I remove all the injector lines so it makes i much easier to access the plugs. Also when you have them out you can use a drill bit or even a tap if you have a set to ream the holes. Yes you need to ream them as this alone can cause a very noticeable inprovement inthe starting of the vehicle wihtout replacing plugs, unless of course teh plugs in question are shot.

Once you have reamed the hole you can turn the engine over with the plugs out to eject any cabon that may have been loosened. You can also lightly coat the drill bit or reamer that you use with grease to help hold on to the carbon that is dislodged. Also when you turn over the engine to eject the carbon place a rag over the IP so that you don't spit fuel all over...

When I ream my GP holes I always do it by hand only so that you won't mess up the threads or cut any metal.

Those are my tips but not necessarily the best tips...

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'99 S420 - Mommies
'72 280SE 4.5 - looking to breathe life into it
'84 300SD Grey - Sold
'85 300SD Silver - Sold
'78 Ski Nautique