What is bumping clearance .

Bumping Clearance is the clearance between the top of the piston and the cylinder head of an air compressor when the piston is at the top dead centre (TDC).

It is generally 0.5 to 1 % of the cylinder bore.

The clearance is needed because due to the following reasons 

  1. To prevent mechanical damage to the compressor.
  2. It provides the necessary space for valve operation.
  3. Provides the space for thermal expansion.

Note:- It must be kept small as possible for achieving the best compressor efficiency.

Procedure for taking Bumping Clearance

  1. Stop the air compressor and take out and do lockout and tag out.
  2. Drain all the cooling water of the compressor
  3. Remove cylinder cover and cylinder head face.
  4. Place the lead wire ball on top of the piston.
  5. Then place back cylinder cover on the cylinder head and tighten all head bolts with proper torque. After that turn the compressor by hand with the help of Tommy Bar and bring the piston to TDC and the lead ball will get compressed.
  1. Then remove the cylinder cover and take the measurement of lead wire ball thickness by using micrometre.
  2. This measurement will give you the Bumping Clearance.

How to adjust the Bumping Clearance ?

  1. The thickness of the cylinder head gasket can be changed accordingly.
  2. Add or Remove Shims between the foot of connecting rod and Bottom end bearing.

Why Bumping Clearance increase ?

1. Wear down of main bearings happen and that leads to lowering of the crankshaft and would result in piston lowering.

2. Wrong gasket fitted during opening up of Cylinder heads.

Note:- Extra clearance would result in a small volume of air being re-expanded every time causing an increase in air temperature. The compressor efficiency decreases and overheating of the compressor takes place

Why Bumping Clearance decrease ?

The crankpin bearing wears down due to use and that clearance can travel right up to the piston that can hit the cylinder head hard.



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