Yanmar 2GM20F:  Engine Template
This page was last updated on 26 July 2001.

NOTE:  On this page, you've actually gone back in time a bit.  I built this engine template prior to hauling the boat and removing the old engine, since this was preparatory work I could do before committing to the project.  But for the sake of chronological continuity, it makes more sense for this page to be at this order in the sequence of work involved in replacing the engine.

Once again, I found myself building an engine alignment template.  Who knew that I'd be doing this again so soon?  It was only a little over a year ago that I built my first template for the Yanmar SB12 engine that I initially installed in the boat. 

The first step, which I took care of before even ordering the new engine, was to find out how much, if any, I would have to modify my engine foundation.  I was a little worried that it would be necessary to cut the foundation down.  However, when I started comparing dimensions, I figured out that, if anything, I might only have to raise the beds a little less than a half inch.  Raising is easy, since I only have to add an oak shim or something.  Obviously, the shaft and the shaft angle are constant, no matter what engine there is, so the only real changes have to do with the height of the coupling relative to the engine beds.  This is good news.

Once the new engine arrived, I was ready to get to work.  I built the template before the boat was even hauled, as it was an easy thing to get out of the way, and the template will be one of the first things I need once the old engine is removed.

Please click here to view a dimensional drawing of the Yanmar 2GM20F.  It gives all the important dimensions necessary to construct a template like mine.

Using the supplied dimensional drawing, I constructed a simple plywood platform, carefully laid out to represent the basic dimensions of the engine.  The most critical part of building the template is to exactly locate a surface to represent the transmission coupling, and to properly and accurately locate the four engine mounting holes, which was easy enough using the drawings. 

When I made the first template, I found it easier to make the plywood rectangle a little shorter than the ultimate dimensions of the engine, and then add extensions to the ends as necessary to represent the exact locations of the critical components, such as the transmission coupling flange.  For the new engine, the forward end of the template is less important, since I already know the engine will fit nicely in my engine bay.  The aft end, however--representing the coupling--remains critical.  To locate the coupling, I randomly decided that my extension would be exactly 2" beyond the end of the plywood rectangle.  Therefore, since the measurements used to locate the four engine mount centers are referenced to the coupling flange, I measured forward and located each mount center--remembering to account for the 2" overhang at the rear.  Then, I added my extension to the aft end of the template (shown to the right).  To account for the vertical piece of plywood at the aft end, which projects down and represents the actual face of the transmission coupling, I subtracted the thickness of the plywood from the 2" overhang measurement.  These pieces are simply screwed together and onto the plywood template.


The vertical piece at the end is designed to exactly represent the transmission coupling, and the offset from the engine mounting flanges (which are represented by the bottom of the plywood template).  This is 2.95" as indicated in the drawing, so I carefully located and marked the center and drilled a 1/4" hole (to allow a centering string to pass through later).  I sawed a slot into the hole to allow the string to slip in.  With the measurements made and the hole drilled, I carefully installed these pieces so that the center of the hole was exactly 2.95" below the bottom of the plywood template.  To make this easier, I drew several reference marks on the piece before installing it.  I added an identical piece to the front edge of the template.  When the template is installed on the boat later, the centering string, which will run though the stern tube into the boat, will run right through these holes, and I will be able to adjust the engine beds or adjustable mounts as necessary.  With these pieces installed, I mounted the template on the four rubber engine mounts--there are two different densities of rubber, so the ones marked "75" go at the aft and, and the ones marked "100" go at the forward end, according to the manual.

Next:  installing the centering line in the boat (after removing the old engine) and modifying the engine beds if necessary to allow for perfect engine placement and alignment.


Click here to continue.


Glissando, Pearson  Triton #381

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