英語検索なら結論があっさりすぐわかるのな。真ちゅうが一番多く、砲金や鉄も使われる。ソフトな性質が好まれるんだそう
Why are synchronizer rings made of brass?
The synchronizer ring mainly relies on friction to work and generally made of brass, which contains alloy additives such as Zn, Al, Mn, Si, Fe and Pb. These additives could improve the overall performance of the material (Li et al., 2016; Garcia-Bernal et al., 2012; Waheed and Ridley, 1994; Mindivan et al., 2003).
FS4W71-B and - later - FS5W71-B gearboxes with bronze/brass (Warner patent) synchros which are still readily available, which leads them to think a gearbox rebuild is viable and relatively easy. Not so if you want to do it with the original type synchros.
It is possible to switch *some* of the Servo steel synchros for Warner bronze/brass equivalents, but this usually requires the gearbox rebuilder to, ahem, think outside the box. It also usually means a full strip-down and rebuild, which costs time and money. That's why people were tending to switch to used C-type 'boxes. It killed two birds with one stone. But now that C-type 'boxes are much thinner on the ground and cost more, it is less easy.
CORROSION PROTECTION
The extreme pressure chemistry used in many gear oils can be corrosive to brass and bronze used in synchronizers and bushings. Most gear oils are corrosive at temperatures of 200°F. Red Line Gear Oils are non-corrosive up to 300°F and the MTL® in excess of 375°F. A corrosive gear oil can shorten synchronizer life by half and can also contribute to rust problems.
The Plus 4 – Morgan’s biggest seller – uses the 2.0-litre Ford Duratec engine. By happy coincidence, that engine was designed by the same person who developed Mazda’s 2.0-litre in the MX-5. As a result the MX-5 gearbox bolts straight onto the engine.