Below is a cut and paste of a post to a similar question from a while back. It has worked flawlessly since install and I am very happy with the results.
I have also added my new sign off/disclaimer which did. It appear on the original post as it seems only appropriate considering many of the responses and PM’s I have received.
After selling our Lagoon
a serious “fixer upper” cat I am in the process of a complete all-systems rebuild/replace. After removing the engines and saildrives for rebuilds, seal/diaphragm replacing, and a bunch of other work
I decided to construct an expansion tank system. Here is what I did: after much legwork and searching I was able to source a 316SS hydraulic plug
of the same thread type/size as the original yanmar
plastic one which then also uses the same size O-ring for its seal. I then drilled a hole in the center and tapped threads to receive a 316SS 1/4 inch hose barb fitting. I found some universal plastic overflow tanks
on amazon with 1/4” barb on the bottom and vented cap for around 10-20 bucks each. A piece of 1/4” fuel
hose and a couple stainless AWAB clamps to finish her off. The pics are a little dark, I apologize as I had to be my own lighting
operator and photographer. I have trouble multi-tasking. Just an option for those considering a similar system. I am sure there are both different and better ways to achieve the same objective. This is just one.
Oops, I forgot to add that I installed the custom “black sharpie” oil level line indicator to assist me in checking it easily at a glance.
And since some then requested info on what I used and from where it was sourced.....
Sorry I should have included these earlier.
The stainless hydraulic plugs are sold
on amazon as these:
Joywayus M33x2 Thread Stainless Steel
Rods by CNC Hex Head Corrosion
Resistant Plugs Pipe Fitting
I purchased are these from eBay:
Universal Radiator Coolant
Reservoir Bottle Overflow Tank Dorman 603-001 New
The hose barb fittings are easily sourced from either amazon or eBay depending on what size threads you desire. I used 3/8” thread and 1/4” barb I believe.
Sorry couldn’t get links to work
or I am too stupid to figure it out. Most likely option 2 is correct. If you search the above they will lead you to what I purchased.
The hydraulic plugs are solid stainless and quite thick so drilling them is a a little bit of a chore. I have access to a large metal lathe which made it easier to drill and tap for the threaded hose barb fitting.
Hope that was at least mildly helpful.
To alleviate the need for the supreme and almighty sailing masters among us to belittle, demean, ridicule, chastise, criticize and in general respond in their usual condescending fashion I will readily admit that everyone here, and most likely everyone everywhere in the world, are much smarter, faster, stronger, better looking, more intelligent, more experienced, more well travelled and in general are much better members of the global society than I could ever dream of being. Please disregard any and all of the above prose, as well as the below pictures, in favor of only listening to the self professed experts among us that are so amazing and magnificent that they have absolutely nothing to be learned from anyone or anything else as they clearly already know all that can be known. Oh and since I haven’t “been there” or “done that” then I am obviously not afforded the opportunity to have an opinion, or so the sailing elite here have informed me. Since they obviously have been everywhere and done everything it appears that they and they alone are allowed the luxury of stating their opinions........oops I forgot they don’t have opinions as they are so smart, whatever they say is fact and shall not be disputed.
Congratulations!!! You win. I lose. I shall now sulk away in shame.
Safe Journeys to most,