That is a silly idea from the start to have alu pushrods without hardened caps on them, same with stainless types, thinking they will "workharden" in use - not so. You want 50 HRC at least on end caps, stainless , carbon steel or alu. So in Vincent case alu is desirable because the rest of engine is alu as well, trying to maintain low valve clearance in hot engine too. Once you got a bit too much clearance you lose all minimal quieting ramps of cams and enjoy the rattle in valve train.
BMW and Horex had alu tubes with steel end caps for long times, never a problem , and they got revved a lot with pushrods ca. 28 cm , while Vincent rods are only 15 cm or so.
In photo 2 at the left are Horex 400 alu top and bottom ends, then BMW alu with looong cap, then BMW stainless - a bit heavy. Some titanium tubes at right, no good, low heat growth !
Vic