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What you need to know about pistons

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IOL mot pic aug24 Corkett Piston

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A high-performance piston and its piston or gudgeon pin.

Ken Corkett

Pistons in motor vehicles have an extremely hard life and, this being so, a lot of serious development and experimentation was necessary to give us the tough and reliable ones in use today. Okay, I said tough and reliable; but apart from normal wear and tear, ham-fisted fitting often plays a large part in the their early demise, so let us take a look at the makeup of the piston and what the DIY man should and should not do when working on and around them.

The top of the piston must withstand the full heat of combustion (around 260 deg C). This being so, it expands more at the top than at the bottom. To allow for this, the top is made smaller than the middle and the bottom.

Although the top of the piston is round, the skirt starts off oval to prevent piston slap when cold. As the engine warms up, the skirt expands across the piston-pin axis and becomes round.

Control of expansion is important in piston design.

Because the alloys of aluminium expand more than steel when heated, a steel strut may be included.

This is also why a properly fitted piston pin is important. If it is too tight, connecting-rod oscillation will be impaired and the piston may be unable to expand - this time in the required manner.

Follow the workshop manual whenever specific clearances are stated. When fitting the pin to the piston, pour boiling water over the piston to expand it, the pin should go in smoothly and will be suitably held when the piston cools down. The pin should be a nice, smooth fit in the small-end.

Pistons and rods must be refitted in matched pairs, so mark them before dismantling.

Pistons are easily damaged and during their working life need all the help they can get, such as accurate matching to bore, good lubrication and correct ring gaps. It’s easy to damage pistons by careless removal and fitting of circlips. Never re-use circlips and when fitting make certain that they fit fully in their slots.

Ring gapping must be done in an unworn part of the bore. You will find that the ring in its relaxed state will be a little larger in diameter than the bore until it is gapped and compressed in the cylinder.

Be sure to fit the taper compression rings the right way up – marked TOP.

To check the fit of a piston in the bore, insert piston and with a feeler gauge measure clearance at skirt bottom on the thrust faces. Follow the manual for clearances.

Lastly, before fitting dip the assembled pistons in clean engine oil and coat the bores with it too. - Star Motoring

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