Valve Job

| May 18, 2009

Any cylinder head work requires expert precision for guaranteed reliability


The cylinder head on your diesel helps contain the high-pressure gases inside the cylinder and allows the intake and exhaust valves to open at the right points in the engine’s rotating cycle. The valves allow the exhaust to blow out of the head after the power stroke of the piston and let in fresh air on the intake stroke.

Properly maintained, the valves will last the life of the engine. But poor maintenance or extreme operating conditions may cause them to fail because of excessive pounding or overheating.

The valves ride inside guides, which are narrow tubes that hold them in position so they move straight up and down. The valve guides in a heavy-duty diesel can be driven out of the cylinder head and replaced.

Valves have a circular bottom with a cone-shaped male face that sits against a similarly shaped female surface in the cylinder head. The seating surfaces are part of replaceable valve seats located in the bottom of the cylinder head.

Rebuilding a head involves forcing out the valve guides and seats, replacing them, then seating the valves in a grinding process that helps them seal effectively. The job also involves dozens of precision checks, such as making sure the head is still perfectly flat on the bottom (machining it flat often is necessary). To avoid errors, most engine dealers send a cylinder head with failed valves or other parts back to a remanufacturing center for rebuilding on a production line. This is cheaper than on-site hand labor and gives the user an excellent warranty that can be used everywhere. So, for most truck users, getting this work done involves having an engine dealer remove the cylinder head from the engine, send it to the factory and replace it with a remanufactured head that is on hand.

REMOVING A HEAD
Drain the cooling system, then remove parts like the intake manifold, turbocharger and any other parts like the air cleaner and intake tubing that will get in the way. Unbolt the cam cover and pull it off.

Disengage the mounting bolts for the engine brake and remove it.

Remove the rocker mounting bolts and the rocker levers that operate the valves and injectors. The technician, Ruben Quesada, removes them with a special tool that keeps the levers level.

The engine is then turned over to align the timing marks on the gears that drive the camshaft so it can be replaced in proper time or relation to the crankshaft’s position. The bolts that fasten the camshaft thrust plate are removed so the camshaft itself can later be removed.

Parts like the fuel lines, injector electrical harness and coolant hoses need to be disconnected.Remove the camshaft-bearing saddle-mounting bolts, along with the bearing saddles themselves. The upper front cover also must be unbolted to free the camshaft. Then the camshaft, its drive gear and the upper front cover can be hoisted off the head.

The 38-cylinder head bolts must now be removed. If any are corroded, a long breaker bar may be needed.

Don’t try this at home! The engine lifting brackets are removed, a special lifting bracket is bolted onto the head, and a crane is used to lift the head off the engine.

A head in the shop was receiving new valves and seats in one cylinder. You can see that the seat insert on the lower right has been removed.

Finally, a remanufactured cylinder head is brought out of the parts department. It will be installed, with a new gasket to seal between it and the block, by generally reversing the removal process. One critical difference is that the bolts attaching the head will be torqued in a specified order, usually in several stages, with a special torque wrench that measures how tight they are. This is precision work!

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