Hey guys,

Recently i stripped one of the rear spark plug holes. I can get the spark plug to grip but it will keep spinning and not lower itself into the thread. The spark plug is sitting roughly 0.5-1 cm above what it should and the spark plug lead will not seal into the head.

Is heli-coil a safe option? can anyone explain to me exactly what i would need to do and what size heli-coil i would need?

cheers

Wil

You'd need a heli-coil of whatever size thread of the plugs you are using. NC30/35 use 8mm plugs, not sure about other NCs. What you do is drill out the damaged thread to the size specified in the kit (pretty sure the right drill bit is supplied), then tap a thread into the new hole. You then use a tool to screw the Heli-coil into the thread you've cut in the head, and once it's below the surface, you can take the tool out. There's a small tang on the end of the Heli-coil that enables you to screw it in. This is removed with special tool or a punch, as it is scored to enable it to be knocked off.

Needless to say, it's not a quick job as you'll need to take the head off to do the it, otherwise you'll end up with loads of swarf in the engine.

For the sake of removing the head, drilling helicoiling etc id just buy a replacement head,

Depending on whether this is your pride and joy, I once knew a bloke who got steel putty and smeared it on the thread before jamming the spark in there, it was a single stroker beach bike though worth about 50 quid . . .

porndoguk wrote:For the sake of removing the head, drilling helicoiling etc id just buy a replacement head,

Well, that is of course the simple option, however how do you know the head you're getting isn't going to turn out to be a pile of crap that's been thrashed to within an inch of it's life? I'd be worried that the engine has been broken because it's knackered, rather than because it was unwanted. Still, I am one of these people who believes in fixing rather than binning if it's an option.

i notice that there is a front and rear cylinder head? is the rear cylinder head easy to remove? i have the haynes manual around but i've never tried to remove a head before

There's more access to the rear than the front. It's 'only' nuts and bolts, but there's potential for it to be a pain in the bum. You'll have to strip the bike right down so you can remove the carbs. You'll probably have to remove almost the whole exhaust except the front header pipes. There appear to be eight bolts holding the head on, and to get at them you'll need to have the cam cover off. Depending on the condition, the various seals and gaskets in the exhaust system and on the cam cover may or may not need replacing.

The Haynes will probably explain all, I don't have it to hand. I don't think it's going to be an especially quick job, for sure!

Definitely not a quick job. The bike will be down for a few days is my guess. Hopefully the thread repair will not go pear shaped.

Tap it out next size up? Maybe ok to do it with the head still on? Tho it would be best to take if off really! Depends on how desperate you are to fix it? What bike is it?
Cheers
Ian

There's no way I'd tap it on the engine. All the metal you remove is going to drop straight into the cylinder, surely?