Question about Klein 21 Spark Plug tool hex drive size

[Quickstep192]

I have a Klein 21 spark plug wrench that I really like for starting spark plugs. I'd really like to do the final tightening with the tool too, but I don't have a tool that fits the hex drive. While looking for an allen wrench or allen socket, I discovered that 10mm is a little small (sloppy fit) and that 11mm won't go in. Is there a special sized hex tool specifically designed to fit the Klein 21 tool?

[Justin@Athens]

A Torx #50 fits almost perfectly and engages 50% of the splines. I don't know if Klein makes a bit or socket to drive the tool other than the L shaped wrench that came in the factory tool kit.

[Justin@Athens]

Upon further research, a triple square bit would be a perfect fit, as it has 12 facets to engage all of the factory tool's spline recesses. I would buy a set rather than just one as I don't know what size bit is required (maybe 8 or 12mm)

Also, many newer models (996 and forward use this fastener with this kind of head) at least in one place I can recall. (door striker plate)
VIM Tools XZN100 3/8" Drive Metric Triple Square Bit 8-Pc Socket Set VIMXZN100

[Quickstep192]

Hmmmmmm. Did later tools have splines? Mine is a hex drive. But - It's not 10mm; it's bigger than that, but not quite 11mm.

[Justin@Athens]

Hmm, must be different form my tool out of the 964.

Here is what mine looks like: