Bolts on a Stihl MS290 Farm Boss

[EWBTus]

I have a Stihl MS290 Farm Boss that needed a new cylinder. The issue I am having is with the head bolts. The originals appear to be 7mm X 1.5 pan head TORX bolts but the threads in the new cylinder (and I have tried 2) appear to be 6mm X1.0. Also, one of the new cylinders has what appears to be a second but smaller spark plug hole. The cylinders claim to be compatible with the original 1127-029 Mahle cylinder but are not due to the head bolts.
Thoughts?
EWBTus

 

[Bob Hedgecutter]

Your saw came from the factory with a non standard system- the cylinders are drilled- but not tapped- the fatter coarse thread screws you have are the original self tapper ones fitted into untreaded holes at the factory when the saw was assembled.
Aftermarket cylinders come tapped for 6mm X 1.0 bolts- so you need to get yourself some new 6mm bolts to make it work- or try to make the original screws tap through the threaded holes and keep your fingers crossed.

Second smaller "spark plug" hole is for a decompression valve to be fitted.

 

[EWBTus]

Much thanks. Does tapping the new cylinder to the original 7mm X 1.5 work? I am not as familiar as I should be on the untapped diameter of metric fasteners. My original saw didn't come with a decompression valve. Do I need to get one or can I just plug the hole?

 

[Bob Hedgecutter]

Much thanks. Does tapping the new cylinder to the original 7mm X 1.5 work? I am not as familiar as I should be on the untapped diameter of metric fasteners. My original saw didn't come with a decompression valve. Do I need to get one or can I just plug the hole?

Personally- I do not know if it will work. I cannot say the thread on the self tappers is 7 x 1.5 and where do you find a tap?
I have never tried to fit original bolts into anything but an original cylinder, have never used a cheap aftermarket cylinder on an 029/039.
Would it not be easier to find 4 new cap bolts of the same length in 6 x 1.0? Granted they are more likely to be hex head than torx- but what the hey.

Yes, decomp valve heads can have the hole plugged and you can run without one.

 

[EWBTus]

I ran down to the local ace and fortunately was able to pick up socket heads in the 6mm X 1.0 size. I had asked the tap question in case I couldn't find them. Ace costs more but they do have a lot of odd nuts and bolts. I'll plug the decomp valve.
Truly appreciate the help.

 

[Bob Hedgecutter]

See, over here we think like the Germans and use metrics- so they are not hard to find or odd.

 

[Bob Hedgecutter]

Now, just before you get too carried away- do you know what and have you rectified what had taken out the original top end?

 

[EWBTus]

I live in Arizona so typically I only used my chainsaw in the fall and winter. When I went to use it this time the starter cord wouldn't move. I took the saw apart and the cylinder and piston were covered in a thick gum. I cleaned everything in my parts washer and then broke a ring trying to put it back together. When I looked online for a set of rings I found a replacement cylinder, piston, ring, etc. was ~$30 and had close to a 1000 "likes" and a score of 4.6 out of 5. So I thought - what they heck - I'll just replace the whole thing. I suspect there is a proper way to put the saw away instead of what I did last year.

 

[Bob Hedgecutter]

We do not have corn syrup fuel- so I'm guessing you should perhaps run your saw dry and drain the fuel before storage.
Do not be surprised if the new top end makes less power than the original.
The cheap versions run- but seldom do they run as well.
Clean, keep the original cylinder and piston- order new rings and do not throw away the original cylinder bolts.