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Author: Subject: Broken plug Thread
belgian2b

posted on 12/8/19 at 09:10 PM Reply With Quote
Broken plug Thread

Hello,

tonight i went to the XTR2 to start it because i have booked a trackday in september.

Engine didn't wanted to fire, so i changed the plug.
Engine started.

I cleaned the old plugs and replaced them on the engine.

And then,.... i broke the thread of one of the plugs.


Engine is a Kawasaki ZX12 A1.

I didn't make any force on it, and when the plugs i took out did not show any damage to the thread.

I tried to take the plug out , but no way.

So, is there a way to make a new thread without dissassembling the engine ?


Gerardo

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coyoteboy

posted on 12/8/19 at 11:01 PM Reply With Quote
I'm confused. If you want to make a new thread you'll need to be able to take the plug out?

And it's possible to chase out a thread without dismantling the head (but not really advised) but I'm not sure on a bike engine like this if there's enough meat around the bones of the hole to do a thread insert?





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Charlie_Zetec

posted on 13/8/19 at 10:08 AM Reply With Quote
If you thinks you've cross-threaded one of the spark plug threads, then you're going to have to either leave the current one in situ and hope it works as it should, or remove it and try to repair. If the latter, then you can look at the possibility of carefully re-tapping the thread (if you've got a full set of taps ie. taper, second and plug) after you've removed the plug, and hoover out the cylinder and inspect with a camera for any swarf. But failing that (worst case scenario) it's a head-off job, and either re-cut the thread or helicoil. Been there with an old Astra GTE....





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russbost

posted on 13/8/19 at 10:23 AM Reply With Quote
Are you saying that the plug has sheared off leaving the thread in the hole?

If so that's not uncommon on modern plugs with the small diameters. If that is the case it's an easy fix, the remainder of the plug should come out easily enough with an easy out or similar - the thread shouldn't be tight in the hole unless it was cross threaded in the first place, you might be able to get it out just by tapping round carefully with a punch - I'd suggest blocking the hole with some tightly packed tissue first to stop metal ingress to the plug hole.





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coyoteboy

posted on 13/8/19 at 11:28 AM Reply With Quote
quote:
Originally posted by russbost
Are you saying that the plug has sheared off leaving the thread in the hole?

If so that's not uncommon on modern plugs with the small diameters. If that is the case it's an easy fix, the remainder of the plug should come out easily enough with an easy out or similar - the thread shouldn't be tight in the hole unless it was cross threaded in the first place, you might be able to get it out just by tapping round carefully with a punch - I'd suggest blocking the hole with some tightly packed tissue first to stop metal ingress to the plug hole.


I'll be honest, I don't have a lot of experience with bike engines but if you've sheared the plug off removing it, the thread is stuck (either crossed or seized) - it takes a fair amount of force to shear the body. And easy-outs are the work of the devil, if anything is guaranteed to break off and give you a hardened steel item that requires head-off machining out, it's an easy-out.

The biggest issue I'd have is if the body has broken into socket and you need to drill it out, you're going to drop ceramic parts into the cyl, which can't easily be hoovered or magnet-removed. If you value the engine, I'd do a simple head pull and do the job properly.





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jollygreengiant

posted on 13/8/19 at 05:16 PM Reply With Quote
IF the plug has a taper seat, then most common cause is that the plugs have not been changed in a while. Also the old plugs were fitted (cold) to a hot or warm engine. this causes the taper to expand into a contracting seat, and it doesn't matter whether the engine is hot or cold, they tend to jam on the taper. I found the EASIEST way to sort it was to firstly remove the ceramic core. This should have come out with the nut section. IF it is still in the head, then try and start the engine and it should blow out. With this out you then need to pack some grease into the hole left in the remnant in the head. This hole WILL be centred and that is important.

You then need a drill bit NO LARGER than the taper. Fit this to your drill and with the flukes of the drill packed with grease (to catch swarf) drill
SLOWLY down into the taper until the ring above the tper releases (this is why the drill bit should NOT be larger than the max diameter of the taper).

Then, turn the engine over again to blow grease (and any swarf) out of the threaded piece of the spark plug.

Now find an allen key bit or torx bit that needs to be gently tapped into the centre of the threaded piece spark plug and this should now come out by turning in the appropriate direction.

Then run a thread cleaner/chaser up and down the plug hole. and fit new plug.

I learnt this from doing HCS (Late Xflow) Ford engine, in the end I got it down to 15 minutes.


IF however it is a spark plug with a gasket seat then you probably have a problem with the plug having been cross threaded when fitted and that is a whole new can off worms and will probably end up with head off.





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T66

posted on 13/8/19 at 06:56 PM Reply With Quote
I had a plug shear off in a Blackbird head, left about 60% of the threaded part in the head. It had been soaked with releasing oil for weeks prior to this. Adding an easy out on the remnants resulted in a snapped easy out. Just when I thought the thing was starting to turn out.

Be careful





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