hello all
when i tighten my no 6 spark plug it alwase comes loose again..
what would be the cause , how would i fix it???
thnax all

Either:

Faulty spark plug thread. Cheap

Damaged thread in head where spark plug mounts. Expensive

meh mate dw about it...locktight it n pray

Yep, pretty much!
If you put spark plugs in or take them out slightly wrong, it can damage the plug and/or the mount thread.

The best thing to do is try a new spark plug. If you still have the same problem, then it's probably the mount thread.

Good luck!

Are you able to supply sufficient torque while tightening the plug? If so, and I disclaim this as theory only, consider this;

Prior to reading this - examine the offending plug for any thread damage or deformation - any variation along the plugs thread should be cause to start-off with a fresh plug plug. You could swap plugs around to prove/disprove this possibility - read on.

A Vee-thread's tightening ability is based on both its wedging tendancy toward its mating surface (by virtue of its angular geometry and percentage of actual surface contact) and the corresponding surface area of shoulder contact that the mated thread is being tightened against. The tapered mating surfaces of both the plug thread and the tapered mating surface that the plugs shoulder butts-up against has a dual effect of both centering and sealing the threaded plug (facilitated in by maximum thread surface contact relative to those mated vee-angles)
Consider now heat expansions of both plug and head and their differing co-efficents of linear expansion (expansion rates) - by virtue of there different manufacturing materials. Varing rates of heated metals expansion, contraction and cooling rates producing a push/pulll effect against those previously discussed mated surfaces.

Just for a laugh, after you thoughly clean the the surface where the plug will mate into the head, colour it in with a black permanent marker. Screw the plug in carefully and torque it up, then remove it for inspection. Are there any small or uneven bright spots to the mating surface evident? Consider also that carborised oil or dirt in the threads could by creating sufficient resistence to prevent the correct torque being supplied to the plug (maybe work it over with a little carbi cleaner and a tooth brush).

It could also be, as previously posted that some thread damage through either wear (thread grease people) or damage; that a minimum thread surface contact is contributing to the problem (If the plug is as loose as a prick in a shirt sleeve right up until final torque-up its a far bet that only a portion to the thread - likely at the crest is in contact). In this case you could consider trying a different brand of plug. The variation in manufacturing tollerances between various brands - ie Thread profile, diameter and thread length might be sufficient to take up some of the limitations imposed on you). You could prove this with the same permanent marker applied to the treads of the spark plug.

Those high (bright) spots previously meantioned can be eliminated by either some careful de-burring or scraping. Obviously block the thread appeture in the head prior to commencing and re-examine with the marker regularly throughout the operation.

Ensuring clean lubricated threads and clean mating surfaces is your best start - Good luck.

The head may be stripped, alloy is fairly soft it may have been overtightened

taking out the spark plugs when engine is still hot can damage the mounting thread.

If it is stripped, get it helicoiled
Exact same problem happened to me with my old alloy headed XF.. worked a treat! Only cost me $40 too!

I agree, helicoils are a superb invention.

what are helicoils sorry

could be that the plug was overtightened, stretched the thread and stuffed the plug seat.
this had happened to me when I got mine, just bought some XR6 NGK's fixed it

yeah i agree, they've gotten me out of many situations
i've never used them on heads, but if it'll work i'd go for it too

Helicoils are a thread replacement. Basically you bore out the old thread to a larger size and re-tap it to the outside thread of the helicoil, then your bolt/sparkplug threads into the inside of the helicoil. Not a job to do with the head still on.

Yeah I have had this problem a few times. At first it just seems that the plug isn't going to fit in properly and is loose after the last turn. But what I do next is I undo it back out,then basically stand over the plug and make dead certain it dosent move while tightening it in slowly. Then it works and seems to e fit in again Tightly. I think because it is such a reach to the back off the No#6 Cyl it is easy to sort of cross-thread it. Sometimes I have this issue on Number 4 Cyl as well.

Any decent mechanic should be able to helicoil the the plug thread with the head on.

I have done hundreds of them and never taken the head off.

Use grease liberally on the drill bit and on the cutter and all the shving stick to the grease and not fall into the bore.

Easy done