Fig. 1: Measuring the ring height clearance

Assessment of used pistons

Measurement and assessment of ring grooves

KS | Kolbenschmidt | Motorservice

Can you continue to use used pistons if you mount new piston rings? How do you measure the ring height clearance? From what point is a ring groove worn? What is the function of the feeler gauge? You can find out here.

If new piston rings are to be pulled on to pistons that have already been run in, the ring height clearance determines whether the piston can be reused. The relevant piston ring is inserted in the cleaned ring groove as shown in Fig. 1 and measured with a feeler gauge. If a new piston ring is to be measured in a run-in piston, the method shown in the figure is better than mounting the piston ring on the piston. Pulling the piston ring repeatedly on and off the piston may cause material deformation on the piston ring, which impairs the function.

Fig. 1: Measuring the ring height clearance


Fig. 2: Worn ring groove

The wear dimension refers to the outer edges of the ring groove to be measured, i.e. it must not be possible for the feeler gauge with a thickness of 0.12 mm to be pushed between the piston ring and ring groove as shown in Fig. 2. In this case, the ring groove is classed as already worn.

Ring height clearance (mm) Suitability of the piston
0.05-0.10 Pistons can be used
0.11-0.12 Increased caution is required
> 0.12 Piston is worn and has to be replaced
Fig. 3: Visual inspection of the groove

It is not possible to check the ring height clearance on keystone rings in pulled-on and untensioned state. Due to the keystone shape, the correct ring height clearance is only achieved in the keystone groove if the piston ring is pressed together on the cylinder dimension or mounted in the cylinder.

This means that a measurement is difficult. For this reason, the inspection must be restricted to a visual inspection of the groove for wear (Fig. 3).