The LH spark plug thread on my Le Mans Mk3 is loose, and the plug will screw in but won’t tighten.
What’s the best remedy? A helicoil? If so, does the head need to come off?
Yes a thread insert can be put in, like a Helicoil, but I would recommend that you remove the head to fit one as there will be swarf produced by clearing out the old thread and tapping a new thread for the insert to sit in.
Thanks Chris, I’ll give it a go!
I’d be taking/sending the head to an engineering shop for that. In fact I’m pretty sure I’ll be doing exactly that with one side of Blue this winter.
Machined thread inserts like Time-sert or E-Z Lock are better than Helicoils which are wound wire (a bit like a spring).
There’s a choice here: http://www.threadrepairkits.co.uk/
But the best solution is an engineering firm with proper machining facilities
I agree with the last two posts, unless you are set up with a decent workshop and tools take the head (or the whole bike) to an engineering works.
Best of luck
Yes, I’m taking the head to my local garage, which is run by keen bikers
Job done! Head removed, Timesert insert fitted by local garage, back together and running!
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Well done 
Sorry for gap in reply date - only recently joined.
I’ve done plenty of sparkplug helicoils in situ. Simply turn engine over to blow out swarf, makes for a good show! Never had a complaint afterwards in 9 years.
What I have done, it was an exhaust stroke set, so exhaust valve opened, and during such work hoover connected into exhaust port. Due to constant draft any swarf migrates out before it sticks to anything inside.