Possibly avoid using a Cometic one unless the deck/head surface are perfectly flat.
I did have a look just then and forgiving gaskets such as Tomei or Nitto don't exist for the FJ20, however the Bar-Tek or Kameari ones look good and might do the trick.
Because pipe threads for pipe things. M threads for bolt things.
Throw some teflon sealant onto it and dust your hands off knowing you did the right thing, instead of going twice around the block to achieve a result using the wrong things.
Of course, all of these solutions are ignoring the fact that the minimum thread pitch we're talking about here is 1.25mm, with the 1/4" in NPT or BPS being out around 1.4mm. You will need to know that the boss you're tapping has enough thickness for at least a few threads. That's one of the reasons that 1/8" is commonly used - because the pitch is <1mm.
And why would you look to use 1/4" NPT in a BSP country anyway? Ugh.
1/8NPT drill size = 8.7mm. Since the hole got messed up by me, it’s probably around 9mm.
So 1/4” NPT would be next step. A drill size of around 11mm would be preferred there which is not way of my M12 (drill 10.5mm and tap M12 threads)
Recommended Posts