Hi, welcome to MyBimmerSpace.
The BMW dummy chain tensioner tool is used when removing the Vanos. It’s job is to prevent primary chain slack and keep timing in check while re-installing the Vanos. Bmw# 90-88-6-113-390
If you want to make your own Dummy Tensioner using your existing primary tensioner than heres how: Pull the piston and spring out and drop in a spacer that will fit into the body of the tensioner. I found that 8 U.S. pennies work great for a spacer, it spaces the piston out about 20 mm, you can add more or less if needed. After you have your spacer in, drop the piston back in without the spring. Basically, all your doing here is shimming the piston so it cannot collapse when you tighten it against the primary chain guide, it’s a very simple tool. Thread it into the cylinder head until it’s finger tight where you feel it stop on its own then nudge it about 15 to 20 degrees further.
Step 1. pennies & tensioner
You may have either one of these pistons depending on the year of your car