Everything on a Toyota vehicle is metric. So it really rubs me the wrong way that a 5/8” spark plug socket is what is recommended. From what I understand 5/8” and 16mm are more or less identical within a fraction of a millimeter. Would not a 16mm spark plug socket just fit better? It makes no sense that oem spark plugs on Toyota would be an imperial measurement unit.
or am I just obsessing on this?

 

5/8” worked on mine, seems to be a standardized thing despite the rest of the car being metric. Measured with a caliper and it was 5/8” too

 

Spark plugs use a Standard Plug Socket, whether the Vehicle is Metric or SAE! In this case it's 5/8" Spark Plug Socket. It's not just the size, but the socket is made especially for a Spark Plug!
I've Never Seen a 16mm Spark Plug Socket, but maybe there is!

 

they do indeed make 16mm spark plug sockets. 5/8” = 15.879mm so .121 mm off. However is the spark plug exactly 15.879mm? Either way either one will work.
I got thinking about this after watching a wrench test video where they said that 16mm and 5/8” are interchangeable. I forget if it was torque test channel or project farm.

 

Thank You,
That's Very Interesting, learned something New Today! A Metric Plug Socket for a Metric vehicle.

 

you got it!
I mean the 5/8” might be a better fit, I’m going to have to verify this when I pull my engine out and get to the spark plugs or I may wait till I get the salvage engine I’m swapping out for and replace the spark plugs in there with new ones. If the spark plug measures 15.87-15.9 mm then he discrepancy is negligible. Ultimately either will work but I have the type of thinking that’s telling me one should work better than the other and why would Toyota spec a non metric component in a vehicle where EVERYTHING is literally metric? It. Just. Makes. no. sense.

 

The difference is .004 of an inch, 5/8" is .625 of an inch and 16mm is .6288 of an inch.
it's Not the difference in size, which is almost negligible, it's how the socket is made.
A "Spark Plug" socket is made especially for a spark plug, it is made to protect the porcelain insulator when removing and installing the spark plug and other important manufacturing details too.
All Spark plug manufacturers have been making 5/8" spark plugs for many Years.
It's kinda like tires,
Tire Diameter is in Inches, eventhough they are used on Import Vehicles! It's just one of those Things!

 

I totally get that but I bet in Japan and Europe tires are sold in metric specs. They definitely make specialty spark plug sockets in 16mm this one looks pretty nice seeing it has a swivel joint and is magnetic

 

[QUOTE=Rx330fnboi;11367743]I totally get tw2hat but I bet in Japan and Europe tires are sold in metric specs. They definitely make specialty spark plug sockets in 16mm this one looks pretty nice seeing it has a swivel joint and is magnetic
/QUOTE]
Yep, something for Everybody!
it's Market Driven, if You want it, Somebody Makes it and Sells it!
I bet someone makes 16mm Spark plugs!
Maybe outside of the US, the plugs are listed as 16mm rather than 5/8"?