MS260 chicken or egg

[openspaceman]

Here's one I could use an opinion on: one of the guys brought in his ms260, which he had used for several years, as it was down on power. It was one I wanted to see as he had tried one of the Oregon 95TXL and found the clutch cover got hot. Anyway as I surmised his clutch sprocket was woefully too worn and the new chain would have been riding on the wall of the rut in the sprocket.

 

 

For its age the saw is in very good condition with none of the damage I often see from being dropped down onto ballast however the spark plug was loose and when I took it out there was a build up of aluminium on it.

 

 

The exhaust port had a similar grey encrustation and the piston rings were lightly seized in the ring grooves at the exhaust side.

 

The big problem is the piston is worn away at the skirt on the exhaust side and a piece has chipped off.

 

 

The bore has slight scoring at the exhaust port and there is a bright spot on the squish band on the exhaust side with a corresponding bright spot on the piston, the base gasket was intact.

 

 

I initially thought play in the bearings had allowed the piston to hit the squish band but can feel no play in the bottom end. I showed it to our Stihl dealer's mechanic and he thinks the 1.5mm wear on the skirt is enough for the piston to rotate on the gudgeon pin and hit the head at TDC.

 

Two questions:

 

Why would the piston skirt wear so badly? The Stihl dealer says fine wood dust entering the intake builds up and pushes the piston toward the exhaust side.

 

Was the loose plug the cause of the trouble or a symptom of the piston hitting the head?

 

We have replaced the saw with a new MS261CM which the guy is very happy with. I'm trying to decide whether to repair with a new piston or put it back together with no piston and do a pressure check. I don't think it is worth my while replacing bearings as I have no tools for splitting the cases.

 

Finally does the MS261 clutch and rim kit fit the MS260 as they appear to be different part numbers, as I have a spare 261 kit?

[njm]

Here's one I could use an opinion on: one of the guys brought in his ms260, which he had used for several years, as it was down on power. It was one I wanted to see as he had tried one of the Oregon 95TXL and found the clutch cover got hot. Anyway as I surmised his clutch sprocket was woefully too worn and the new chain would have been riding on the wall of the rut in the sprocket.

 

[ATTACH]211742[/ATTACH]

 

For its age the saw is in very good condition with none of the damage I often see from being dropped down onto ballast however the spark plug was loose and when I took it out there was a build up of aluminium on it.

 

[ATTACH]211743[/ATTACH]

 

The exhaust port had a similar grey encrustation and the piston rings were lightly seized in the ring grooves at the exhaust side.

 

The big problem is the piston is worn away at the skirt on the exhaust side and a piece has chipped off.

 

[ATTACH]211744[/ATTACH]

 

The bore has slight scoring at the exhaust port and there is a bright spot on the squish band on the exhaust side with a corresponding bright spot on the piston, the base gasket was intact.

 

[ATTACH]211745[/ATTACH]

 

I initially thought play in the bearings had allowed the piston to hit the squish band but can feel no play in the bottom end. I showed it to our Stihl dealer's mechanic and he thinks the 1.5mm wear on the skirt is enough for the piston to rotate on the gudgeon pin and hit the head at TDC.

 

Two questions:

 

Why would the piston skirt wear so badly? The Stihl dealer says fine wood dust entering the intake builds up and pushes the piston toward the exhaust side.

 

Was the loose plug the cause of the trouble or a symptom of the piston hitting the head?

 

We have replaced the saw with a new MS261CM which the guy is very happy with. I'm trying to decide whether to repair with a new piston or put it back together with no piston and do a pressure check. I don't think it is worth my while replacing bearings as I have no tools for splitting the cases.

 

Finally does the MS261 clutch and rim kit fit the MS260 as they appear to be different part numbers, as I have a spare 261 kit?

