grrrr - sparkplug fail :(

[buffalo606]

In the past month both a 261 and 660 blew their sparkplug out - kindly threading the hole.  I've got a piston kit going in the 660 and trying a helicoil in the smaller saw.

 

It's mildly annoying and costs a bit, so wondered how to reduce the chances of this happening again?  As the cheap sparkplug is always going to win against the softer metal of the expensive engine. 

[Paddy1000111]

Are you torqing it down properly? Unless they're being cross threaded when put in, done up too tight or coming loose they shouldn't strip? The only other cause is worn out threads from continuous insertion/removal? 

Edited February 3, 2021 by Paddy1000111

[spudulike]

The plug shouldn't undo if done up tight, the HT cap should also help hold it in place. That and checking it during regular cleaning will help.

BTW, leaving it loose to make the engine run better is a load of bollocks from a past age!!

If you get an engine where the plug keeps coming loose, you could try a little thread lock on it but it really should not be needed but is better than the thread being taken out.

Oh - one last thing, use a quality tap and not a £6 ebay one. The lead-in (pilot nose) taps are good and Vokel make some nice quality taps - cheap taps give cheap results and "cheerful" isn't included!

[Stubby]

Just the pedant in me but when you " thread it " you are making a thread . Its when you strip the thread that you have a problem .

[skyhuck]

17 minutes ago, Stubby said:

Just the pedant in me but when you " thread it " you are making a thread . Its when you strip the thread that you have a problem .

Thank you!!! Was driving me nuts too!

[peatff]

So the thread in the cylinder failed not the spark plug stripping the thread. I bought a MS460 with a stripped thread someone had tried to repair, you could have fitted a bigger plug in it by the time there was enough metal left to cut into. I bought a genuine Stihl cylinder and piston and when I stripped it found the piston in it had a cracked land so it needed the full repair.

[buffalo606]

20 hours ago, Stubby said:

Just the pedant in me but when you " thread it " you are making a thread . Its when you strip the thread that you have a problem .

Absolutely correct - doh

[spudulike]

He meant de-threading, stripping the thread or in engineering terms.....Oh F£$k it!!!!

[Paddy1000111]

18 hours ago, spudulike said:

He meant de-threading, stripping the thread or in engineering terms.....Oh F£$k it!!!!

On soft metal caused by over eager tightening. Tighten up till you hear a crack then back it off 1/4 turn and leave it for the next guy 😂

 

Or the german torque limit-  Guten-tighten

Edited February 4, 2021 by Paddy1000111