Biscuit-Tin Treasures: Sifting Good Hepolites from Autojumble Shelf-Fillers

Autojumble tables are a wonderful place to buy someone else's abandoned project. A piston looks like a simple lump of aluminium, which is exactly why so many tired ones get sold as good ones. Hepolite is one of the names that makes British enthusiasts look twice: Hepworth and Grandage pistons were fitted to plenty of British singles and twins, and a genuine Hepolite can be a useful find. But a name on the crown is only the start. Whether you are at Stafford, Kempton or a muddy field in Netley Marsh, the same checks apply.

Read the stamping before the shine

Look for the Hepolite name and a part number, plus the size marking. Standard, plus twenty, plus forty, and so on. A piston that has been bead-blasted to a silvery shine may have lost its markings and its history. Original part numbers let you check the application; a number that has been ground off or a crown with no marks is a gamble. Also check that the piston and any rings are a matched size. A plus-twenty piston in a standard bore is a shelf ornament with extra steps.

Ring grooves and lands

Run a ring into each groove and check side clearance with a feeler gauge. The ring should be free but not rattling. Look for a stepped groove, a broken land or a worn top groove that lets the ring rock. Carbon can hide a lot, but it can also hide a crack. A chrome-faced top ring with peeling chrome is not worth fitting. If the rings are missing, price the job as if you will need a new set, because NOS rings can be brittle and pattern rings vary.

Skirt wear and shape

Pistons are not round; they are cam-ground and taper slightly. Measure the skirt at the specified height, at right angles to the gudgeon pin, and compare with the book figure. Deep scoring, a collapsed skirt or a wear ridge near the top means it has done its work. Light polishing is normal; a bright patch with a lip you can catch a fingernail on is not. Check the gudgeon pin bore for slop and the circlip grooves for damage. A pin that is seized in the piston can sometimes be pressed out, but budget for a new pin and have the bore checked.

Corrosion and cracks

Aluminium corrosion shows as white powder, pitting and a general roughness, especially in ring grooves and under the crown. A piston that has lived in a damp shed for twenty years may look fine until you clean it. Inspect the skirt for cracks at the pin bosses and the crown for valve damage. Dye penetrant can help on a suspicious area. If the piston has been hot enough to soften, it may not hold its shape; a seized pin or a blue tinge is a warning.

Matched sets and weights

A set of Hepolite pistons should match in part number, size and weight. Weigh them on a kitchen scale; a big difference suggests a mismatched pair. For a twin, matched weight matters for smoothness. For a single, you still want the right piston. Check that the valve cutouts, if any, match your head. Don't mix a Hepolite with an unknown piston just because they look similar.

Questions for the seller

Ask what it came out of, whether it is standard or oversize, whether it has been run hot, and whether the rings and pin are included. Ask if you can measure it. A seller who says it will clean up is telling you it needs measuring. A seller who has a bore gauge and a micrometer is worth talking to. Red flags: a piston rolling loose in a biscuit tin with no rings, a freshly painted crown, or a price that assumes it is made of gold.

The biscuit-tin test

Before you hand over cash, picture the piston at 70 mph on a British A-road. If you would not trust it, leave it as a paperweight. A good used Hepolite can be a sensible buy if it is the right size, within wear limits and sold with the right rings. A shiny unknown piston is garage sculpture. Buy the measurements, not the story.