 

Loose plug would lead to the saw running lean , Husky 550 and 560 are very guilty for digesting dust but i doubt very much this pushes piston towards the exhaust !! it will wear out the piston leading to slap and skirt failure IMO , 1.5MM is a horrendous amount of wear on a skirt the saw must have been rattling , no doubt the crank and bearings are well shot , from the pictures i would say the saw was running lean and has been used very hard

Edited September 25, 2016 by njm
info

[flatyre]

i'm no mechanic but would the piston not hit the end of the plug before the head?

[njm]

i'm no mechanic but would the piston not hit the end of the plug before the head?

 

No wont hit the plug

[flatyre]

my bad:blushing:

[openspaceman]

my bad:blushing:

 

 

Not at all, it was a good question and NJM provided a good answer, with a picture painting a thousand words.

 

You will see if the piston skirt wears thin the piston can wobble around the small end bearing and then the clearance at the squish ban reduces.

[Stubby]

Not at all, it was a good question and NJM provided a good answer, with a picture painting a thousand words.

 

You will see if the piston skirt wears thin the piston can wobble around the small end bearing and then the clearance at the squish ban reduces.

 

Reduces on one side but increases on the other .....but when it goes bang it evens out to a little more both sides

[openspaceman]

Loose plug would lead to the saw running lean

 

 

 

Yes I thought that

 

, Husky 550 and 560 are very guilty for digesting dust but i doubt very much this pushes piston towards the exhaust !!

 

OK

 

 

it will wear out the piston leading to slap and skirt failure IMO , 1.5MM is a horrendous amount of wear on a skirt the saw must have been rattling ,

 

Yes I'm surprised it was not noticed earlier but the guy is very conscientious, just slipped up a bit on the clutch sprocket but I can understand some reticence to change parts because it's a ballache trying to get spare parts approved by the accounts department. I wonder how long the spark plug was loose and I reiterate would the piston wear and touching the head have been the cause of the spark plug coming loose or the weak mixture from air sucking past the loose plug being the cause of the piston wear?

 

The thing is I am not a fan of frequently checking the plug and they don't come loose for no reason, so if it hadn't been checked recently...

 

 

 

no doubt the crank and bearings are well shot , from the pictures i would say the saw was running lean and has been used very hard

 

Yes it would have been used hard which makes me think it was not a chronic problem. As I can detect no play in any of the bearings and most of the aluminium would have found its way out without getting into the crankcase I wonder.

 

However Is it worth 20 quid for a meteor piston and an hour's time to put it together or and a couple of hours' time to run a pressure test?

[openspaceman]

Reduces on one side but increases on the other .....but when it goes bang it evens out to a little more both sides

 

Stubby I don't understand that, sure it has reduced on the exhaust side and slaps around plus one can see aluminium build up on the plug, piston, head and exhaust port but no signs on the inlet side. There is a slight recent scratch on the inlet side and a "polishing" mark on the ring lands by one transfer port which I took to be from the triangle of aluminium from the piston skirt being sheared as it left the crankcase. The piston inlet side is just worn smooth from long use.

 

It didn't go bang and despite the rings being stiff in the grooves they were not gouged and the saw actually still ran. Most I get back are horribly seized, either because someone was sloppy with adding oil to the fuel or mostly from someone tweaking mixture a bit lean, a problem I should not see with the MS261CM.

 

Incidentally has anyone else had problems with the FS460C EM with the carb and engine management/ignition module failing just after one season, and in our case a few days after warranty expiry. The fitter told me, on the side, they had had a number refused for warranty claims which required ignition unit and carb at £250 because they would not pick up from tickover.

[spudulike]

The 026/MS260 are susceptible to piston slap and if left, the piston wears to the point the piston skirt can shatter. I put this down to the cylinder not being extended in to the crankcase as on some saws which does reduce this.

 

The wear on the piston and all that aluminium will have found its way in to the main bearings and will normal leave them in a real bad way. You can try flushing them out but from what you say, it sounds like they are past reclaiming.

 

Looks like a full rebuild to me, the cylinder may be OK if the plating isn't worn through so check this thoroughly and clean the ports and bevels before reuse.