Project: Exhaust System

Exhaust Riser Replacement

My wife and I are owners of "Flying Colors", Catalina 34, Hull #80.  A "tech note" in the May 1997 issues of Mainsheet reminded me that  periodic inspection of the exhaust riser was a good idea.  To my surprise, cursory inspection under the vanity in the head revealed soot buildup on under the sink and on the interior side of the bulkhead to the aft stateroom -- not a good sign.  It was a surprise because neither my wife nor I noticed or smelled diesel exhaust in the head or the main salon. 

With Mainsheet article in hand, I called Catalina and discussed my options for replacement of the exhaust riser.  Because hull #80 featured a black-iron riser as original equipment, it was recommended that I replace the riser with the black-iron version ($115).  This recommendation was largely driven by the fact that the stainless steel riser used on more recent Catalina 34s results in the riser being centrally located between the interior bulkhead of the aft stateroom and the aft bulkhead of the engine compartment.  (As I would later learn for "Flying Colors", the output of the upgraded riser was about 3" forward of the muffler input).

Ordering the riser was very straightforward, and was accomplished by telefaxing critical information including the desired riser material, hull number, and model year to Catalina.  Catalina advised me that the flange and gasket for mounting the riser on my M-25 engine had to be purchased directly from the local Universal distributor. 

To my delight, both the riser and the flange arrived within 7 days.  However, as I would soon discover, the easy part of this job was over. Examination of the existing flange revealed that it was secured onto the exhaust manifold by three 9/16 nuts.    While the top two nuts were easily accessible, access to the bottom nut was less than optimal and, of course, the bottom nut was the most seriously corroded and required the most attention.  To improve access to the lower nut, I unfastened the heat exchanger mounting bracket (with heat exchanger attached), and lowered the entire assembly approximately 3 inches away from the riser.  I found that access to the lower nut was then best accomplished through the large access panel under the aft berth.   However, after several applications of liquid wrench, various wrenches, several sockets, and assorted vice-grips, I concluded that the lower mounting nut had become an integral part of the boat’s structure, and wasn’t going anywhere without greater leverage and persuasion. 

Discussing the situation with a few very reputable marine diesel mechanics, I was consistently advised that my encounter with a frozen lower nut was very common.  The consensus opinion was that I should remove the entire manifold with the riser still attached.  Once the assembly was clear of the boat, I was able to remove the mangled nut by hammering a smaller socket onto the nut and applying torque by using a 2-foot socket handle.  Persuasion worked!  However, I soon learned that the entire flange had seized to the manifold due to an extensive rust buildup.  A local engine shop was able to apply some heat with a blowtorch and remove the flange from the manifold by using a small sledge hammer.   At this point, I employed the engine shop to clean the mating surfaces of the manifold.  I also took the shop’s recommendation to remove the rusted mounting stud, and use a stainless bolt for attaching the new flange (the other two studs were in good shape).  I had the shop screw the new manifold into the new flange - a good decision since it took two men, a vise, and two pipe wrenches to apply enough torque to screw the riser sufficiently into the flange.

Upon returning to the boat, I remounted the flange/riser assembly and utilized new manifold gaskets which my engine shop had been nice enough to cut.  At the recommendation of the mechanic, I applied "Anti-Seize" to the nuts and bolt in order to help preclude the future seizure of mounting nuts and bolt.  Upon remounting the manifold/riser assembly on the engine, I discovered that the new riser was misaligned with the inlet port of the muffler by about 3 inches.  My choices were to remove the stainless riser and order a black-iron riser, or replace the muffler; I decided to replace the muffler. The boat’s muffler was the original box-type waterlift which measured approximately 13"x13"x10".  The muffler was screwed and epoxied in-place on a piece of plywood which was form-fit to its base.  After removing the mounting screws and breaking the muffler free from its foundation, I found that the height of the inlet and outlet ports severely restricted the muffler’s egress. 

Consequently, I cut-off the inlet and outlet tubes, and was able to remove the muffler through the larger access port under the aft berth. The new muffler which I selected was manufactured by Veralift, and, similar to the old muffler, featured inlet and outlet ports on top of the unit (not a commonly used configuration anymore – it required a special order).  However, unlike the old muffler, the Veralift was cylindrical in shape, with a diameter of approximately  8", and a height of 9".  Consequently, it was convenient to work with, and I would later be able to install the muffler through the access door under the vanity in the head (i.e., I would work right-side-up!  What a novelty!) As a foundation for the new muffler, I selected 3/8" marine plywood and cut it to a dimension of 17" x 13".  I then aligned the plywood such that the 17" length ran fore/aft and overhung the forward end of the old foundation by 3".  I then screwed the plywood onto the old foundation; the 3" overhang would allow for mounting the new muffler in alignment with the new riser. With the new foundation in place, I maneuvered the new muffler through the head-vanity access door and reconnected all hoses. 

As I started the engine and checked all connections, I realized that my five-day adventure was almost over.  The only other action would be to retorque the flange bolts after a few hours of engine use. 

Important: The opinions expressed here are those of the individual contributors to this page, and not those of the Catalina 34 National Association or Catalina Yachts, Inc. Additionally, this material has not been reviewed by Catalina Yachts, Inc. for technical accuracy. This page's maintainer cannot guarantee the accuracy of this information or the desirability of suggested modifications or upgrades. Please obtain assistance from a competent marine mechanic or boatyard prior to making any significant modifications to your vessel.