We have found with the GX120 (same collett) we were dropping valves 2 or 3 times a season. After a while it became apparent that engines were actually failing quite soon after preventative maintanence. We discovered that the collets coming in as spares are not the same as the ones fitted to new engines. The Honda collett has two countries of origin, Japan and Thailand, same part number just different packaging. The Japenese Collet is the one you normally get if you order a spare part both in the UK and in France, although I here there are now also some Chineses ones (not clone parts)about.
The colletts actually look completely different,(Thailand one is shinier) but as you would expect the Japanese one is much better quality, but this includes sharper corners on the contact point with the valve. In race use this saws away at the valve until eventually the top comes off. Since only using the Thailand colletts (fitted from new to both the GX120UT1 & GX160UT1) we have not had a failure.
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