What's on your bench today?

[Baldbloke]

On the MS461, there is a strange alloy bracket that bolts across the crankcase mouth and goes right up in to the piston. There has been a lot of speculation on what it does but the clever money is on dome sort of diverter creating a strato effect on a relatively standard engine design. See pic below.

I believe that the piston probably had a very fine crack in it after I tried to remove the central boss from the clutch after an employee of the saws owner didn't fix the e clip on correctly. The clutch grenaded and did the boss up super tight. I used pretty much all the force I could muster to shift it and ground it off in the end with a diamond burr after I couldn't shift it - it lasted a few months before letting loose. The stop I used was the Stihl one followed by rope when the damn thing didn't shift.

Got it back together again and hope it lasts now, all the parts looked good and measured OK but it is damn difficult to tell if components have micro cracks or a bit of stress damage. I know a few tricks in checking engine components and agreed with the owner that the repair bill was worth paying to see if it lasted - the other option was a complete rebuild with new components which didn't make sense.

 

Could that strange alloy bracket merely be an additional brace between the crankcases to ensure the crank runs true? Even though the barrel effectively does this, the brace is lower.

[spudulike]

No and definitely NO, it is shaped to go in to the underside of the piston and would have zero bracing effect from its design. It is to do with making the saw a little cleaner and has nothing to do with the strength of the cases. 

[Toad]

I assume it is designed like that and bolted in as they need to machine the crankcase flat for the cylinder to go on. Quite clever design really.

[spudulike]

The picture doesn't show it but the centre section of the flange goes in to the underside of the piston around an inch.

[bmp01]

It's a stuffer then ? To move air-fuel gas around under the piston.... Piston cooling ? Fuel preparation ? Pin bore lubrication ? 

Or can it change the air-fuel movement in the crankcase, for better scavenging or fuel preparation?

 

Is it solid, this bit that goes up under the piston...? Are they reducing the volume in the crankcase (if its solid) or only displacing the air-fuel volume under the piston (hollow underneath)?

 

Got to be the result of some computer modelling I think. 

 

 

[spudulike]

It is a solid alloy of some sort. My thinking is it is added for cleaning up the emissions but wasn't as effective as the strato type saws as the 462 came out to replace it. Some sort of bridge between the 460 and the 462 that gave Stihl a bit of time to develop the later machine.

 

[peatff]

On 27/11/2018 at 09:26, Baldbloke said:

Could that strange alloy bracket merely be an additional brace between the crankcases to ensure the crank runs true? Even though the barrel effectively does this, the brace is lower.

It might help stop the piston skirt wearing on the bore.

[spudulike]

No, it is too an extreme mod just to do something like that. When the saw came out, the Arboristsite guys were trying to work it out and think they eventually came to the conclusion that it cleaned the saw making it more environmentally friendly. They also stated that if they manufactured an extremely clean saw, it gave them extra EPA points so they could make a less clean one and the MS461 was one of those. How true....no idea!

The flange would come in to action on near the bottom of the downward stroke, the carb has no fresh air strato system so I can only think it is to reduce the amount or fuel air reaching the transfer ports thus making the saw more fuel efficient as all the scavenging would have already happened by then - I guess they fine tuned it with the flange rather than changing the size of the piston windows and transfer port sizes which would be more difficult, costly and time consuming.

That is my take on it for all it is worth!

[billpierce]

I did have a vague feeling it was yours, it was running like a bag O S and not revving, the coil sorted it whilst you were supping Prosek in Croatia I believe!

Just whacked a cheap 13quid coil in that arrived today. Appears to be running normally again. Tried adjusting flywheel gap on the OEM one that was giving me bother and ended up with no spark, even when set back up to normal business card gap! So guess it was on the way out. Couldn't bare to spend nearly 90quid on oem coil if they only last a few years!

Could early failure be linked to flywheel or anything else?

[billpierce]

Pressure check the carb. The throttle being open or closed will have no effect on the carb leaking fuel through to the cylinder.

Came back to this, started first pull, ran fine for 10mins. Restarts fine hot or cold. Must be a gremlin somewhere but seems fine the now