Originally Posted by trc.rhubarb
That's about it right there. The new tensioner device is filled with oil, has an internal spring, and has a ball check valve at the bottom of the piston bore. (You can see part of the check valve ball at the bottom of the bore.)
When in service, the one-way check valve allows the new tensioner cylinder to fill with oil easily. You know, it lets oil in but won't let it out. Once the cylinder is filled with oil the piston is very hard to compress. The reason for the difficulty is the only path for the oil to escape is between the tight piston-to-cylinder clearance. I don't mean the original tensioner body piston, but the piston clearance on the new tensioner 'insert' you are installing.
The new tensioner is shipped filled with oil. What you can do is get the oil out of there. That makes the tensioner insert (and the original screw-in tensioner body) easier to compress by hand while installing it.
Here's one way to remove the oil. Clamp the new tensioner in a VERTICAL vise or other VERTICAL clamping method. Clamp it between the jaws with the open end of the piston pointing down, and the cylinder pointing up. Note: You're doing this with only the tensioner insert in the jaws, not the whole screw-in tensioner body.
Slowly close the vise and watch oil weep out from between the piston-to-bore clearance. The first portion of the compression process is usually easier because there is often some air inside the tensioner along with the oil. And if so the air will compress much easier than the oil will squeeze past the piston clearance. Turn the vise handle a little and wait a few moments for the oil to begin weeing out. When the oil flow stops, turn the handle a little more.
After the piston is compressed and the oil has found its way out, the piston will be much easier to compress by hand. It still won't be easy, but it will be easier.
BUT WAIT there is a caveat to all the above.
The issue may just be that the cam chain is pushing up on the chain guide, which reduces the available space for the tensioner body. To solve that, put the gearbox in a high gear and turn the rear wheel forward a little. Doing that rotates the crank and cam chains in the forward direction, and ensures that the slack side (the side the tensioner is on) of the chain is slack, and allows the maximum room to install the tensioner body.