spark plugs and threads

[handball]

Last fall when winterizing my two mercruiser engines (350 chevys) after running antifreeze thru the salt water circuit and fogging the engine with west marine spray can I pulled the plugs to spray some in each cylinder. On the starboard engine 2 of the plugs were extremely hard to unscrew, after finally memoving them I found that they had striped some of the threads from the head. My guess is that the yard mechanic from the previous owner had cross threaded the two plugs installing them.I went to the local auto supply store and bought a retapping die and after a lot of sweating managed to straighten the threads out and install the new plugs.I also have had this same thing happen on aluminum heads on both a vw and a mazda, and am positive I did not cross thread the plugs when installing them, and always put them in by hand in order to get them started before using a socket wrench.<br /><br />Is there anyway to avoid this problem? and does it matter whether you remove them with the engine hot or cold?<br /><br />Also I wonder if putting additional fogging oil in each cylinder is really required , after running it thru the carb.

 

[Elk Chaser]

Re: spark plugs and threads

I don't think it makes any difference if you pull the plugs hot or cold. sounds like you already know what happened to your threads anyway.<br /><br />Fogging each cylinder is a good idea because most of the oil is burned when pulled into the carb while running.

 

[deputydawg]

Re: spark plugs and threads

If the plugs are froze in or seized it sometimes helps to warm the engine a little. The cars you talk about probably had aluminum heads. Any time you screw steel like spark plugs into aluminum, you find a lot of seizing. These engines will definately help to warm the engine before removal. The aluminum expands at a lower temp.<br />You shouldn't have to put extra oil down the cylinders after fogging. If you fog theough the carbs it will suck enough oil into the engine to coat the cylinder walls.<br />When you re-tapped the threads, make sure it left you with enough threads to hold the spark plug. I worked on several motorcycles that had this problem. After tapping the hole, as soon as it was run at high RPM's, the threads gave and the plug popped out. Also make sure you didn't get any metal filings in the cylinder while running the thread tap down the holes.

 

[Otter0575]

Re: spark plugs and threads

I am not a boat mechanic but I am a motorcycle mechanic and almost all the heads are made of aluminum and are subject to a lot of heat since they are air cooled engines. But anyway Anti sieze should be used any time that you install plugs. Just ask for it at your local auto parts store. It is simply a graphite goo that you put on the spark plugs that prevent welding that spark plugs to the head.

 

[Yepblaze]

Re: spark plugs and threads

Aluminum heads must have plugs serviced while cold and a cast iron head might be a little easier when slightly warm.<br /><br />Use never sieze compound on the plugs and head threads each and every time.

 

[TwoBallScrewBall]

Re: spark plugs and threads

Agree with yepblaze, with an aluminum head, the expansion of the head is so much greater than the expansion of the plug metal that the head will clamp down on the plug when hot. My dad and I learned this the hard way working on his old 1994 dodge dakota (only a year or 2 old at the time) when he tried to pull the plugs and snapped one clean off in the head. <br /><br />Always a good idea no matter what the material to let the engine cool first. Almost mandatory with an aluminum head.