Which is a concern when one is changing a clutch disc?

When changing a clutch disc, the concern related to asbestos is significant because many older clutch discs were manufactured using asbestos as a friction material. Asbestos fibers can become airborne when the clutch is removed or handled, posing serious health risks through inhalation. Extended exposure to asbestos is linked to respiratory diseases, including asbestosis and mesothelioma. Therefore, proper safety measures, such as wearing appropriate personal protective equipment and ensuring adequate ventilation, are crucial when working with these components to minimize exposure to hazardous materials.

In contrast, emissions are not a primary concern during the physical replacement of a clutch disc, as the process itself doesn’t directly involve processes that would produce emissions. Fiberglass is also not commonly associated with clutch discs; while it may be used in other automotive components, it's not relevant in the context of classic clutch systems and their materials. Synchros or synchronizers, primarily relevant to manual transmission operation, don’t directly pertain to the replacement of a clutch disc, focusing instead on gear engagement. Thus, the concern about asbestos remains the most critical in this scenario.