I do have a Yanmar and have thought why nylon/plastic
for sealing in oil. the surfaces are so slick over-tightening would be easy and an issue. Also you have the heat over time making the plastic very brittle. How many times have you broken a plastic cap closing a tube of grease or silicone?
Fortunately I have a bit of experience with hydraulics and associated stuff. On my 4JH4E (56hp) the filter is a spin on so no biggie. I don't like the plastic priming pump though.
Always remember to treat plastic parts with extreme care. Plastic parts ALWAYS undo with far more stress than they are tightened. This is due to the softness of the plastic and heat combine to allow the soft plastic to "form" on each other for an increase of torque required in breaking it loose~~be careful.
Neil, I may look into your polishing system when I go to 2 Racors for switching the feed and adding another tank for cruising
Dave Neptune
