A four-hour strike over wage reforms stopped production at Hyundai and Kia factories on Friday.

The strike, voted for by 70% of Hyundai’s union workers, cost Hyundai-Kia Motor Group the equivalent of around US$64m in lost production according to calculations by the Yonhap news agency.

Friday’s action followed 17 rounds of talks with the management over the last two months. The dispute centres on whether regular bonuses should count as part of ordinary wages which would increase the amount paid for overtime by about 10% according to Hyundai’s union.

South Korea’s Supreme Court has already rules that regular bonuses should be part of ordinary wages for the purpose of calculating overtime pay, holiday shift pay and paid annual leave, as well as pensions.