Learn Finnish: Säästimme tämän painavan osan vanhemman kellon heiluriksi. - We saved this heavy part to be the older clock's pendulum.

This is a great example of the translative of intended purpose. The part isn't physically transforming into a pendulum right now, but it is being saved to act as or for the purpose of being a pendulum later. When a noun represents a role, use, or purpose something is intended for, Finnish uses the translative ending -ksi.

Could I use the postposition varten instead of the translative case here?

Yes, you could say vanhemman kellon heiluria varten (for the older clock's pendulum). However, using the translative case (heiluriksi) is a very concise, natural Finnish way to express that something is meant "to serve as" something else.

Why is the adjective vanhempi (older) written as vanhemman in this sentence?

Vanhemman is the genitive form of the comparative adjective vanhempi. Because it modifies kellon (clock's), which is in the genitive case, the adjective must match its case to show that they belong together.

Why is the object tämän painavan osan in the accusative (genitive shape) rather than the partitive case?