How to compress chain tensioner to install
 

I'm trying to replace a chain tensioner, I compress the piston slowly with C clamp, but as soon as I remove piston pops up immediately.

How do I get it to stay so I can slide it in

You need the little pin that holds it in place. I made mine from 1/16 music wire. Coat hanger wire would be ok.

There is a hole the wire goes into. Piston has to be in almost the entire way.

New tensioners come with the pistons retracted and a nicer pin

Cotter pin or finishing nail will also work.

Cheers

Or Allen wrench

I used an Allen wrench as well.