How to check our Bumping Clearance (Clearance Volume) of air compressor.! & why should we care.?

As you already know, its the gap between the cylinder head and top of the piston (TDC position)

Bumping clearance is given for the piston to expand when the compressor starts running.

Pic: A shiny Piston head
Pistons are of Aluminium alloy with cast iron compression ring


1). For time being we are not talking about multistage. just a single-piston cylinder arrangement

2)When the piston reaches the TDC and after the delivery, a small amount of high-pressure air is trapped inside!

[ Keep in mind that ]

 1, Our valves are operated on the difference in pressure [ie no mechanical actuating parts] wherever there is low pressure, air flows into that side. it's that easy.

2, In order to take suction from the atmosphere, the cylinder has to have pressure less than atmospheric pressure.

What happens if the clearance volume is Excessive?

The trapped air must expand below the suction pressure (ie atm pressure)
for that, the piston has to move down even further. 
Thus a loss in effective suction stroke, [leads to lesser volumetric efficiency

what happens if the clearance volume is less.?
Yes. piston bangs into the head.

air compressor
Pic: a baby air compressor

How bumping clearance changes overtime?

1) Placing an incorrect gasket between head and cylinder at the time removal of suction and discharge valves (for some compressors head has to be removed)

2) Wear on the main bearing.!
If this happens, then the whole crankshaft comes down and lowers the piston (leads to increased bumping clearance)

3)Wear at the crankpin bearing
If this is the case then, the piston can hit the head during the unloaded condition.
this particular type of wear down can be easily identified as the compressor makes banging noise a
in the unloaded condition (including at the time of starting and stopping)

what are the 3 methods of checking bumping clearance?

1. Wire method / led ball method
2.Dial gauge method
3.Feeler Gauge Methode

So, how much clearance should it be?

Generally, its 0.5% to 1% of the Bore of the cylinder

Ok now, How to adjust Bumping clearance.?

1) Change the gasket and make the clearance make it within the acceptable limit specified by the manufacturer.
2)If the bearing has worn out adjust with the gasket temporarily. and replace the bearing when you get it.
3)Only add shims if it's a marine palm type Con Rod.!
Click here for Marine Palm Bolt: Diagram

Episode 3

Why should we care?
Incorrect Maintenance of Compressors took many lives and fingers. especially if its a belt-driven.
handle with care, and always use proper PPE.

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