It works on the bike if you're just changing a broken valve spring, but to replace seals or measure spring height you need to pull the head. It is still way better than using the old c clamp type spring compressor so yeah it still gets 5 stars.
Very nicely made tool for the M8 engine. It's what I would expect from a Jim's tool. This tool will allow me change out my stock valve springs to the Star Racing valve springs so I can put in my Star 30-30 cam or or Star F35A cam. Cant decide which one but with the racing valve springs installed I can choose either and not have to worry about coil bind or valve float.