Does anyone know the thread size of the oil drain plug on a 500efi? Are they the same on all merc motors? I want to put a line from the pan that will come out of the plug at the rear of the boat. Does anyone have any experience with these? Should I stick with the suction on the dip stick?
Thank you!

The 525's come with a drain line. You might just be able to use that part # if threads are the same.

Mercury has a retrofit kit. There are only two threads. When I put one on my 500EFI years ago, I bought both of them and returned the one that didn't fit. www.bammarine.com would be where I'd look online.

It uses the metric sized kit.

Does anyone have any experiences with these

They come on ALL Mercury motors now. They are THE BEST way to change the oil. Sucking it out the dipstick tube leave at least 1/2 quart, maybe more in the pan. I usually open the drain into a container when I come home from the water on the weekend; oil is warm and circulated. I let it drain over night and fill back up one night during the week. As long as the oil is warm, it drains out rather quickly though. Some folks even hook a drill pump up to their oil drain tube to pull it out even faster, but it's not necessary.

There are several older discussions on this here on OSO, check out the search tool.
http://www.offshoreonly.com/forums/g...rain-plug.html