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ENGINE MOUNTS
Dimensions
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Yanmar Pleasure Boat Marine Engine Help

Overview - Flexible Engine Mounts

Firstly, let us make sure we understand that, working from the bottom up, the engine beds, the flexible engine mounts and the engine mount brackets, are separate identities. When installing a new engine please take into account that you can:

  1. Alter the engine beds
  2. Adjust the flexible mounts

You SHOULD NOT, however, modify the engine mount brackets (the pieces of metal that stick out from the engine that sit on the flexible mounts) so that the distance from the center of the crankshaft to the mounting hole is different from that supplied.
graphic of mount effect Flexible engine mounts placed between the engine mount brackets (part of the engine) and the engine beds (part of the boat) considerably reduce noise and vibration.

The selection and position of flexible mounts is important for effective dampening.

Flexible engine mounts supplied as Yanmar accessories and listed in the original Yanmar brochure and in the parts books as optional parts, are extensively assessed and tested by Yanmar before they are included as a Yanmar part. In my experience, these mounts, fitted as originally intended, are a perfect match for the engine.
When re-powering a vessel, always change the engine beds, don't change the engine mount brackets! The engine mount brackets are often erroniously modified as the person who installs the engine is an engineer, who doesn't know about boat building. 'He' will always find it easier to whip off the mount bracket and weld on an extension than find a boat builder to bolt, glue, glass an additional pair of bearers into the boat. He knows about welding, he doesn't know about glue or fibreglass. (Please excuse me if you don't fit this explanation.)
From the above picture you can see that a properly mounted engine will always rotate around the crankshaft centerline. If a mount bracket, or brackets, are extended the engine will screw itself around the centerline causing the alignment to be thrown out as soon as the load is applied. It would be like giving ' Long John Silver ' one long and one short crutch to walk with!


mount amplitude diagram
Sometimes a customer will complain that the engine mounts are too soft, here's why they are as they are:
If the mounts are replaced with stiffer mounts the peak amplitude, as shown on the graph on the right, moves further to the right, above the idle RPM. Vibration becomes very noticable. If your boat vibrates while coming into a bay or the dock, the engine mounts may be too stiff (or faulty, the idle rpm is set too low, the engine is not aligned correctly, the prop is crook, the shaft is bent, etc.). Do not operate the engine in this rpm range until the problem is fixed as the engine may eventually fail. Also, you can see from the peak amplitude point on the diagram that the engine really bounces around as it comes to a stop. This is normal.

Offshore safety tie

drg for offshore safety
The mount on the left is the type used under 2GM to 4LH series engines, if you need to pass a Cat 4 inspection for off-shore, put a wire or small chain through the centre 'nose' and over the top of the top metal piece and 'Talurite', crimp or u-bolt the ends together. I have never seen this mount fail because the bonding failed from reasonable use. The failures were always incorrect installation, age, or they became covered in diesel.




Mount Dimensions


This drawing is for GM, HM, JH and LH series engines.
LY and CX mount drg.
This drawing is for LY and CX engines. The bottom table has the dimensions for these mounts.

On all models, try and keep the engine foot or bracket closer to lower end of dimension 'H' if possible.

engine mount dimension table
another mount dimension table

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