The best blade I've found so far is made by Milwaukee called "The Torch" They're about 7" long and about $6 each. They just sing through the metal: I cut a rear 1/4 out of a w116 in perhaps 45 seconds.

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Over 21 years I owned several:
w108
w110
w111
w115
w116
w123
w124
w126
w212