Stihl spark plug problem

[Robert kirkland]

Hi everyone, I have a stihl ms391 and the spark plug threads are damaged on the engine side i think is there a remedy for this ? Probably come loose and blown out

[Stubby]

28 minutes ago, Robert kirkland said:

Hi everyone, I have a stihl ms391 and the spark plug threads are damaged on the engine side i think is there a remedy for this ? Probably come loose and blown out

Helicoil or similar . Spud used a better one but I cant remeber the name . 

[Alan Smith]

Timesert is usually considered to be a bit better than Helicoil.

[Stubby]

53 minutes ago, Alan Smith said:

Timesert is usually considered to be a bit better than Helicoil.

Thats the one. 🙂

[spudulike]

TBH, both helicoil or Timesert will do an effective job if done correctly. One bit of advice is to use a decent quality tap and usually a taper one otherwise there is a distinct chance that the tap will go in at an angle causing sealing issues.

I use Volkel taps as they are decent and not OMG expensive, avoid all these cheap Chinese alternatives as the job is as strong as the quality of the receiving thread.

[Mark_Skyland]

4 hours ago, Alan Smith said:

Timesert is usually considered to be a bit better than Helicoil.

From what I've read they are the best option. Expensive but good. I've repaired a fair few cylinders using a helicoil and they haven't yet failed. I will only do this for a good customer and I wont guarantee the work as opposed to replacing the cylinder which I know sounds like a cop out but when someone has already stripped the threads before I'm not having them come back in a year with the same issue and blaming me. New cylinder, its 100% their fault.

 

 

[Macpherson]

Personally for thread repair in general I see a Helicoil as the proper engineering solution for stripped threads, and some of the top engine builders use them as standard in alloy cases in new engines.

 

But I agree that with a short reach sparkplug with only 4 or 5 threads a Timesert is probably a better solution for most folk that don't have the correct tooling or experience.

 

Without trying to blow my own trumpet over my working life I repaired thousands of damaged threads and I can say that I detested finding Timeserts in casing threads of an engine that I was repairing for every possible reason. 

 

 

[Muddy42]

I've done a few helicoils and they have lasted fine.  I use cheap taps but I go slow, use lube and lots of reversing to manage the swarf.  On that check for metal swarf in the cylinder, cotton buds work well for picking it up.

[Stubby]

I helicoiled the spark plug thread in a VW Fastback once . Took the engine out to get at it . I asked an old VW air cooled expert ( long time dead now ) called Rudolph Krouzer how I should get the swarf out . He said put the engine back in and start it up . The mag/alloy swarf will just burn away . It did . 

Edited 18 hours ago by Stubby