How to hold rear derailleur sprocket arm extended This is about inserting a chain, either a new chain or reinstalling the old one, like after cleaning as in my case.  The rear derailleur arm is fighting me of course because of spring tension.  The chain in my case has a master link now.

Is there a tool, or something I can make, or some trick to hold the RD arm extended while I get the new chain on and the master link clipped?

A third hand would be great but that's a project for the future.

Thanks.
Assuming you've trimmed the chain to the right number of links already...

First, put the gearing in small-small, whatever that is.

Thread the rear first.  Use a short length of old spoke through the end of the chain to stop it pulling through.

Then thread the front.

Pull both ends together at the bottom of the chain-path and use a double hook of old spoke to hold them together with a small bulge of slack chain is upwards at the joint.

Stop here, and check that your rear chain-path is correct.  The little tongue in the middle of the cage is a common culprit.  If not right, rethread.

Do up your master link or pin according to instructions.  Then check for a stiff link at the joint.