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Hey everyone. I’m working on a -81 R100 with a dual plug setup. Trying to replace the lower plugs but I cannot for the life of me get them out. I have a set of new NGK BP6HS plugs which take a 13/16 (20mm) socket easily but unfortunately that socket size doesn’t fit in the cylinder. Is there a special socket? I’m thinking of grinding down a spare socket to try and fit. I really hope I don’t have to pull the heads

I may try and order a thin wall 13/16 socket off Amazon or something and give that a go too.

There’s not much room to get anything in there.

Maybe a set of deep well sockets? Seems mine are thinner than the stock ones.

Kurt in S.A.

Kurt in S.A. wrote: ↑Sat Mar 26, 2022 5:07 pm Maybe a set of deep well sockets? Seems mine are thinner than the stock ones.

Kurt in S.A.

Right on…I may have to go out and see what I can find. It appears there’s like a 2mm or less clearance between the cylinder fins and the spark plug outer.

All airheads had only two upper plugs (one per side) at birth.
These are standard plug wrench size of 21mm (actually 20.65mm).

The lower plugs were added later and are often smaller (16mm) than the upper ones.
Maybe this is your problem?

Curious. My 1978 R100/7 with dual plugs takes the same spark plug socket above and below. Clearance on the outside for the socket isn't a problem and I don't think the head's been cut except for the threaded hole and a sealing surface.

melville wrote: ↑Sun Mar 27, 2022 10:05 am Curious. My 1978 R100/7 with dual plugs takes the same spark plug socket above and below. Clearance on the outside for the socket isn't a problem and I don't think the head's been cut except for the threaded hole and a sealing surface.

Interesting…so someone possibly did a half as job on this. It’s on both sides. It definitely looks bigger then a 16mm socket.

I take that back--my earlier statement was related to feeling around down there and not finding any sharp edges. I went back, laid on the floor, and took a pic. The fins were definitely relieved for socket clearance:



You might try the stamped steel plug socket from an old VW (aircooled era) tool kit. That might be thin enough to clear the space available.

Melville has a good idea there. I forgot about the original Airhead tool kit. It has a spark plug stamped tool and a smaller one. The smaller one has one end for the acorn nut on the valve cover...the other larger end I'm not sure where it fits. But the spark plug tool is kind of sloppy for my plugs...but the plugs aren't in that tight so sloppy works.

Kurt in S.A.

Moto D's head (left) beside Melville's head (right).
I will refrain commenting because I can't see them in person.
Maybe it's just different camera angles and old eyes.
Something looks wrong to me.

That’s interesting to see them side by side. This is my right side cylinder as well…like I said. Something looks wonky for sure