Repairing a valve usually means discarding old fasteners and replacing them with new ones. But it takes more than just matching thread diameter and pitch to make the correct fastener selection.
First let’s discuss why a bolting change would be necessary. Nuts and bolts serve two purposes: they hold components in alignment or they provide clamping force. In some cases, they do both. In the case of valve body/bonnet joints, the fastener’s function is to clamp the two pressure-containing valve halves tightly together with a gasket between them to prevent fluid leakage. When a bolt is tightened, it is stretched a small amount. If it was not stretched, there would be no clamping force.