who's done this before assemble the riser while you hold the shop light and offer encouragement.
  • Do a trial run connecting the pipes before using the sealant.
    • My original riser had a 4" nipple between the Street Elbow and the first 90 degree elbow, so that was the size I bought. After a trial assembly, it was obvious a 5" nipple was going to work much better. Another run to the hardware store.
  • Street elbow to flange (flange off the engine): It has to be oriented correctly, therefore "tight" will not necessarily be the correct vertical position. With the flange off of the manifold, do a test screw in to see how far you need to screw in the Street Elbow, then unscrew, apply sealant, and screw it back in the flange.
  • Use 2 pipe wrenches to connect and tighten the pipes.
  • Permatex on both sides of exhaust flange gasket, Permatex on threads of pipes, Permatex all over your fingers.
  • Exhaust flange nuts: Tighten the 3 nuts uniformly in several passes.
  • Double clamp hump hose connections (2 clamps on each end).
exhaust riser fiberglass wrap after 2 hours of motoring
One layer of thermoglass wrap after 2 hours of motoring

Part 6: Wrap the new riser with Thermoglass tape

On Reality Check (Catalina 36 MKI), the original riser had a wire cage around it which was wrapped in insulation, safety wire laced around that, and then wrapped in fiberglass tape. The insulating tape around the Onan water jacket actually rested against the back berth shelf, and this heat protection installation was problem free.

Riser insulation

Apparently the new way to wrap exhaust risers is just with fiberglass tape. I didn't want to touch the riser with only one wrap of insulating tape - with a second wrap of Thermoglass tape I could touch the hot riser coming out of the exhaust flange. I still had 20' of fiberglass tape from the original riser and wrapped this third layer for extra heat insulation.

YaHOO! The riser build is DONE! Now it's time to reinstall the heat exchanger, fill the manifold with coolant, check for leaks and ... oh heck, there's more work before this project is wrapped up.

Part 7: Reinstall the heat exchanger and add coolant

How to reinstall the heat exchanger, add the coolant, and bleed the air from the system is on the removing and reinstalling the heat exchanger page.

Part 8: Check out your new exhaust riser

You've started the engine and are now checking for leaks around the exhaust flange and hump hose. If the exhaust flange leaks, you have to START OVER. If there's some water leaking from the hump hose connection to the water lift muffler tighten the hose clamps.

Part 9: Take the boat out and burn off the fumes and smoke

Fumes and SMOKE? After one wrap of the fiberglass tape on the brand new riser, we took the boat out for a couple of hours to burn off the fumes from the galvanized pipe. I expected fumes, but SMOKE? For one and a half hours smokey fumes were coming off the riser. Found out later that the smoke is from the fiberglass insulating wrap and smoking wrap is normal for a little while.

One wrap of fiberglass tape wasn't enough heat protection. Another day and another layer of new tape (plus a third layer of the original old tape) and we took the boat motoring again for a couple of hours. This time I expected the smoke and sure enough the new tape was smoking for another 1 1/2 hours. No fumes from the galvanized pipe though. The smoke doesn't show up in a photo, but it filled the cabin. Need a lot of fresh air blowing through here until it stops.

hose bracket to secure hose underneath shelf
Hose brackets to secure hose underneath back berth shelf
Secure hoses to insie of back berth shelf
Secure hoses to inside of back berth shelf. Old hose for chafe guard.

Part 10: Finally - put the shelf back in and attach the hoses

Secure the hoses to the inside of the back berth shelf. I moved the hose from the heat exchanger to the front to keep it from touching the exhaust riser.

Make sure you have the back berth access panel in place before you finalize the hose positions.

