Can anyone shed some light on my puzzle,

2mnth ago i put a set of plugs in , Yesterday taking off from work the rear 1 blew out still  connected to  lead .

I tried to put it back in but just kept spinning, when i go it home i checked the threads and both were fine, So i swapped em over and no prob

Started it up and no probs, Then i thought i would swap em over again and check, And it wouldn't tighten up. So im confused                                                                                                                                                                                                                                              

 

Cheers Finny                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     

 

Finny, not good, if the threads on the plugs are OK, then you gotta assume that the thread in the rear head is partially damaged/stripped.

Best get it checked out before more damage is done.

If the plug is already being blown out it suggests that the thread is already too damaged to re-tap, dealer or engineering shop will be able to let you know if a heli-coil is available for the plug size,,,, or if you're gunna be up for a new head.

Grab some verniers and check against the plug that works ok. Sounds like its been loose a while and vibration etc as it works itself out has 1. Worn the plug thread 2. Worn the thread in head. Without looking at it hard to say but thats my guess to the puzzle of why the good plug is ok. You may need to get a plug insert put into the head

 Thnx for replying, The thread on the plug is fine , I think there might be something else going on in side. The gap on the plug was squashed so i think the piston has hit it. And yes the thread on the head is stuffed. Now the question up size it or get a new head, Maybe stage 2 SE kit.

 

Cheers Finny

 

Heli-coil inserts work just fine so long as they're fitted correctly.

But what you say about thinking that the piston has hit the plug needs thorough investigation before choosing any of the options.

 

(just thinking out aloud,  has the whacked plug still got a washer on it? )

A wise man once said "if plug thread fucked, take opportunity to enhance engine" I doubt the piston kissed the plug, more likely to have hit something when it flew out. that would be confirmed when you swapped plugs over and it was OK. unless, of course, you did not start it?

Mate, sounds like the incorrect plugs were sold to you. Incorrect thread length and maybe even diameter. If it did get hit by the slug then DaveP has the right idea.
Regards