Piston to bore clearance

Crank, rods, sealing, pistons, block, flywheel etc
Guy Croft
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Joined: June 18th, 2006, 9:31 am
Location: Bedford, UK

Piston to bore clearance

Post by Guy Croft » July 5th, 2006, 11:02 am

The piston runs with the skirt in constant contact with the cylinder bore, indeed apart from the piston rings this should be the only part in contact.

Pistons are barrel-shaped across the skirt and oval viewed from the top around the skirt region. The only circular section is the ring land region and this will be considerably smaller in diameter than the bore.

Bores are machined and honed to suit piston size or vice - versa. If reboring always check availability of pistons and rings BEFORE boring.

Piston 'size' (sometimes erroneously called diameter) is the width across the skirt at the 'gauge' height (see data sheet). A racing piston manufacturer - or supplier - should state this or at least specify the bore size and tell you what the running clearance should be. I am not going to state these figures, it is unique to the piston.

Aftermarket cast piston sizes (99% of petrol engine pistons are cast, I don't know of any production engine fitted with forged as standard) should be etched or stamped on the piston and marked on the box - and this size can be either the piston size or the bore size.

Measure your bores and check the piston-to-bore fit with a feeler gauge. Before you fit your rods and pistons. If you are building the engine you are the 'engine builder' and it doesn't matter how inexperienced you are this is one of YOUR jobs! Better safe than sorry - manufacturers and machinists do make mistakes, we all do.

You need a bore gauge to measure cylider bores. You cannot measure them any other way (eg with caliper) so don't even try to. Only a bore gauge with a spring loaded anvil - like the one below (that I am using to measure a 2 liter TC block) will find true diameter. Smallest diameter is always true diameter. It is calibrated to a preset diameter, say 84mm, and it measures the difference in the bore - over or under to 0.01mm.

The best way to check a piston-bore fit is with a feeler gauge as shown. The blcok must be finish honed, chamfered lightly at the top, cleaned. I have put the feeler gauge in the bore and slid the piston onto to it to avoid damage to the gauge. You just start with a thin gauge and go thicker till it is snug - not massively tight. (If you force it, you might have real trouble getting the piston out without scoring the skirt).
NE HSR piston side clearance side view.JPG
Checking GC forged piston-bore clearance in a 2.3 liter Vauxhall HSR block, with a feeler gauge (in this case 4 thou") against the skirt.
NE HSR piston side clearance side view.JPG (151.1 KiB) Viewed 14223 times
NE HSR piston data sheet.JPG
A data sheet from my preferred supplier CP Pistons. All my pistons are drawn on CAD and emailed. A proper drawing eliminates a lot of quality problems. Order without specifying things at your peril. If anything is wrong - who's fault is it?
NE HSR piston data sheet.JPG (109.49 KiB) Viewed 14217 times
bore size check.JPG
Checking bores with a bore gauge.
bore size check.JPG (22.08 KiB) Viewed 14209 times


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