[ROCKLOBSTER]

There is one part of American cars which have always been metric. Does anyone know what it is?

14 and 18mm spark plugs.

[yarddog]

Yes that is it.

Spark plug thread size has always been metric.

I suppose some engine somewhere has used SAE but for the most part spark plugs have always been metric thread.

[yarddog]

Yes that is it.

Spark plug thread size has always been metric.

I suppose some engine somewhere has used SAE but for the most part spark plugs have always been metric thread.