Bumping Clearance in Marine Reciprocating Air Compressors: How to Check and Adjust

Bumping Clearance in Marine Reciprocating Air Compressors: How to Check and Adjust
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What is Bumping Clearance

Bumping clearance as the name signifies is a clearance given so that the piston of the marine reciprocating compressor would not bump into its cylinder head. In new compressors this clearance is adjusted by the manufacturers and the marine engineers are blissfully unaware of its importance. However the ship does not remain new for ever and every machine demands overhauling and that is where the problems start. Even routine jobs like lifting the cylinderhead to change the low pressure or first stage valves can change the bumping clearance if the correct thickness gaskets are not used or if the head is overtightened thus squeezing out the gaskets. Many engineers miss this vital adjustment during overhaul of the compressors and efficiency and free air delivery of the compressor suffers.

Bumping Clearance Changes over Time

The bumping clearance in a new machine is set properly by the manufacturers during construction but over a period of time the clearance changes because of the following reasons:

  1. Wear at the crankpin bearing. The crankpin bearing wears down due to use and this clearance can travel right up to the piston and an unloaded piston can hit the cylinder head. This type of wear can be recognized when the compressor makes impact sounds running unloaded at the starting and stopping operations. This type of wear would also be accompanied by a slow decrease in oil pressure over a period of time.
  2. Opening up of cylinder heads. In certain types of reciprocating compressors the cylinder head have to be removed for the changing of the first stage suction and discharge valves. When the cylinder head is put back the correct thickness of the cylinder head gaskets should be used otherwise it would change the bumping clearance.
  3. Wear on the main bearings. Over all wear on the main bearings would lower the crank shaft and would thus lower the piston and increase the bumping clearances.

Significance of Bumping Clearance

The bumping clearance must be adjusted properly otherwise there is risk of damage and loss of efficiency. If the bumping clearance is less the volumetric efficiency would increase but there is risk of the piston hitting the cylinder head, especially when the compressor is unloaded during start and stopping.

On the other hand to play safe, the engineer gives few millimeters of extra clearance, the volumetric efficiency of the compressor would decrease, the free air delivery will fall and there will be a fall in pressure. The extra clearance would result in a small volume of air being re-expanded every time causing increase in air temperature, fall in efficiency and overheating of the compressors. This would endanger the ship during maneuvering by sudden loss of propulsion.

How to Check Bumping Clearance

The bumping clearance can be checked by the following methods:

  1. In case a suitable opening is available the piston can be barred to the top dead centre and then feeler gauges can be put inside and the clearances checked at two three points.
  2. The more convenient method is to take lead wire from the engine store and make a small ball based on the expected clearance and put it between the piston and the head from the valve opening. Then the piston is slowly turned to the top dead centre with the help of a Tommy bar. After that the piston is again turned down and the lead wire ball is extracted and the thickness measured with the help of a micrometer. This measurement would give the bumping clearance.

The caution which must be observed in these methods is that the clearances of the main and the crank pin bearing have not been taken into account. The correct method is thus that after turning the piston to top dead centre the piston connecting rod must be jacked up with the help of a crow bar. It is only after this hidden clearance has been accounted for, will the correct bumping clearance be found.

How to Adjust the Bumping Clearance

The bumping clearance once found to be incorrect would have to be adjusted. The methods of adjusting the bumping clearances are as follows:

  1. The cylinder head gaskets can be changed to a different thickness thus altering the bumping clearance.
  2. The shims between the foot of the connecting rod and the bottom end bearing can be changed thus changing the bumping clearance.

However after adjusting the bumping clearance the clearance should be checked once again to make sure that there is no error and the clearance is within the range as specified by the manufacturers. It must be stressed that compressors are unforgiving and incorrectly maintained compressors have claimed many a lives


  • General Engineering Knowledge by H.D. McGeorge
  • Introduction to Marine Engineering by D.A. Taylor
  • Reciprocating Compressor: Wikipedia Commons

This post is part of the series: Marine Reciprocating Compressors

This series discusses the maintenance, operation, design, overhauling, and the troubleshooting of Marine Reciprocating Compressors.

  1. Marine Compressors: Operation, Maintenance, and Troubleshooting
  2. Marine Reciprocating Compressors: Correcting Piston to Head Clearance