MS361 Problems

[BenC]

Hi

 

Im having problems with the exhaust bolts snapping. I put new bolts in and 2 days later they snap off. I then extract the bits of bolts out and put new ones in. The bolts snap in different places each time and have been replaced 3 times. Does anyone have any idea why this is happening.

 

Regards

 

Ben

[AndrewS]

Hi Ben,

 

Assuming they're genuine spares, or of known origin and not far-eastern plasticine, I would guess that they may be too tight. It they're starting to yield at room temperature, thermal expansion of the exhaust as it heats up may be enough to stress them to the point of failure.

 

Best,

 

Andrew

[Megatron]

Are they genuine bolts? Have you installed a new muffler gasket? What about the bolt holding the muffler to the case and are the threads still in good order in the cylinder?

[BenC]

The parts are Stihl bolts from other Stihl chainsaws. The gaskets are in and if you dont tighten the bolts upto the required torq they undo.

[AndrewS]

Ben,

 

I would replace them with new bolts - I think that will cure it. You're absolutely right, the bolts do need to be tight to pre-stress them so that they don't vibrate loose, but I wouldn't re-use them. They are almost certainly failing through fatigue - they are highly loaded when installed, they are then subject to millions of vibration cycles which will cause any small defect through the section of the bolt to grow into a crack.

 

When you then remove the bolts and refit them, the preload will cause these "micro cracks" to grow rapidly as a result of two factors: 1. the pre-stress in the bolt is effectively higher because the cross section of the bolt taking the load is reduced (the area bearing the crack will obviously carry none of the load, leaving the full load to be carried by the surrounding, sound metal), and 2: because the cracks have extremely sharp edges.

 

The rate of propagation increases almost exponentially because as the crack grows, the load-carrying section reduces and the stress rises until a critical point is reached when the stress exceeds the yield strength of the bolt and it fractures.

 

This is why you never re-use cylinder head bolts for example.

 

Hope that helps? Look up "low cycle fatigue" for a better explanation ;-)

 

Cheers,

 

Andrew

[landyjim]

i had the same problem with mine i and the only thing i found to fix it was to drill it out and tap it i believe it is a m4/5 i cant remember it was a bit ago now but i tuck it out to a m6 and that sorted it out it not easy and u need to know wot your doing i cant say why its happening i'm sorry hope this can help but only do this as a last resort after trying everything else!!

[MattyF]

Recommended torque with thread lock??

[spudulike]

If they are standard allen bolts, use stainless ones!