Complex sentence: Thermal expansion was a problem for these long rod stays, both in their expansion tending to elongate them and reduce their staying effect, also as the inner firebox expanded in length, this would bend the stays forwards.
Split into simpler sentences: Thermal expansion was a problem for these long rod stays, both in their expansion tending to elongate them and reduce their staying effect. Also as the inner firebox expanded in length, this would bend the stays forwards.