"The Chamberlain 41C5141-1 Square Rail Trolley Kit is a replacement part manufactured specifically for the use with Chamberlain garage door open. This trolley is made of heavy duty materials to guarantee that your garage door operates as it should and for a long time. It is very simple to install and is compatible with other Chamberlain models to bring a better performance to the gara... https://www.wholetoolbox.com/chamberlain-41c5141-1-square-rail-trolley-kit