E: Engine spark plug 'ole

vibrac

Well Known and Active Forum 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.
 

Marcus Bowden

VOC Hon. Overseas Representative
VOC Member
Tim my handsome, if "B" Nighted reads this post you might receive in a few days my hand JIG for doing just that, putting in a 2nd plughole, I made it at sea about 35 years ago and you only need use handtools.
bananaman.
 

Cyborg

Well Known and Active Forum User
VOC Member
Same as the valves I think IIRC. Do you have a rotary table? Set the mill up using the existing plug hole and then turn the table 180.

Somewhere there is a discussion about the angle, but a search wasn’t successful.

After drill and tapping, I was able to find a old valve seat cutter that I could use to shape the top of the hole, so it looked the same as the original.
 

vibrac

Well Known and Active Forum User
VOC Member
F
Tim my handsome, if "B" Nighted reads this post you might receive in a few days my hand JIG for doing just that, putting in a 2nd plughole, I made it at sea about 35 years ago and you only need use handtools.
bananaman.
Fingers crossed...
(Bike is safe)
 

Robert Watson

Well Known and Active Forum User
VOC Member
OK Tim

I feel bad. I "may" have a 90 piston so before I dig through the pile the question is,,,,,,,, do you need one oversize and if so what??

Robert
 

clevtrev

Well Known and Active Forum User
VOC Member
All the holes are at the same angle, If you have a plate with the four stud holes mounted on an angle plate, set at 32 1/2 degrees, clock in a valve hole, lift the head off the plate turn through 90 degrees and you are centered for the plug hole.
 

Robert Watson

Well Known and Active Forum User
VOC Member
OK Tim

I just had this feeling that there was something out there. I have one slightly used Omega 3273 piston Bore 90.0 Yours for the asking if it's any use. No rings and no pin and keepers.

Robert
 
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