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I acquired my Mio Mechanical from On Rope 1 in 1997. I acquired another in 2017 as part of Bob Thrun’s collection.
My Mio Mechanical is 136 mm. long, 109 mm. wide, 65 mm. high, and weighs 641 g.
The shell is a piece of 2.9 mm. stainless steel bent into a "U." The cam assembly is more complex, consisting of a cast stainless steel cam face that pivots on a pin that passes through the eye assembly. This assembly is made from two stamped pieces of stainless steel that are spot-welded together. This assembly is loosely riveted to another stamping that carries a roller at the top. The "rivet" is tubular so that the eye can pivot on a standard quick-release pin. A weak spring acts to close the cam, and a pin passing through the eye limits the motion. Finally, a bent rod attaches the assembly to the shell.
The front is stamped with an up-pointing arrow under the word "UP," "MIO MANUFACTURING CORPORATION," "MADE IN USA," and "PAT.NO.5,156,240." The rear is stamped as follows:
1/2 IN DIA.
DON’T USE ROPE
At first glance the Mio Mechanical looks like a normal U-frame Type 1 Lever Cam Ascender, but the rope bearing surface is a separate piece that has 3 Z-shaped teeth. This piece is not curved to fit the rope circumference, and there doesn't seem to be any good reason why it couldn't be. This design spreads the load over a greater rope length than most normal Type 1 Lever Cam Ascenders do. This could easily be incorporated into a caving ascender.
Opening the Mio Mechanical rope grab is interesting. After the quick release pin is pulled, the cam and roller are slid upward on an external rod (visible below the roller in the second picture), and then rotated out of the way.
The manufacturer seemed to have some difficulty deciding what size rope this device was to be used with. The original stamping said 5/8" (16 mm.) rope, but this has been restamped to say 1/2" (13 mm.) rope.
The Mio Mechanical rope grab is protected by U.S. Patent 5,156,240.