I have a HAYTER POWERTRIM |(407C001001), engine runs well. 

Unfortunately, the Guide Assembly, Metal Height () disintegrated due to rust. I have a replacement but cannot remove it from the Shaft, Cutting (). 

Does anyone have experience of changing the guide assembly/metal height? or advice.. 

been trying for a year, getting old. 

See attached photo. Thanks.

I gather it's the bolt and cupped shaped washer your trying to remove?
 
. A pic of the replacement part might help too.
 
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Thanks for the reply.
 
Yes, that bolt is welded to a metal cup which broke and screws onto the shaft.
 
I've attached photos of the older metal cup and the replacement.
 
Since posting this YouTube has been offering videos. One suggests using a 50/50 mixture of acetone and motor oil to loosen the bolt.

I've had it soaking in oil for days at a time previously to no affect.
 
 

Couple things,
 
      First, your going to have to lock up the engine to keep the crank from turning- Do something with the flywheel.  Any looseness, no solid "stop" (when turning loose) will defeat you..
 
Can you hammer a 6 point socket onto what appears to be a nut?  With the crank locked up, you may be able to crack it loose.  Some medium impacts to the bolt can help loosen it. 
 
A strong air gun may work.
 
If you access to torches, or at least MAP gas, heating the crank , where the bolt goes
thru should help loosen the grip...
 
There's some other options, but try ^^ first..
 
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