Piston Replacement

Thanks to William Bailey and Larry Rouse for assistance.

    When the U.S. Coast Guard outlined the specifications for these new cutters, they wanted high-speed (for that time) diesels. Winton's engines of this size had a maximum RPM of 350, but the USCG wanted 450 RPM in order for the vessels to have a 16 knot top speed for their design purpose of seizing offshore supply vessels during Prohibition.

    In order to avoid unmanageable inertia stress on the wrist pin, connecting rod and rod bearings from the higher piston speed, the pistons on these engines were made of a then-secret experimental alloy of aluminum, with beryllium and copper added to give the aluminum adequate strength, toughness and durability.

    When it is necessary to pull a piston for replacement, the engine is first drained of water to below the top of the cylinder liner. Then the water outlet rail, fuel common rail, starting air valve, injection valve, intake valves and exhaust valves are all removed from where they mount to the head, which is separate for each cylinder.

    By removing just two nuts, the rocker arms are lifted off their stanchions as an assembly, allowing the exhaust valve water cooling jumpers, the valve operating tee pieces, the valves (in their cages), and the fuel injector to be easily removed.
 
    The cylinder head (which weighs about 700 pounds) is unfastened and lifted off with a chain fall on a lifting bracket bolted to the valve mounting studs. It is then placed on its side for scraping and cleaning. The cooling water enters through the smaller holes between the head stud holes around the cylinder, and through the slots around the exhaust elbow (top). It exits through a larger flange (not visible in picture) on the exhaust elbow .
 
    In order to pull the piston, the connecting rod must be separated from the bearing box. Working through the crankcase access doors, after the tension is relieved on the rod bolts, the setscrews on the bearing halves are tightened so that the rod bolts are held in position without separating the bearing halves once the castellated nuts are removed completely. The rod foot can then be slipped right off the top of the bearing assembly.
 
    So as to lift the 14" diameter piston off, an eyebolt is threaded into the tapped hole provided for that purpose, and a chain fall is hooked on. The piston, wrist pin and connecting rod assembly together weigh about 500 pounds.
 
    Once the piston is out, another chain fall is attached to the connecting rod foot, and the piston is swung around and inverted. Then the wrist pin (which is semi-floating, clamped to the rod but floating in the piston) is removed and the rod pulled off. There are removable covers over the holes in the piston where the wrist pin is removed, and small holes 90° from them for socket wrenches to reach the wrist pin clamping bolt.
 

    The top of the water box (which carries the wet-type cylinder liners) is cleaned, and the head studs are tested for cracks the "low tech" way (tapping with a hammer to see if they "ring" properly).  A tapered "ring funnel" tool is placed over the liner, to compress the piston rings and guide them into the liner as the replacement piston is lowered.

 
    After assembling the connecting rod and wrist pin to the replacement piston, the rings are installed and placed, then the piston can be lowered slowly, guiding the rod and making sure the rings compress properly as they slide into the liner.
 
    The squiggly line to the right of the chains is not a crack, it is an oil drain groove. The hole above it is for access to the wrist pin clamping bolt.
 
    The connecting rod has an oil line with check valve fastened to it. It is vital to make sure that the oil tubing and valve are clear of obstructions, as they provide vital lubrication and cooling for the wrist pin and piston.
 
    The rings are repositioned, if necessary, just before they slip into the liner.
 
    Sometimes, the piston needs a little "persuasion" weight to slip into the liner. The Oiler is a handy tool to use for this.
 
    The nuts and spacers temporarily put on the connecting rod bolts are removed, letting the set screws temporarily hold the bearing box halves together. The connecting rod foot is guided onto the bearing assembly as it is carefully lowered, with the proper piston clearance adjusting shim between them.
    As the piston is lowered, one person guides the rod onto the bearing box, while another guides the rings in.
    Once the piston is installed and the connecting rod bolts have been properly tightened and cotter pinned (after first loosening the set screws, then retightening them after the rod bolts have been tensioned), the thoroughly-cleaned head is reinstalled.

    The head gasket, a properly-shaped copper ring, must first be softened so that it can be reused. Unlike most metals, copper is softened, rather then hardened, by heating and quenching. So the ring is heated red hot with a torch, then dropped into a large garbage can full of water.

     It is vitally important to install new rubber O-rings around the water spigots that protrude from the water box into the head, and not to forget any. The exhaust elbow asbestos gasket may be reused, although we usually use twisted strands of lampwick coated with Permatex® to ensure a positive seal around the water passages, which are outside of the exhaust opening.

    Once the head nuts are tightened in the proper order, the valves, injector, water connections, starting air valve and air jumper pipe, etc., are replaced in the head. The copper valve gaskets are reused after softening, as was done on the head gasket. The asbestos-lined copper injector gasket is replaced.

    The engine is then test run at the dock, and everything is checked for proper temperatures and normal running sounds.


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Copyright © 2002, Brian Bailey, W4OLF
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