UPDATE
I replaced the spark plugs and wires last weekend on my 2004 350 MAG. I know this is a standard repair but I wanted to give some tips for anybody else doing this job. The forum been good to me providing information to perform this task. Keep in mind it's not the process of replacing the plugs, rather it’s the procedure for plug replacement. BTW- This is a fresh water boat and I understand what that means. Some tips follow:
1. Setup. Get conformable. Remove everything that makes sense to remove on the Port and STBD side of the engine so you have plenty of room. You need to have access. Take the time to do it!
2. 5/8” Socket. Modify a spark plug socket as BT Doctur states above (most important). Reason: the socket needs to fit normal to the plug. This means the socket has to fit square over the plug and not be off-center or cocked. The socket modification allows room for you to adjust the socket, as necessary, over the plug and adjust for any misalignment, i.e. normal to the surface of the plug. If you have a plug buried against the manifold, you cannot adjust the socket (buy a cheap wheel grinder someplace to grind the socket - you'll use again)
3. Socket Extension. I did it with a 3”- 3/8” socket extension + socket height. I wish I had a 4”-4.5” extension to clear the manifold so I could turn ratchet handle 360 degrees past manifold. HOWEVER, the longer the extension the more misalignment you introduce (no longer normal to the plug). Using double extensions (2 Extensions) may introduce too much wiggling.
4. Plugs. Gap and make the new plugs available to you. Reason: you need to grasp them with one arm. I shouldn’t have to tell you this, but remove one plug at a time.
5. Access Removals. On my engine, I had to disconnect a cooling water line that was in my way of the forward plugs. Suggest you disconnect the water line first and lit it drain and do the aft two plugs first. Then when you do the FWD plugs it only dripping - stick a rag in the hose outlet to keep things dry. Tie the lose end of the hose up out of the way (I use telephone ties). I add Vaseline to pipe connection to aid in hose re-installation – one hose clamp broke.
6. Plug Boot Removal. Maybe the toughest part of the job because you only have two fingers to do it. Rotate the boot 90 degrees each way first before you try remove. Reason: brakes any heat seal between boot and plug. Pull straight. A spark Plug boot puller tool may help too.
7. Plug Removal. After you get the socket over the plug keep the ratchet and the socket normal to the plug. Place one hand on the ratchet handle and the other over the head of the ratchet head to keep everything square (normal) to the plug just below the exhaust manifold. (I loosened the plugs without a persuader over the wrench handle – am not strong.)
8. Plug Removal. When the plug is loosen, I removed the plugs by hand. In one case, I unscrewed it with a 9’ socket extension (I put the socket between my knee pulled out the 3” extension and put in the 9”). ONE HAND IS ALWAYS ON THE PLUG or HOLE.
9. Plug Removed. The second most important thing. ALWAYS ALWAYS ALWAYS keep one hand on the plug or the plug hole. Don’t lose the plug or plug hole, and you can look at the old removed plug later. Had had to get a flashlight when I took my hand off. Grap the new plug with your other hand.
10. Fingertip Distances (Horizontal & Vertical). This is sort-of difficult to describe and most important thing also. Measure distances with your fingertips. If you do this, it helps with reinstallation of the plug and boot. Measure the distance between the plug tip and the bottom of the exhaust manifold as you remove the plug. When you reinstall the plug, you maintain the same “fingertip” distance. When you reinstall the boot, measure with your fingertips the distance between the plug base and the edge of the boot over the plug, i.e. the plug boot seat (all the boots should have same plug seat - check).
11. Rag. Place rags under plugs so it catches the new plug if you drop it (doesn’t end up in bilge –happened to me once, BUT my bilge is dry.
12. Plug reinstallation. Use hand only to thread new spark plug – can’t stress this enough. Maintain the fingertip distances you noticed during removal. My threads where long, tapered, and took a while to thread- be patient. When the threads start and your tightening my hand you may have to wiggle the loose plug a little in the threads to get over high points in thread. For me, the plug stop immediately when they were fully threaded in plug hole (bottomed-out). I hand tighten them with my socket and 3” extension only and check fingertip distances.
13. Boot reinstallation. Easy, check distances
Hope this helps somebody. I probably forgot something. I installed a new wire set after the plugs where done.
