Measuring piston ring groove clearance:
Pistons are often fitted with two compression rings and one oil scraper ring. When we replace piston rings, the clearance in the piston ring groove and the ring end gap must be checked.
The following image shows the measurement in which the clearance in the piston ring groove of the piston is checked. A feeler gauge and a new compression ring are slid into a piston ring groove at the same time. In most cases, the clearance may only be between 0.025 and 0.050 mm.
Measuring ring end gap:
The ring end gap is the space between both ends of a piston ring when installed. To measure the ring end gap, we place the piston ring in the relevant cylinder. We keep the piston ring relatively high up in the cylinder; any wear on the cylinder wall (often around the middle) has no effect on the measurement.
With a feeler gauge we check how much space (read: ring end gap) is present between the ends.
- an excessively large ring end gap results in a lot of blow-by gases and thus loss of compression;
- with too small a ring end gap, or no gap at all, friction and thus wear, or even seizure, can be caused.
As a rule of thumb, a clearance of 0.05 mm per centimetre of piston diameter applies.
Installing piston rings:
During assembly work, close attention must be paid to the correct installation position. In most cases the piston rings are marked with: “Top” or “Oben”. This marking must then face the piston crown (top side).
The piston rings must also be positioned correctly. In the example, the piston ring gaps are offset 120 degrees from each other. At all times, prevent the piston ring gaps from being positioned directly in line with one another.
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