How to Tighten Belt on a MerCruiser

by Chris Stevenson
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Drive belts on the MerCruiser engine run all of the components at the front of the engine, including the power steering pump, alternator, seawater pump pulley and circulating pump. The belt must be in pristine condition so that it does not slip or chatter. Salt, oil and general weathering can destroy the integrity of the belt, leading to breakage and glazing. After new belts have been installed, a critical adjustment must be made to the belt tension, so it will drive the pulleys without stretching, snapping, slipping or putting undue stress on the component bearings.

Step 1

Unclasp the engine cover cowl snaps and tilt it back in the full reclining position. If you have bottom flange bolts holding the engine cover to the deck, use the appropriate socket to remove them, pull the engine cover off and place it to the side. Turn the ignition key to the "Off" position and remove it. Turn off the main fuel supply line that goes from the gas tank to the engine. Disconnect the negative battery cable with a socket.

Step 2

Remove the bolts or screws to the top engine plenum with a socket or screwdriver. In most cases this is a decorative plastic cover that protects the top of the engine. Put the plenum cover aside. Refer to your owner's manual for the location of the idler pulley. Some MerCruisers have dual idler pulleys. You will need the one with the 5/8-inch locking nut and the 5/16 adjusting stud. It should be the top idler pulley with a dual arrangement.

Step 3

Wipe the entire inner surface of the serpentine belt with a clean rag and examine it for cracks and frayed fiber. You may wish to replace if it looks badly worn or glazed. Loosen the 5/8-inch locking nut on the pulley adjustment stud with an end wrench.

Step 4

Unscrew the 5/16 adjusting stud with and end wrench to loosen the belt. If you need to replace the belt, make a note of belt orientation around the other pulleys, or consult your repair manual for their positions. Place the new belt around the pulleys and slip it over them.

Step 5

Turn the 5/16 adjusting stud until the belt begins to tighten. Measure the deflection of the belt at its longest travel length between pulleys. Push your thumb lightly against the belt and measure the movement with a ruler. You should have no more than 1/4 inch, or 6 mm deflection.

Step 6

Take a more accurate reading with a belt tensioner gauge. Place the gauge hook in the middle of the long part of the belt and pull back on the tool. The gauge needle will measure the pull movement on a scale. Again, no more than 1/4 inch or 6 mm can be allowed. Hold the 5/16 adjusting stud with an end wrench while you tighten the 5/8-inch locking nut with another end wrench.

Step 7

Replace the plenum cover over the engine and refasten the bolts or screws with a socket or screwdriver. Set the main engine cover box over the engine and refasten the clasps, or secure the deck bolts with a socket and wrench. Reconnect the negative battery cable with a socket. Replace the ignition key and turn on the main fuel supply valve.

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