the Company’s guidance range of $80 to $90 million primarily due to increased product sales in Asia Pacific. For the full year 2018, revenue was $384.6 million, a year-over-year decrease of $70.9 million, driven by a decrease in product revenues of $86.0 million due to the reduction in demand for offshore exploration and production equipment, especially subsea equipment, as a result of sustained low oil prices. This was partially offset by an increase in service and rental revenues year-over-year of $15.3 million, largely due to increased technical advisory assistance as customers draw down on product purchased in prior quarters.
Western Hemisphere revenue for the fourth quarter of 2018 decreased from the prior quarter by $1.0 million, or 2%, primarily driven by low activity levels in Brazil offset by additional project activity in North America. Eastern Hemisphere revenue increased by $1.1 million, or 4%, in the fourth quarter compared to the prior quarter due to increased activity in Norway and Africa. Asia-Pacific revenue for the fourth quarter increased sequentially by $4 million, or 47%, due to increased customer sales offset by lower aftermarket activity.
Cost of sales for the fourth quarter of 2018 was $68.7 million, an increase of $3.0 million compared to the prior quarter. For the full year of 2018, cost of sales was $271.5 million, a reduction of $33.9 million, or 11%, compared to full year 2017 cost of sales of $305.4 million.
Gross operating margin for the fourth quarter of 2018 was 29%, in line with the third quarter of 2018. For the full year of 2018, gross operating margin was 29%, which was slightly less than full year 2017 gross operating margin of 33%.
Selling, General and Administrative Expenses
Selling, general and administrative (“SG&A”) expenses for the fourth quarter of 2018 was $25.0 million, a reduction of $6.5 million compared to third quarter of 2018, primarily due to the execution of the first phase of Dril-Quip’s transformation project. For full year 2018, SG&A expenses decreased by approximately $12.3 million, or 11%, to $104.0 million from $116.3 million in 2017. The year-over-year reduction was primarily due to lower employee costs, lower insurance costs and favorable foreign exchange impacts.
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