Thread size for spark plug bore

[00reg00]

Hi i have a four cylinder mercury 50 hp 1979,can anybodey help with the size of the spark plug bore as i need to cut a new thread for the cylinder,i do not know what size tap to buy.When i removed number 0ne plug some of the thread come out and now i can not replace the spark plug,
Thank Reg

 

[justme65]

Re: Thread size for spark plug bore

If I am understanding you correctly you have striped the threads on the head & now want to re cut them to oringinal size? Thats wont work. The striped thread is now a bigger hole than it should be so any recut thread would have to be a bigger size. BUT dont panic yet. Dont know about where you are but here we have a thread repair kit called a helicoil. You basicaly get all you need in the kit to recut a larger thread & then line it with a stainless steel insert that has the correct sized thread inside it for the plug (actualy in the helicoil kit its almost like a spring but much higher quality, some kits are a solid threaded insert but I have heared that they come out when removing the plug if your unlucky).

Justme65

 

[Chris1956]

Re: Thread size for spark plug bore

Reg, The plug is likely a 14MM. However, it is likely you will need to retap the hole and install a helicoil or solid insert. I like the solid ones, although I epoxy them in to keep them in place. The tricky part is to keep the metal 'chips' out of the motor when you retap. The obvious way is to strip down the motor to a bare block.

 

[00reg00]

Re: Thread size for spark plug bore

Hi i have not Completely stripped the thread,i just can not get the spark plug to start in the cylinder head,the first part of the thread seems to be damaged is there any way to clean it up,i would say that 75 % of the thread is allright.
Reg