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rgnprof 02-07-2009 09:46 PM

'86 560SL spark plug thread problem
I am having trouble installing one plug - I just can't get it started. It looks like most of the threads are there, just the one right at the top of the hole seems to be messed up. Can I try a thread chaser - anyone think that may work? Or do I risk messing it up more?

Is this a 14 x 1.25 thread?


Arthur Dalton 02-07-2009 10:03 PM

<Is this a 14 x 1.25 thread?>

If you are handy, you can make a chaser from an old plug by filing a groove lateraly on the thread at c/l with the flat cutting edge on the tightening direction side.
If you are not, then get the tool....and go slow, turning backwards first to seat the's feel thing.

rgnprof 02-07-2009 11:26 PM

Thanks Arthur,

I will probably buy the tool - I'm just not sure if I need the thread chaser or should I get a tap?

I think I might try a thread chaser first - and if that doesn't work, I might then try the tap? Does that make sense?


Arthur Dalton 02-07-2009 11:38 PM

Hard to tell w/o seeing it...screwed up threads are on a one on one basis..they all take some finesse and everyone is different..the rule is slow and easy...........and don't force anything. Try to keep the tap or chaser straight up at 90 degrees on x and y axis is of the utmost can do it..... try an old plug by turning it backwards untit you feel it click in and then tighten...that sometimes works, but if they are bad , it will require a tap.

Strife 02-07-2009 11:49 PM

I would buy the tool (I have one, I use it to chase out the old anti-seize).oil the heck out of it, and operate it with your fingers ONLY a little at a time once you have it started right. A mistake here is very, very expensive. Take your time!

rgnprof 02-08-2009 04:49 PM

Got it!

Tried the thread chaser first, but no luck. So I decided to tap it - slowly - with lots of oil and grease (to catch the debris) and it works fine now!

Thanks for the help.


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