Guzzi fiddling tonight.
Someone converted the heads to dual plugs a long while ago. The previous owner told me that he had never been able to get the plugs out, no socket would fit.
Here is the offending hole:
Aha! I thought, I'm a -ing rockstar, I have the solution, this person simply doesn't know about Snap-On thinwall sockets!
Nope. None of my sockets fit either.
The 5/8" goes down the hole easy but won't fit the plug. The 18mm won't fit down the hole.
Hmmm... I wonder if the 5/8 is too thick to get all the way to the bottom of the hole
Nope. Machined the socket down and it STILL wouldn't grab the plug.
Strange.
No sacrificial 18mm deep socket lying around.
Well, 11/16" is pretty much 17.5mm and I have no idea what size this damn plug is. 11/16" will work on a 17mm with a bit of wiggle and will -probably- work on a 18mm if I push on it hard or give it a bonk with a hammer. Here we go...
The world's thinnest 11/16" socket, only 0.030 left in the corners.
Down the hole, tap with a hammer to seat it on the plug and out it comes!
The original 18mm hex plug is on the left, the new, correctly sized 5/8 hex plug is on the right. For future reference for anyone who needs it DC is the NGK prefix for the 5/8 hex plugs and D is the prefix for 18mm.
Chances are, sometime in the past a parts counter monkey subbed one for the other, not realising the havoc it could cause.
Looky here, a 5/8" conventional plug socket fits just fine.
I think the center electrode is a bit eroded on this one: