Hitchcocks Forum

I thought something wasn't right when I fitted the spark plug on my last sevice. This time, when I went to change the plug I saw the thread was stripped & the new plug kept turning. Does this mean a new head or can it be repaired?

Any decent engineering shop should be able to repur it for a reasonable price, by either a helicoil or recoil threaded insert or boring it and pressing a bush in and rethreading it .
This should not cost much.
Most motorcycle shops should also be able to either fit a helicoil or will get it done for you.

Don't buy a new head

Regards

Rich

A local place (Nottingham)small works did my head for £15..I took just the head in and said no rush but this week. and he did a insert for £15 well cuft was I,

You can buy helicoil and other insert kits on eBay quite cheaply. The idea is you run the supplied oversize tap down the existing hole and then fit the insert. Sounds simple - but it isn't. I couldn't get the tap to bite and when you look into it more you find the more expensive kits have a much more sophisticated tap which have a lead in from the remains of the existing thread before starting to cut the new one. I gave up and paid an engineering shop £10 to do it for me. No problems since.

Helicoils are good and actually stronger than the original. For a plug, which tends to come out more often than most fastenings, I think a solid insert would be better. You have plenty of room round the plug hole to fit an insert. Helicoils come into their own when you don't have the room.

That's good news, thanks guys.

If there is enough meat around the plug hole, then you can bore the hole out and tap it and machine a bush that you can screw in and drill and tap to the spark plug thread .

Yes pathfinder but you would need a fairly well equipped workshop to do it. As well as a lathe to make the insert you would need a mill (or a good pillar drill) with a tilting table and proper clamps to hold the head.
You could pile in freehand of course but your accuracy would go out of the window.

Just out of interest, would a brass insert do the job you engineers out there?

Yes of course, any small engineering workshop should be able to do this for you.
I would use Phosphor Bronze over brass.

helicoils are good for a on the road repair to get you home.



for a definite repair a insert like time sert or an aluminum insert made on a lathe is better.

time sert interlocks itself in the end, so in can’t go deeper an can’t come back out again.

keep in mind that different materials have different heat transfers, stick with the two above.



if you want it to last a repair like this should be done in a workshop, its impossible to get it straight and true by hand.