Piston Ring Gaps

Why Piston Ring Gaps are Crucial in Preventing Engine Damage

Federal-Mogul explains why it is important to understand the acceptable degree of piston ring gaps in preventing blow-by and oil consumption, and ultimately, preventing engine damage.

Installers occasionally have queries concerning apparent differences in piston ring gaps from those recommended in engine manufacturers’ literature. The main fear of the engine builder stems from the thought that gas and oil may pass through the gap and therefore the gap must be as small as possible to prevent blow-by and oil consumption. However, a slight variation is not as hazardous as it might seem.

Observing Fitted Gaps at Operating Temperatures

It is critical to observe the fitted gap at the operating temperature because the ring material will expand as the temperature rises. If the gap is too small, the ends of the ring may collide which can lead to the ring breaking, scoring the bore, and eventually lead to complete breakdown.

However, piston rings can be made from a wide variety of irons and steels with very different coefficients of expansion. So, a slight variation in the gap is acceptable.

The Perfect Pre-Gap

All Goetze piston rings are pre-gapped for ease of installation, and to ensure the gap precisely suits the material specification, engine operating conditions and cylinder bore size. They’re also checked before dispatch, in gauges conforming exactly to the relevant bore size. So you can be sure the Goetze piston ring you are fitting is the right one for the job, without removing the rings for checking. Even so, Goetze piston gaps may not be the same as other ring manufacturers may recommend as it depends on the materials, any surface treatments, and even the groove location.

Tests by OE engine builders have proved that a slightly larger gap will not always have a negative effect. If the cylinder bore and ring grooves are the correct size, and the ring grooves correctly dimensioned and finished, a new ring on a new piston in a new bore will not suffer from increased blow-by or oil consumption – even with a ring gap larger than the manufacturer’s recommendation.

On the other hand, when fitting new rings to worn or re-grooved pistons, it’s important to inspect the condition of the grooves. As a rough guide to wear limits, a 0.006″ (0.152mm) feeler gauge blade should not be able to enter the groove when the new ring is in place. If it does, the wear is excessive and the piston should be replaced.

Aligning Ring Gaps

Unless pegged, all piston rings slowly rotate round in their grooves while the engine is running. For this reason, it isn’t important to make sure the ring gaps are aligned – though you may find it easier for fitting if you stagger the ring gaps before you apply a ring clamp. However, no harm will be done if the gaps temporarily align in service.

So despite the dire warnings often given about the importance of ring gaps, you’ll find that the side face flatness of the piston rings, freedom from chatter, and the wave form of the ring grooves, are far more important in preventing blow-by and oil consumption. And those are all features which get close attention during piston and ring production at Federal-Mogul.

Further information can be found in Federal-Mogul’s Service Bulletin SB2129. Goetze piston rings are available same/next-day from FPS via the MAM Autocat and the F:Drive. For more information, speak to your local FPS representative, log on to www.partinfo.co.uk or www.fmecat.eu


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