1. Prepare yourself! Changing manifolds can be a little dirty and may
take an hour or so to do.
2. Ensure you will have the correct tools to do the job. Have you got the
correct spanner size or wrench size sockets? Do you have a scraper to remove
old gasket material from around the mouths of the cylinder head exhausts?
Old cast iron manifolds and risers can be very heavy- do you need somebody
else to help you lift them out? Changing manifolds is also an excellent time
to replace spark plugs and check engine compression.
3. After you have removed the old manifolds examine the extractor
entrances. If they are nice and clean then there is probably no problem, but
if you see signs of water, rust, corrosion or anything else that seems out
of place, it could be a sign to inspect further. Look inside each exhaust
outlet on the cylinder heads- do you see any corrosion or rust? Do they all
have the same appearance?
4. Remove all the spark plugs. The spark ends should be characterised by
a light brown chalky colour. If they are wet or appear rusted, then water,
either salt or fresh has entered the cylinder at some stage. If you have a
compression gauge, remove and ground the HT lead from the coil, have
somebody turn over the engine at least four revolutions (it is a four stroke
engine after all!) and write down the compression. Look in your engine
owners manual for the correct compression rate. Do the readings for each
cylinder approximate this figure? As long as all the cylinder
pressures are within say 20psi or each other, there is nothing major wrong.
A very low compression may indicate worn cylinder rings or improperly
seating valves. Unfortunately this generally calls for major engine surgery.
5. If you have not already realised, your manifolds might come in two or
even three sections - the manifold section and the riser section. This is
because many boats, such as ski boats, have a straight through wet exhaust
that exits via the transom or from an open engine configuration and as such
do not need a riser.
Risers are used to raise the level of the exhaust water above the water
line of the boat so as to ensure no sea water flows back into the engine via
the manifolds. In effect the exhaust is channeled along the manifold and up
into the riser and then downwards where it is mixed with the raw water and
through to the exhaust.
If your boat uses risers, you will now need to bolt the components
Place the exhaust gasket between the manifold and riser and bolt securely
together. It is possible to use a sealing compound, however check that it
will not deteriorate the gasket. Use an anti seize compound when
tightening all the bolts.
You are now ready to attach the manifold to the engine. Coat the manifold
attaching bolts with a non-hardening substance such as gasket maker. Gasket
makers are useful if some bolts screw into the internal water compartments
such as on a Chrysler 318 engine. Hence they will not rust, seize or leak.
Loctite produce a range of suitable anti-seize products for this purpose.
After you have tightened all the bolts, ensure the exhaust is secured. It
is now time to connect the plumbing. Depending upon your engine, you should
be able to use your existing hoses.
6. Double check everything! Did you loose any cooling water when you
removed the bolts- if so replace the water/cooling fluid. Check the oil. (If
it shows milky streaks then there is water in the oil.) Ensure bellows and
hose pipes are secure. Start the engine and listen for the water pump to
fill the manifolds, risers and flow out the exhaust. The engine may sound
different your new manifolds. Run the engine at fast idle until the engine
is warm- look for leaks or anything else that seems out of place. Turn off
the engine and retighten all the bolts and hose fitting while the engine is
7. You are ready to go! Enjoy worry free boating!