Engine Trouble Shooting

This article is about the damage that is commonly caused by pistons being damaged by the circlip. It

piston and valve damagecontains information about why the piston can run at an asymmetrical angle, causing damage to the piston, damage through axial thrust and also other similar causes damage does by having a damaged circlip.

Piston Circlip Damage Continued

In this case the Piston has been running at an angle, which has caused the piston pin circlip to be hammered out. As a result of the piston running at an angle in the cylinder and the one sided load on the piston pin, the piston pin has broken (Fig. 6), and subsequently the piston has been broken as well (Fig. 5). The asymmetrical running pattern resulting from the piston running at an angle can be seen very clearly (Fig. 4)

Damage Assessment

Whether as wire circlips or seeger type circlips, the piston pin circlip can only be forced out or hammered out in operation by means of an axial displacement of the piston pin. However, this is based on the requirement that the circlip has been correctly inserted and has not been damaged. Axial thrust in the piston pin always occurs when the piston pin axis is not parallel to the crankshaft axis during operation. This is most commonly the case when a bent connecting rod leads to the piston running at an angle. The reciprocating movements of the piston result in an alternating axial thrust which effectively hammers out the circlip lying in the main pressure direction. Once the circlip has jumped out, it is then clamped between the piston pin moving in an outward direction, the piston and the cylinder running surface. There, it is worn away and finally breaks into several fragments. Under their own inertia forces and the reciprocating movements of the piston, the fragments hammer into the piston and break it down within a very short space of time, as can be seen in (Fig. 2). Individual fragments also move through the hollow piston pin causing corresponding destruction on the opposite side of the piston.

Possible causes of Damage

Axial thrust of the piston pin during engine operation caused by the following - Bending or twisting of the conrod, Axes of conrod small ends not bored parallel to crank shaft axis, Cylinder axis not rectangular to crankshaft axis, Excessive connecting rod big-end bearing clearance, particularly in conjunction with asymmetrical connecting rods, use of old or damaged circlips or improperly installed circlips.

For more Trouble Shooting information, please see the following;

Valve Damage


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