Most open hole Yamaha flutes (and some other older flutes such as Haynes) are problematic because the bushings fit so poorly (the cup chimney is tapered). Metal bushings leak and make it hard to shim & re-shim a pad with any accuracy without damaging the pad.
The JS bushing upgrade kit solves this problem. New stainless steel sleeves are pressed over the old cup chimneys so they are even at the top (use red loctite to secure). The original cup chimney must be 7.8mm or smaller. The delrin/plastic bushings provided slide smoothly over the resized chimney for an accurate and airtight fit. Shimming a much easier task. The upgrade kit is essential for high quality JS pads. We do not make an upgrade kit for the Yamaha 300 & 400 series.
Also use JS screw retainer seals to eliminate leaks between the screw head and retaining washer.