the chain tensioner is removed from the engine the piston is fully extended
by the spring inside the body under the piston. The oil in the tensioner
combined with a spring clip prevents the piston going back down inside the
body. To reuse the tensioner lever the piston out of the body using a
screwdriver in the groove in the piston just above the body, this will
release the spring clip from the top groove in the body. Drain the oil from
the body, collect the spring and clean all parts.
The grooves in the body are slightly tapered on their top edge to allow the spring clip to pass for the piston to extend but the spring clip prevents the piston going back down into the body by being opened by the taper in the bottom piston groove and locating in the bottom of the grooves in the body, this is nesessary to keep some tension on the chain when the engine is started before the oil pressure takes over to apply the tension.
To re assemble the piston the spring clip has to be fitted to the top groove of the two in the piston as this groove is deeper and allows the spring clip to compress and go past the grooves in the body.
On the top of the piston is a smaller spring clip in a groove, this clip holds it all together until the piston is pushed down and released to apply tension to the chain.
At the bottom of the top groove of the piston there is a ledge around the groove that is wider than the rest of the groove, this ledge pushes the sring clip into the top groove of the body preventing the piston from coming out. When the piston is pushed down to apply tension to the chain the spring clip comes off the ledge and contract into the groove allowing the piston to come out under the pressure of the spring.
To push the spring clip onto the ledge when fitting the piston a tool is needed that fits in the gap between the piston and the body. I made one up using 0.50mm feeler blade and bent it around a small socket until it was the same diameter as the piston.
Assemble the tensioner by inserting the spring in the bottom of the body followed by the piston with both spring clips fitted.(larger clip in top groove of the two together). Push the piston against the spring using a 'G' clamp with the cylindrical feeler around the 'G' clamp thread and the end of the thread on top of the piston. Continue compressing the spring by scewing in the 'G' clamp until the piston is just proud of the top of the body (where the top groove of the piston and the body are in line). Insert the feeler between gap of the piston and body and tap the top of the feeler to push the small spring clip onto the ledge. Slightly release the pressure off the clamp and tap the feeler again to make sure the spring clip is located on the ledge and in the groove of the body. Finally, lightly clamp the in a vice between the bottom of the body and top of the feeler to make sure the spring clip is fully home on the ledge.
(Warning , be very careful that the piston doesn't fly out, always keep the piston pointing away from you).