Thanks roebuck, I looked at the lower one and misunderstood it initially, I thought it was the type that works by levering against the engine block. However it is cleverer than I thought. I now believe you rotate the retainer until the keyhole is pointed away from you, then just press the upper side of the tool against the underside of the retainer, lift it slightly, then pull it toward you. That will move the keyhole across to the release position so it falls off when you relax the upward pressure. As long as the valve spring is weak, it should be dead easy. You can put it back on just by reversing the procedure. You do need to carry out a spot-facing operation to make the tool though, so making their own might not suit people who don't have suitable cutters.

I once bought a heavy cast iron version of the upper one, on a scale suitable for rather large engines, but threw it out after a few years, having never used it. (It came in a load of mixed blessings I bought at a clearing sale for a couple of bucks.) In this case, Briggs recommends the upper one, which is their part number 19063, for all of their side valve engines:
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Last edited by grumpy; 07/12/13 09:03 PM. Reason: Add detail