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Coastr,
I think you have the idea. The Haynes manual doesn't get it right. Once you've got the piston/cylinder purged, filling the upper reservoir can be done with the bleed screw in and the unit sitting upright. When you push the aluminum sealing plug down onto the oil in the reservoir, it should pretty well eliminate any air under the plug. The bleed screw is really only necessary if you can't get the circlip in place above the steel retaining cap, and then you can leak a drop or two of oil from the reservoir to get it all to fit. I thought I recalled the sealing plug protruding a little through the steel cap when done, but your photo of the one with the retaining clip looks right, and you can see how the aluminum plug is up snug against the underside of the steel cap. That is why there is the deep relief groove machined in the sealing plug for the spring to reside.
Well, in any case you've got the concept now, which is to have the piston/cylinder purged of any air, and a spring loaded oil reservoir above the piston to prevent (for a while anyway) air from being drawn in when the piston extends after having been compressed. It is really just a shock absorber, and exerts surprisingly little spring pressure against the tensioner arm. As you noticed, you can actually compress one with just finger pressure (your coin is a good idea to prevent a sore thumb). It just moves extremely slowly under that pressure as the oils moves through the check valve, and if you relax for even an instant it pops back up. Now that you know it secrets, you will never have to buy another 911 chain tensioner.
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