Spark plug thread repair
- Thread starter Mtoliva
- Start date
pf1
Hero member
- Location
- Portsmouth UK
The good bit might be the cast iron skull?
Exmoor beast
Hero member
- Location
- Minehead,Somerset, ENGLAND OF COURSE
Stop now & take to some one who can “try” to chase the thread with a special tool. Personally I don’t think it’s saveable & you will need a helicoil/ Timesert, doubt anyone will attempt it with the head on.
Schurminater
Hero member
- Location
- Surrey Hills
Stick "Spark plug thread" in the search box (top right) and see what comes up
Exmoor beast
Hero member
- Location
- Minehead,Somerset, ENGLAND OF COURSE
Worth Timesert
I guess this may be the center pot under the frame tube. I recommend to use a very small amount of anti-seize compound on spark plug threads, and then possible a little bit reduce the torque. I consider cylinder heads must last 100 years or more, and they don't if I don't do this. 
Some manufacturers recommend to keep them dry and eventually the threads fail.
Especially if this head has skulls, I consider the best possible repair for a loose head would be to nicely drill off a couple of upper turns from the thread, and this in a tilted drill/mill in the correct angle, and not with too big drill. Some 1960's Rambler AMC cast iron cylinder heads are originally made that way, the upper half of the thread missing. You can just drop the plugs in place and turn - they never go to wrong threads. The thread don't need to be that long.
The thread is M14x1.25x19
The usual thread tap with H7 fit would made them too loose if turned too deep.
I use a thread tap from the side of the combustion chamber for loose heads.
In this case without removing the head - for a first aid - I would just nicely remove all chips and burrs in wrong places until a new plug goes in.
Note that the old plug may copy some damage if used, you can bin it or clear it with a thread die and use it elsewhere, but not as a first plug back on this cylinder.
This with small screwdrivers and possibly very carefully with a thread tap, that is turned by hand. Can take hours, good lights and vision, and taking care any more intact thread is not removed or damaged.
Some manufacturers recommend to keep them dry and eventually the threads fail.Especially if this head has skulls, I consider the best possible repair for a loose head would be to nicely drill off a couple of upper turns from the thread, and this in a tilted drill/mill in the correct angle, and not with too big drill. Some 1960's Rambler AMC cast iron cylinder heads are originally made that way, the upper half of the thread missing. You can just drop the plugs in place and turn - they never go to wrong threads. The thread don't need to be that long.
The thread is M14x1.25x19
The usual thread tap with H7 fit would made them too loose if turned too deep.
I use a thread tap from the side of the combustion chamber for loose heads.
In this case without removing the head - for a first aid - I would just nicely remove all chips and burrs in wrong places until a new plug goes in.
Note that the old plug may copy some damage if used, you can bin it or clear it with a thread die and use it elsewhere, but not as a first plug back on this cylinder.
This with small screwdrivers and possibly very carefully with a thread tap, that is turned by hand. Can take hours, good lights and vision, and taking care any more intact thread is not removed or damaged.