Automotive Engine Rebuilder RPM Improves Product Quality with Flex-Hone
RPM, a Holman Enterprise company and a leading manufacturer of automotive parts, has implemented a metal finishing system in recent years that has improved the quality of the cylinder bore surfaces of its overhauled engines.
The new process involves a special deburring/finishing tool that is used to process the cylinder wall after a rigid honing operation. Results show that the company’s overhauled engines exhibit fewer engine failures due to insufficient piston ring conformity to poorly machined cylinder walls compared to competitors’.
RMP, a Holman Enterprise company (Pennsauken, NJ), overhauls car and truck engines as well as associated parts, including master cylinders, water pumps, carburetors, and alternators.
As an authorized manufacturer for Ford and Chrysler, RMP distributes its products through three East Coast branches, which in turn supply Ford and Chrysler dealerships. With over 300 employees, the company generates annual sales of more than $20 million.
In a typical month, RMP overhauls approximately 1,300 gasoline-powered engines (70% for trucks, 30% for cars). The warranty for truck engines is 12 months or 12,000 miles; for car engines, it is 6 months or 6,000 miles. Previously, RMP machined the engine cylinders with a standard honing machine, but found that the surface finish did not meet the company’s specifications. The engine cylinders range from 6 inches to 7 inches in depth and from 3 inches to 5 inches in diameter.
According to Dale Houghton, head of RMP’s engine department, the engine cylinder walls had an unclean finish, with cut, torn, and folded metal remaining along the cylinder surface. This type of finish could ultimately lead to engine failure.
This problem could occur during the break-in period of the newly overhauled engine if the finish adversely interacts with the piston rings. The principle of interacting metals states that the pressure on the surface of the cylinder wall is equal to the load divided by the projected bearing area. For example, if a pressure of 500 lb./sq. in. exists on the lower surface of two interacting metals and both surfaces are theoretically flat, the pressure on the surface is 500 lb./sq. in.
Since no surface is perfectly smooth, some abrasion inevitably occurs between the surfaces of the cylinder walls and piston rings. If the surface is 80% smooth or flattened, the pressure is 625 lb./sq. in. If the surface is 10% flattened, the pressure increases to 5,000 lb./sq. in., which can lead to severe engine damage.
In search of a solution for its surface finishing problem, RMP turned to Brush Research Manufacturing Co., Inc. (Los Angeles, CA), which supplied the company with a novel deburring/finishing tool, known as Flex-Hone®. This system consists of a flexible hone with abrasive globules at the ends of high-density nylon filaments. At RMP, the hone is inserted into a handheld air tool and secured with a standard chuck.
RMP’s improved honing process works as follows: Engines arrive at the company via a dealer exchange program or from core suppliers. The engines are then completely disassembled, cleaned, and inspected multiple times. The engine block is bored after passing various tests.
Between 0.001 inches and 0.006 inches are removed from the cylinder wall. After boring, the engine block is machined with a semi-automatic rigid honing machine, which removes 0.0001 inches from the cylinders.
Immediately after this step, the Flex-Hone tool is used. To break down the remaining metallic peaks in the cylinder walls, an operator applies three to four strokes of the tool to each cylinder.
The honing tools vary in diameter from 4 1/8 inches to 4 1/2 inches and are equipped with 120-grit silicon carbide. RMP uses an average of 280 Flex-Hones per year, or 60 engines per hone. The cost for the units is only a few cents per engine. As a result of using the Flex-Hone tool, RMP has been able to maintain high quality standards at minimal cost. According to Houghton, RMP’s overhauled engines exhibit faster break-in time, better oil retention, and reduced blow-by effects due to the plateau finish in the cylinder walls.
RMP is so satisfied with the Flex-Hones, says Thomas Carter, the company’s purchasing manager, that they also use them for finishing their overhauled master cylinders. For this honing process, operators use air-powered Flex-Hone tools ranging from 1 inch to 1 1/2 inches, with 180-grit silicon carbide.
