Spark Plug Hole Thread Size?

Spark plug hole thread? Anyone know what size tap to use on the F 134 engine. I want to clean up the Spark plug holes, threads on my head. I’m guessing it must be a national pipe to thread (NPT). Any ideas on what size tap.

 

Not NPT!

Metric, IIRC. You can buy a plug thread chaser at your FLAPS.

 

14x1.25mm

 

I will loan you a tap if you want, you can probably buy one for less that shipping though.

 

I expect that the original spec was inch-fractional, and it's so close to a metric spec that the metric description has been universally adopted.

9/16"-20 is 0.5625-0.050 and 14-1.25 is 0.5512-0.492 ... close enough(?)

https://www.amazon.com/spark-plug-thread-chaser/s?k=spark+plug+thread+chaser

 

My thought too, since they were invented in France and Germany. Although... Model T Fords have a tapered thread.


Also, the specified tolerances on metric plugs are different than a standard metric thread, so I hear. Best to use a dedicated sparkplug tap.

 

There is some history here:

• https://www.hemmings.com/stories/article/fire-in-the-hole-3

Spark plugs have had standardized metric threads since the 30s.

 

Wow......glad I didn't bet anyone they would be metric.
Never looked at the thread chaser I have.
Learn something new every day....now if I can just remember what I just learned.