All photos of this project

Build a new exhaust riser

hump hose
Hump Hose

Parts & Supplies

  • Exhaust Hump Hose 6", Product #Z2152 - Dimensions: 6" long X 1 5/8" ID (2" OD) - $31.49, CatalinaDirect.com
  • Rectangular Exhaust flange gasket, Product #Z2593 - $3.95, CatalinaDirect.com, or $3.20 A to Z Marine (San Diego)
  • Maybe: Exhaust flange - you may be able to re-use your original flange, I did. If not, a new one is around $83.00, Z2605, CatalinaDirect.com, or A to Z Marine (San Diego)
water injection nipple
Water injection nipple
  • Universal Exhaust Water Injection Nipple Part #299693 -1 1/4" black iron (i.e. steel) pipe - $64.00, A to Z Marine Services
    • 6 inches x 1 1/4" NPT
    • smooth end - 1 5/8"
    • injection nipple pipe - 5/8"
  • 1 1/4 inch Black Steel Pipe or Galvanized Steel Pipe - about $25, local Ace Hardware
    • 90 degree street elbow
    • 5" nipple
    • 10" nipple
    • (2) 90 degree elbows
  • New hose clamps $12.00, Ace Hardware
  • Permatex form-a-gasket non-hardening pliable sealant Item #80016
  • Thermoglass tape - 1/8 x 2 inch - 45 feet@.80/foot - $36.00, San Diego Marine Exchange
    1. 25 feet for the 1st wrap
    2. 20 feet for the 2nd wrap
  • Stainless Steel Monel seizing wire - $7.09, San Diego Marine Exchange
  • 1 gallon 50/50 Antifreeze/water mix (Prestone green) to refill coolant system - $12.00, O'Reilly Auto Parts


  • 9/16 socket wrench and 9/16 box end wrench
  • Screwdrivers
  • Hammer
  • Vise
  • 2 pipe wrenches

What should you know about the parts?

  • Silicon Hump Hose
    • This hose is flexible, taking up the vibration stress from the engine
  • Hump Hose clamps (CatalinaDirect.com)
    • Should you get the pair of special hose clamps for $9.98? The kind recommended for the hump hose are smooth so the clamps don't grab the silicon hose while you're tightening them. This is supposed to make the clamps fit tighter. I didn't buy these, double clamping and tightening the regular clamps on the hump hose worked fine.
  • Exhaust flange gasket (CatalinaDirect.com)
    • Should you get 2 gaskets? in case you spoil one? Heck yes, if you're buying from CatalinaDirect.com get all your parts in one shipment and save some $.
  • Black Steel Pipe or Galvanized Steel Pipe?
    • A lot of people build the riser out of galvanized steel pipe, and so did I. BUT - fumes off of galvanized pipe are toxic, AND after hours(?) of motoring...
      • ...eventually the galvanized coating on the galvanized steel pipe burns off, leaving you with what? Plain old black pipe.
      • However, on the plus side - galvanized pipe is more readily available than black pipe at the local Ace Hardware stores. Good thing, because I had to run out and get a 5" nipple and would have been out of luck with that if making it out of black pipe. Now that I know what pipe pieces are needed, I'd build it out of Black Pipe next time.
  • Thermoglass or fiberglass insulating pipe wrap
    • Fumes AND smoke? The fiberglass insulating wrap will smoke for the first hour and a half on the engine, so now you have this in addition to the smell of the fumes from the galvanized pipe. Imagine seeing smoky fumes coming directly off of your new exhaust riser as it heats up. A little disconcerting.
    • Wear protective clothing when installing fiberglass wrap - long sleeves and gloves. A dust mask or charcoal mask is also recommended. The fiberglass tape can release tiny fibers which are really irritating.
  • Universal water injection nipple
    • The material is black steel. Black Steel pipe and Galvanized Steel pipe are both iron pipe and OK to connect one to the other.
  • Permatex Form-A-Gasket #2
    • This sealant is slow drying and non-hardening.

Build, or buy?

Is it worth it to build your own exhaust riser? The parts used to create the riser (including the hump hose) cost about $170.00. A new riser from Catalina Direct is $358.00 (as of December 2011), plus you still need the hump hose and exhaust flange gasket - so, about $400 for parts. Plus whatever shipping charges to send them your old riser and ship the new one.

Related topic: Checking the exhaust riser


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