Plugs on the top of the cyl head what are they for?

[alanfox55]

97 30 hp serial 0G465407
I used the top plug to mount a cyl head temp gauge sending and that worked out perfectly but I'm a little concerned about what I don't know about that plug. How far should the plug be turned in? I eye balled how far out it was and put it back to what looked liked the same depth,it was not all the way in. How tight should it be screwed in? I think it's plenty tight but it will go more. Thanks for any help.

 

[alanfox55]

What are these holes on the cyl head for?

97 Mercury 30 hp serial 0G465407
Two holes one on top of the cyl near the spark plug and one on the bottom near near spark plug with bolt like plugs in them. Can someone please tell me what they are there for? I ask because I used one to mount a cyl head temp gauge sending unit and I want to make sure I'm not going to do something to mess up my engine. Thanks for any help.

 

[Silvertip]

You need to determine if it is a "plug" or a "bolt". Just to the right (the gold thing) is a bolt. It would appear the "plug" you refer to is indeed a plug but as such, has tapered threads meaning the threaded area is tapered and as it is tightened, the taper is what seals the plug. It also appears that hole goes into the water jacket which is where a "water temperature" gauge or an overheat sensor would screw into. Tapered plugs are not designed for, nor should they be used to secure anything. They are for sealing a hole. Your cylinder head temp sensor bracket needs a solid ground against the head. That plug looks chewed up and corroded due to leakage and cannot provide a reliable ground over time..

 

[alanfox55]

Thanks for your response. I'm pretty sure it's a plug and not a bolt because it doesn't secure anything it only plugs the hole and your correct it is tapered. The plug is suppose to have flats on the head for a wrench but the previous owner buggered them up so I had to use pliers to get it out. It looks like it's corroded but it's actually brass. So then if I had the usual head bolt to mount the sender the head bolt would become a needed ground? This sender is for a Faria gauge and it uses a ground wire but the body of the sender still needs to be grounded correct? If that's the case then a brass plug is not going to work. Maybe I can replace the brass plug with a steel one and that would work for a ground or could I attach a ground wire to the body. The problem I'm having is there are no head bolts I could use to mount the sender and this plug amazingly fit the sender mounting hole so perfect that it snugged it right down on the head or in this case the water jacket.

 

[Silvertip]

Brass is a good conductor so yes -- it works as a ground. You are correct that a head bolt would be a better choice to secure the bracket but since there are none, you are stuck with what you have. If you remove that plug, does water come out? If so, buy the water temp sender that screws into that hole. Problem solved.

 

[Sea Rider]

Alan,

Probably are flush adapter ports to connect a water hose on them right on powerhead. If you unscrew them how deep are they ? Recently discovered on mine that exhaust cover had a hidden flush port nicely ignored per Owner's Manual. Can even install a water temp gauge there.

Happy Boating

 

[alanfox55]

Thanks for both of your responses. I haven't looked very close to see how deep it goes but it doesn't seem very deep and I haven't run it with the plug out to see if water comes out. I would rather have a cyl head temp gauge than a water gauge and I already have it so I don't have to spend more money but if that's what I have to do that's what I'll do. Thanks again for the info I would've never guessed that hole was for a water temp gauge and I couldn't find anything in my service manual.

 

[Silvertip]

With the water jacket and cylinder head so close to each other the temperature difference between the two is minimal. In fact the same sensor and gauge are used for both. Even if the hole doesn't go into the water jacket, the bottom is just a fraction of an inch away from it so a standard screw in sensor bottoms in the hole and you have the same thing you have now -- but a better installation. Both systems work just fine, but yours is just not a very good way to mount that bracket. The bracket would not be needed with the screw in sender.

 

[alanfox55]

OK thanks for that info.