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E: Engine spark plug 'ole


Well Known and Active Website User
VOC Member
As I am retrofitting a 84bore cylinder I need a new second spark plug hole in my cylinder head for my decompressor
Its not something I have done before but I have a mill and an angle plate so...
I have worked out is a 14mm X 1.25 tap and a 12.8 mm tapping drill but does anyone know the angle from the cylinder base of the hole? by eye it looks like 45 degrees.


Well Known and Active Website User
VOC Member
For the countersunk area on the outside, I found that an old Kowa (Honda) valve seat cutter would round off the bottom of the pocket so that it was almost identical to the original. It wasn’t used to cut the actual countersink, just finish it off. Made a pilot, similar to the one with that nice jig. Basically a gutted spark plug with a bushing in it.


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