Yamaha 40XMH Assembly Instruction Manual

MODEL BC FOR 40 HP YAMAHA SERIES 

ASSEMBLY INSTRUCTIONS 

YAMAHA MODEL 40XMH, 2 CYLINDER, 2 STROKE, 42.9 CU. IN. 

1

1.  Place the engine on the transom of your boat in the normal fashion.  Disconnect the shift rod coupling.  Remove 

the 5 bolts holding the gearbox to the exhaust housing and remove the gearbox assembly. 

2.  Remove the water pump assembly from the propeller drive, including the lower stainless steel plate and the 

dowel pins. 

3. Install the jet driveshaft assembly into the spiral pump housing locking it in place with the four ¼-20 x ¾ bolts 

and lockwashers.  Use grease on the threads.  Tighten to 8 ft-lbs. 

4.  Insert the clear plastic ½ long x 3/8 dia. Sleeve in the 3/8 hole in the main housing, just ahead of the cooling 

water pump. 

Install the water pump assembly on top of the stainless steel plate, using 4—M8 x 30MM bolts and the special 

steel washers under the bolt heads.  Be sure also, to install the water pump impeller drive key and dowel pins.  

Grease the threads.  Before tightening the bolts, press the 2 ¾ long x ½ dia.  Plastic shift rod guide into the clear 

plastic sleeve, through the stainless steel plate, until it bottoms.  Tighten the bolts.  

Remove the nut and hex coupling from the gearbox and motor mid section shift rods.  Thread the nut and 

coupling all the way onto the shift rod guide #1911, and tighten the nut against the coupling.  Thread the 

coupling onto the motor shift rod and tighten it against the shift rod.   

5.  Remove the plastic water tube guide and rubber sleeve from the outlet of the cooling pump.  Install the 5/8 dia. 

X 3/8 long plastic spacer in the outlet, followed by the rubber sleeve and the brass water tube extension.  If 

there is a burr on the end of the motor cooling tube, file it smooth so the “O” ring inside the water tube 

extension will slide on.   

6.  The large ¾” adapter plate is attached to the exhaust housing to hold the jet drive.  Two 6 x 16mMM dowels 

locate the plate, four M10 x 35MM bolts with lockwashers and one M8 x 30 MM bolt with lock washer secure 

it.  Grease the bolt threads. 

7.  Next, attach the jet drive to the motor. Use two 5/16-18 x 2-1/2 bolts (front), tow 5/16-18 xxx 2-3/4 bolts (rear) 

from below with lockwashers, one 3/8-16 x 1-1/2 bolt and lockwasher, above rear, inside the motor mid-section 

and two 3/16 x ½ dowel pins. 

Grease the bolt threads, driveshaft spline generously, and rubber water tube pilot and guide the jet into place. 

Tighten the 5 bolts. 

8.  Next, install the impeller. Grease the shaft threads, key and impeller bore. Place the plastic sleeve inside the 

impeller, hold the key in the nose of the impeller with your forefinger and slide onto the driveshaft. Install the 

eight shim washers and nut retainer on the shaft, up against the impeller, and bring he nut up snug by hand.  

Place the water intake in position and secure with 2 bolts. Observe the clearance between the impeller blade 

edge and the intake liner. Then remove the intake. 

When, after use in sand and gravel, the blade clearance becomes more than about 1/32 inch between the 

impeller edge and the water intake liner, one or more of the stainless shim washers can be transferred from the 

bottom stack to the top of the impeller, which moves the impeller down into the tapered casing to reduce the 

clearance.  

Shims should not be used above the impeller on new installations, where no wear has occurred, unless the blade 

clearance exceeds 1/32 inch. Insufficient blade clearance will do more harm than good from any performance 

gains it might provide.