Oil Field Engine News

Piston Position


A 22-1/2 HP 2-stroke Bessemer.

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I have received many inquiries as to the correct position of the piston on a 2-cycle engine such as a Bessemer. Many of the crosshead 2-cycle designs operate in the same manner, such as South Penn, Ball, Oil Well Supply, most all of the half-breed cylinders and many, many others. The best advice I know to give in response to this question is a quote from the Bessemer manual:

"It is important that the piston head be set in the proper relationship to the transfer ports. There are two lugs on the end of the piston which must come on either side of the transfer ports. These lugs or deflectors, guide the transfer mixture along the top of the cylinder. If it were not for these deflectors part of the transferred mixture would travel around the end of the piston down toward the exhaust ports and not only waste part of the mixture, but cause frequent back-firing."

If you take the head off the engine, these lugs can be easily positioned correctly, or they can be observed somewhat through the transfer port cover. Other types of engines might not have so called "lugs," but maybe a dip or slope in the shape of the piston, or maybe even some sort of fin. The point to remember regardless of the make of the engine is that these are always intended to direct the intake gases away from the exhaust ports, thus these should always be positioned bearing that in mind. Some makes of engines exhaust on the top or the side. The piston deflectors should be positioned accordingly as needed for your style of engine so that intake gases are directed away from the exhaust ports.

In rotating the piston on its threaded rod to position it in relation to the ports and deflectors, we must also take care to adjust the piston correctly in relation to length of stroke. The Bessemer manual states:

"When the piston is at the bed end of the cylinder on the extreme stroke, the transfer ports should be wide open and the edge of the piston should match the edge of the transfer ports as nearly as possible. However 1/16-inch in either direction from that preferred will answer."

After any adjustment in the stroke, the engine should be slowly and carefully rolled over, taking note to ensure the piston does not bottom out at either end of the cylinder. Obviously any adjustment in the stroke will affect compression, and problems such as preignition could possibly be solved by reducing compression. The Bessemer manual also states:

"The piston may be screwed in a turn or even two turns without seemingly interfering with the best operation of the engine. Usually, however, when this trouble exists, it is best to decrease compression pressure by inserting a ring between the face of the cylinder and the cylinder head, thus allowing the piston to remain in its true relative position to the transfer ports."

After careful investigation of the manner in which your engine cycles, you should be able to determine the best manner in which to adjust the engine. Different styles are designed to cycle in other ways or locations, but most operate on the same basic principles.

Contact the Oil Field Engine Society at: 1231 Banta's Creek Road, Eaton, OH 45320-9701;