Chain does not move on the ms170 stihl chainsaw

I assume you have the correct chain for the bar and that the sprocket on the end of the chain bar goes around. Check the bar for any defects like pinches where the bar has been caught. Check the chain drive cogs for any burs on the edges - often happens if the chain has been run too slack and then come off!

Check the chain drive teeth thickness which should be the same as the chain bar can accept. Check also you have the right pitch chain for the drive teeth on the clutch sprocket. Assuming the chain brake is not on then you should be able to move the chain around the bar when all the slack has been taken out. If the chain and bar checks are all ok then there is a problem with the clutch. With the chain brake off the clutch with the chain drive sprocket on should spin reasonably freely. If not then you need to remove the 'E' shaped circlip in the centre of the clutch drum , remove the washer behind it and slide off the clutch drum. Then remove the roller bearing and be prepared to replace it if it will not run freely. Clean out all the muck in the clutch housing and check for anything obviously broken. If all appears ok then put a few drops of 2 stroke oil on the roller bearing and replace it on the shaft. Clean the clutch drum and replace it on the shaft. Then the washer and 'E' clip and re-fit the bar and chain. Re-fit the cover and tension the chain and try again. If that does not do it then there has to be a problem with the chain bar.