Stihl ms251 Spark plug thread repair

[Weesa20]

Hey guys- -any guidance on using a helicoil or similar to repair spark plug threads on Stihl ms 251? Guidance on technique? I guess I'll need to pull the cylinder?

Thanks.

Weesa

 

[ironman_gq]

Definitely pull the cylinder, I don't like helicoils I much prefer the insert type like timesert. Very easy to use, they have a piloted reamer that threads into the existing plug hole and it cuts and threads the hole for the insert. Thread in the insert with a little sealant on the threads and use the expander tool to expand it, let the sealant cure overnight and get back to sawing.

 

[HarleyT]

 

[HarleyT]

Here is the ms251.

 

[HarleyT]

I use helicoil myself.

 

[alexcagle]

I use timesert, but I only have the 14mm at the moment. If more saws start having 10mm, then I guess I'll bite the bullet and buy the kit.
I install the inserts with the cylinder on the saw. Unless I happen to be taking the cylinder off anyway. Just position the piston where the exhaust port is barely open, and pull the muffler. Wheel bearing grease on the tap will trap a lot of the shavings, then hold the crank while you blow through the exhaust port to expel any errant pieces. Then alternately blast compressed air in the spark plug hole, then the exhaust port several times. When done, pull the starter a few times slow, then fast to confirm that there are no hang ups.
The soft aluminum won't hurt anything, at least I've never seen evidence of the contrary. I've done dozens of them too.
If you are OCD, or just scared to try it, then hang the saw upside down while tapping.
The timesert uses 266 Loctite sealer. I don't really bother letting it cure before starting, the heat cures it.

 

[Weesa20]

Thanks guys. I pulled the cylinder off last night as I bought it off CL for $80 in this state and was worried about where the original threads went plus wanted to check out the innards before dropping $$ to repair the hole versus buying a new and P&C if scored. Piston looks perfect with manufacturing lines still on it. Bought the m10x1.25 timesert kit on amazon last night. Hopefully back together by Monday night.

Weesy

 

[rocketnorton]

home made "timesert". cutting wax traps cuttings/chips better than grease, imo. you need 18m1.5 & 14m1.25 taps, corresponding tap drills, 5/8 & 1/2 iirc., + 18mm plug. ngk ab6 in this case, hi heat Loctite.

2100 husky is victim here.

 

[alexcagle]

home made "timesert". cutting wax traps cuttings/chips better than grease, imo. you need 18m1.5 & 14m1.25 taps, corresponding tap drills, 5/8 & 1/2 iirc., + 18mm plug. ngk ab6 in this case, hi heat Loctite.

2100 husky is victim here.

View attachment 615915 View attachment 615916 View attachment 615917 View attachment 615918 View attachment 615919

*
Can you expound on that?
Are those spark plug adapters, or is that what you mean by "homemade"?
*
I've encountered several saws in the past that have had inserts already installed, but had also stripped out.
I had no other option to oversize again, but this looks like it could be a solution.

 

[rocketnorton]

the insert is made from the 18mm plug.
cut crimp off just above hex
knock porcalin out w/punch
drill out what's left & thread 14m1.25
after tapping, cut to length
hole in head needs to be 5/8 to tap 18m1.5
hi heat Loctite hole &/or insert, screw into head using 14mm plug.

 

[alexcagle]

Excellent. Thanks.
I love MacGyver tips.

I didn't know there were 18mm plugs.
What's that 18mm plug from, an old tractor?
When I "engineer" things, I try to make it look as though it could have came from the factory that way if possible .
When I see my handywork still holding and working perfect after a couple years, it gives me a warm fuzzy feeling.

 

[rocketnorton]

no idea what the plug goes in. some ol ferds used big [tapered seat] plugs.

I did same repair on 797 bout a yr ago. sfsg. even tried to get the insert out of it. wont move. time will tell...

 

[rupedoggy]

the insert is made from the 18mm plug.
cut crimp off just above hex
knock porcalin out w/punch
drill out what's left & thread 14m1.25
after tapping, cut to length
hole in head needs to be 5/8 to tap 18m1.5
hi heat Loctite hole &/or insert, screw into head using 14mm plug.

Can't you just use the bigger spark plug after you tap it out or does it hit the piston? Mike

 

[earlthegoat2]

That's a neat solution.

In OPs case though, the spark plug the MS251 uses is M10. To use your homemade solution the OP could probably start with an M14 plug to use as the insert.

 

[rocketnorton]

Can't you just use the bigger spark plug after you tap it out or does it hit the piston? Mike

that ab6 is a tall plug. too tall for the 2100 & 797 ive done that to. sure it would work on some.

 

[rocketnorton]

That's a neat solution.

In OPs case though, the spark plug the MS251 uses is M10. To use your homemade solution the OP could probably start with an M14 plug to use as the insert.

didn't know bout 10mm. in that case, id just tap it 14mm, use 14mm plug.

 

[alexcagle]

A little off-center of topic, but does anyone know what size and pitch heli-coil works for standard decompression valve hole that's stripped?

 

[SteveSr]

Interesting concept but what does this do to the heat range of the plug now that it is surrounded by much more steel than was intended in the original application?

 

[L34]

I think you cut the adaptor to the correct length before fitting.

 

[heimannm]

For the compression release, most Euro saws are 10 mm (1.25 or 1.5mm pitch, I'm not sure which but my 10 mm bolts fit properly) and most domestic saws are 3/8-24.

McCulloch uses a 3/8-16 for the PM800 saws for a short while before adopting the "Q" port instead of compression release.

Make sure the helicoil is threaded in just below the surface or the valve seat will never seal.

Mark