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Re: Spark Plug Thread Size
Norcoastal #789537 11/08/19 9:58 pm
Joined: Aug 2001
Posts: 5,234
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With a helicoil you get a sized clean up drill, Timesert the same. Both are way better than fuucked threads. I thought it would be a good idea to fit Timeserts into my old head, the threads were OK, but I knew the plugs had been in and out a lot, there was a bit of waggle about 1/2 way My LBS put Time serts into my A65 head, I watched Ali do it , proper job. , I thought they might be better than helicoils, the plugs are still not a grade A fit , somewhat loose , OK , but some perceptible waggle at 1/2 in ( same as before). I cleaned the plug , combustion chamber interface up, the plugs were more recessed than I like and an old thread was in free space , this would have invited pinking. YMMV. Unless your threads are totally fecked , dinnae bother. A helicoil might be better because it will give original plug depth. If you must do something, have the head bolt hole shoulders spot faced, my centre mount was miles off the batter.

Last edited by gavin eisler; 11/08/19 10:00 pm.

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Re: Spark Plug Thread Size
BSA_WM20 #789539 11/08/19 10:12 pm
Joined: Jan 2014
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Originally Posted by BSA_WM20
Originally Posted by NickL
I think a lot of you lot are a bit anal,, i've fitted successfully dozens of helicoils to cars and bikes over the years
and never had any problems with them. If you are careful and stake the top they work fine.
Just my 2c of course.
I agree inserts are nicer but i've not got any.


Helicoils are by nature not oil or pressure tight so my general rule is solid inserts when oil tightness or pressure tightness is essential, wire inserts everywhere else.
Having said that I do fit Recoils to the plug holes on mowers & chainsaws but a leaky plug on your lawnmower aint going to leave you stranded in outer whoop whoop.

All of the head bolts on the M20 are helicoiled as are most of the gearbox holes.
However the sump gets solid inserts.



No parallel thread is guaranteed to be either gas or pressure tight, that's why sealing washers exist. I've never seen a tapered threaded spark plug but itv wouldn't surprise me if
the idea was tried at some stage. Most plug threads are stripped due to over-tightening, for some reason people think they are required to be 100 ftlb.
The last time i had to personally use a helicoil was at a race meeting, on checking plugs after practice my passenger was a bit heavy handed when replacing one.
I used plenty of grease on the helicoil tap and did the job in-situ. We then went out and won both our races for the day. The head still had that heli-coil in when the bike
was sold many races and years later.
I suppose you blokes would have abandoned the race meeting, driven home, taken the head off, taken it to a machine shop, etc. etc. (No bloody way .)

Re: Spark Plug Thread Size
Norcoastal #789543 11/08/19 11:26 pm
Joined: Dec 2013
Posts: 5,117
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i helicoiled a spark plug once on a small block chevrolet motor without taking it apart. never saw it run afterwards but the job was straightforward.


every day you do not take a chance is a day of your life that you will never get back.
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