I just pulled one off one of the junker 18RC's I've got in the shed, and the spring from the top tensioner measured around 70mm uncompressed, so it looks like you're missing a bit!
I'll have a look tomorrow to see if any of the tensioners in these motors are in a decent condition, let me know if you're after one and I'll put up some photos. The spring I just pulled out is full length but is a bit wonky in the middle, I've got another two 18R's I can have a look at tomorrow though.
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