Spark Plug Substitution



My 1992 Toyota Tercel has stripped threads for the #1 cylinder. I tapped out the hole and put in replacement threads, but after many thousands of miles, the replacement threads blew out too.

The original plugs are 14mm. Rather than find another head at a junk yard, I’d like to tap out the hole for an 18mm plug.

I am having trouble finding information. I assume that if the 18mm plug has the same reach and heat range as the 14mm original, it will work OK.

Does anyone know where to find the information?



Here you go:


Looking for that info could get pretty time consuming so I’ll just leave a comment or two.

Strictly from memory here, but I think all? or most? of the 18mm plugs have short threaded shanks and the plugs on your Toyota are long shanked plugs are they not?

The engine would run, more than likely with no problems, but there could be a few potential problem areas.
One could be a sharp lip on the edge of the spark plug hole which could get hot and cause detonation since the plug would not protrude all the way through the spark plug hole.
Since the plug is firing inside a hole basically, it’s quite possible the flame front would be affected and this could affect performance and mileage to some degree. That’s very hard to say.

I’ll think on this some more but I’m having a difficult time thinking of a long reach 18mm plug.

No matter what, if you manage to do this the plugs should be pulled after a day of driving and the tips inspected; just in case.
My opinion would be to do a proper Heli-Coil job and run the plugs that are called for. It could be easier and safer.


Bring the old tap to the parts place especially if you bought the cheap kit with the aluminum thread repair. The one for the metal threads is a bigger tap.


One word…Helicoil.


I think you’ll get into the valve seats if you go to 18mm plugs. There isn’t much room between the valves here.