Chainsaws

Joe,

I have Not worked on a Model 51, but I have worked on other Husqvarna's as well as many other brands and models. Here is what I always do to find and solve  a problem  with a chain tensioner:

First, clean everything, Bar and chain, tensioner screw and nut, bar shim plates, and clutch brake cover.

Next, Remove the tensioner screw and separate the nut from the screw. Lay the tensioner screw on a flat surface (I use a tool anvil but any flat surface will do), roll the screw back and forth on the flat surface and see if its bent. I have replaced alot of bent tensioner screws, and usually what happens is the nut binds up on the bent screw and feels tight when your turning the tensioner screw but will not tension or move the bar.

Next, Check the tensioner nut, or on the model 51 part #  501 45 41-01, and make sure the liitle tab that extends into the bar slot is not badly worn, and possibly no longer extends into the bar. It has to extend into the bar, when everything is put together with the bar inplace or it will not tension.

Also remember, to properly tension, your chain has to move freely on the bar, (Thats why you need all the parts including the bar cleaned), then as you turn the tensioner screw, since its captive by the chainsaw housing, the nut should not rotate but move forward and back on the threaded screw. When you put the tensioner screw and nut back together (I always use a little lithium grease on the tensioner bolt threads) make sure that once the screw and nut are in the housing that the nut does not simply rotate, if it does you may have a worn chainsaw  crankcase housing where the nut is suppose to be captive and only slide to and fro. If that is your problem, you may have to get creative with a little plastic epoxy, and a thin narrow strip of metal (I use tin soda cans cut into strips, then epoxy into the channel where the nut fits, just don't fill it with too much epoxy or you wont be able to get the tensioner nut too fit, I hope that makes sense, and alot more reasonable then buying a new crankcase housing.

Refer to the linked IPL drawing "J", which will show the tensioner screw, nut, and crankcase housing: /images/ddoc/HUSI/HUSI2000_AAaa/HUSI2000_AAaa_I0000076_.pdf

I hope this helps,

PS I am not a Husqvarna Representative or an Authorized Repair, just an experimental chainsaw person.