i have a yamaha 250 street bike and i cross threaded the spark plug whats the easiest way and cheapest way to fix it. i cross threaded it into the head so it only goes in half way
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Try a new sparkplug and hope it was the threads on the spark plug that were crossthreaded and not the head.. Otherwise you're looking at a helacoil after you pull the head.

 

how much does one of those heli coil cost and how hard is it to install

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The helicoil itself is cheap, but you need a special tool to insert it, plus a drill bit of a certain size and a tap of a certain size. You will have to pull the head to do it, or wreck a good motor with metal shavings in the cylinder. Now, as long as you are taking the head off, try to run a tap or a bolt in the spark plug hole from the other direction, you may get lucky and undo what you did.

 

They are not too hard to install. But you should remove the head as you will have to drill out your old sparkplug hole. You dont want shavings going into your cylinder.

you basically take an oversized drill and drill out the hole. you then take the tap is for the helicoil and tap the hole you just drilled. You then thread the helicoil into the head.

I would use a product called a keysert instead though. The keysert locks into place once you install it so there is no worry about it backing out as you change your sparkplugs. Its the same process, but for once the keysert is installed, you just tap 4 bars down and those cross the outter threads and it locks it.

here is a link to them.

http://www.newmantools.com/key.htm

 

I wouldn't worry about a helicoil backing out. It is something of a spring, a bit oversize for the hole. If you were to try to put it in backward, it'd never go in.