Valve Guide
I had a JPI FS-450 fitted to mine. It has an FAA STC for R-22 and R-44, amongst others.
Absolutely accurate, also gives fuel used and fuel remaining as well as fuel flow. If tied to a GPS also get fuel remaining at destination, etc.
I found it almost indispensible.
https://www.jpinstruments.com/shop/fuel-scan-450/
John