Big Rig Basics: Remanufacturing cylinder heads
Having a cylinder head rebuilt in a shop environment will fix it but likely will not bring the head back to as-new condition.
Craig Wood, Vice President of Business Development and Marketing for Detroit Diesel Remanufacturing says, “The typical rebuilt cylinder head is only repaired to the level of failure. That means any components beyond this level are left intact. Also, testing procedures vary greatly among individual rebuilders. Detroit Diesel Remanufacturing’s reliabilt engine components are returned to the latest blueprint specifications and tested to original equipment standards.”
Steps to remanufacture a reliabilt cylinder head:
1. The head and its parts are totally disassembled, down to removal of pipe and freeze plugs.
2. The head is baked in an oven for an extended length of time which removes oil, soot, and grease, and then cleaned with a process called shot blasting. With shot blasting, soft particles are ‘shot’ against all parts of the casting to thoroughly clean and remove corrosion that may be present.
3. The head is thoroughly inspected and a process called Magnafluxing is used to finds cracks. Damaged threads are repaired.
4. If a head became too hot, it may have warped. The firedeck, or bottom sealing surface of the head, is checked for flatness and machined if necessary so it will seal properly.
5. Valves and valve seats are machined to original blueprint specifications. The surface finish and angle of these components is critical to ensure proper sealing of the combustion chamber. A good seal between the valve and valve seat is critical in optimizing the engine’s operating efficiency and emissions and ensures proper cooling of the valve during operation.
6. Valves seats and guides are inspected and, if necessary, machined to return surfaces to perfect condition. With the seat, as with the valve itself, perfect surfaces are necessary for sealing and proper cooling. In the case of the guide, proper geometry and the correct inside diameter (where the valve rides up and down) is critical if the valve is to seat smoothly, and also to control oil consumption. Seats and guides that cannot be brought back to original spec’ will be replaced. Truck diesel cylinder heads are designed so seats and guides are separate parts from the head itself, and can readily be forced out and replaced without damaging the head.
7. Valve springs are inspected with special equipment to ensure that they produce the proper tension at the proper height so valves will operate smoothly, following the motion designed into the cam precisely. If not of the latest design, they’ll be replaced with new ones.
8. The valve stem seals are replaced with new seals to further control oil consumption and emissions.
9. The firedeck is resurfaced using the OEM process to ensure the bores that carry the camshaft bearings will be perfectly aligned. All individual cam bore dimensions, as well as all other critical ones, are checked to ensure they meet specifications on a device called a “coordinate measuring machine.” This helps ensure long camshaft bearing life and smooth operation.
10. All the parts, including valves, springs, and seals, are then reassembled, using only parts of the latest design. All wear surfaces are pre-lubed to protect them at initial start until the engine’s oil reaches them. A heat tab is attached to indicate overheating, and the assembly is sealed up in a plastic bag to protect it from contaminants, which could produce unnecessary wear during shipping.