Inboard Engine Alignment.

Since most of us are working on a boat that has had an engine and drive train installed before; this section won't cover an engine, shaft hole, strut, and prop shaft installation from scratch.

Before starting engine re-installation; be sure to test the condition of the wood stringers and the wood around the holes from the old motor mounts. If the wood looks soft or you can dig a pocket knife more than 1/4 into the wood, at the lag bolt holes; you should back-fill the old holes with epoxy and re-drill new holes.

The prop shaft and flange should be installed. Use small wedges to center the prop shaft in the shaft hole where it comes through the hull. The packing gland and the rubber boot cannot support, and keep centered, the weight of the prop shaft.

The engine is then lifted into the boat and temporarily blocked in place with the propeller coupling in as close alignment to the coupling half on the propeller shaft as possible. Center the threaded motor mounts, so you can do the adjustments and still have plenty of adjustment left over for corrections in the future. 

With the engine securely blocked in place, fasten them temporarily with "C" clamps. Slide any pre-cut permanent engine shims in place, between the motor mounts and the stringers and align any predrilled holes from the old installation, if possible. A single Oak, or equivalent hard wood, permanent shim can be fabricated for each mount, if necessary. Bore holes for the mounts the length of the engine bed, through the stringers and shims. One-half inch carriage bolts are then inserted in the holes and securely tightened. Use washers under all nuts. In some cases, it will be possible to drill these holes without disturbing the engine; in others, the engine must be removed. Drill these holes using a drill 1/8 inch smaller than the lag screws to be used for fastening the engine in place. Three-eighths-inch lag screws which will project three inches into the engine beds will be sufficient, Insert the engine hold-down bolts with washers under the heads and screw them to within three or four turns of being tight.


Sample engine bed shims on stringers.

Note: prop shaft center usually matches trans output shaft center and not necessarily engine crankshaft center.


The temporary blocking can now be removed from the engine and the engine lined up with the propeller shaft. The engine will be in alignment when the faces of the two halves of the propeller coupling are parallel within .003 of an inch or less. A feeler gauge is used, checking all around the two faces and shims added under the engine hangers until the two faces are in alignment. If no feeler gauge is available, four narrow strips of paper can be placed between the two faces at four points around the circle. Any variation of alignment can then be felt by the looseness of any one of the strips. 

Use the threaded motor mounts to adjust the engine up and down until the alignment is correct. If you have to loosen the top motor mount nut to adjust height; be sure to re-tighten the nut and then re-check the alignment. Each time a shim is added or removed, tighten all hold-down bolts and recheck alignment. Extra care at this time will pay off in terms of future performance,


Do not attempt to bring the two faces of the coupling together by springing the propeller shaft. When the engine is in perfect alignment, bolt the two halves of the coupling together and remove the wedges from around the propeller shaft. With the engine in neutral, the propeller shaft should turn easily. If not, check for tightness in the stuffing box or slight misalignment in strut location or angle.


Work in progress


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