I don't think that there would be any SAFE way to do this - locking it and then pushing the engine to high RPMS might result in alloy fan blades flying through the engine compartment and/or someone's head. I really don't think the fan could take 5-6K RPM, even if it was perfectly balanced. Imperfectly balanced, it might lead to premature water pump bearing failure.

I have wondered, though, why no one has made a replacement non-clutch flex fan. The ones installed in 70's hot rods looked nice and were supposed to fold at high RPMs, reducing drag; they were very light. I had a stainless steel one in my hotrod.

(Of course, they did an excellent job of surgically cutting off a lot of fingers also; fortunately, not mine.)

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86 560SL
With homebrew first gear start!
85 380SL
Daily Driver Project

http://juliepalooza.8m.com/sl/mercedes.htm