I have found that the (sleeve, or whatever it is called), that the plug slips into is leaking, is there a way to seal this,or what? This is in a good ol' Bell Boy.
The replacements that I have seen come in two pieces that screw together. One from inside and one from outside. I usually just use plumbers putty or silicone to seal them when I put them in. Just make sure that you get the two pieces together very tight.
Don't get in a hurry there will be someone else along with another opinion. Never hurts to have them. I would be more worried why it is leaking. could be that your transom is rotten and putting in and taking the plug out along with the wiggling it requires made it loose because there was nothing to hold it tight.
You will get better input if you tell us a bit about where this sleeve goes. I assume you are asking about the transom plug? There is a tad bit of difference between an alumnium boat and a glass boat - - and between a 12'er and a 31'er.
