Guides

STD, +0.25, +0.50: reconditioning sizes explained

By the Agroparts engine specialists · Last updated: 2 July 2026

STD marks a part made to the engine's original dimension. Oversize pistons and liners (+0.25, +0.50 mm) fit a cylinder rebored to a larger diameter. Undersize bearings (-0.25, -0.50 mm) fit a crankshaft ground to a smaller one. The size the machine shop records tells you which part to order.

Why reconditioning sizes exist

Reconditioning sizes exist because engines wear and machining removes metal. The machine shop rebores a worn cylinder to a clean, larger diameter and grinds a worn crankshaft journal to a clean, smaller one. Parts at the original size no longer fit either surface, so manufacturers make repair parts in fixed size steps.

The steps are standardised. Metric engines use +0.25 mm and +0.50 mm for bores, -0.25 mm and -0.50 mm for journals. Engines built to imperial dimensions, common on Perkins, Ford and other UK and US designs, use steps of 0.010, 0.020 and 0.030 inch.

STD, short for standard, is the size the engine left the factory with. An engine that has never been machined takes STD parts. After the first machining pass it takes the first repair size, and so on until the block or the crankshaft reaches its machining limit.

Oversize: pistons, rings and liners at +0.25 and +0.50

An oversize piston fits a cylinder rebored beyond its original diameter. A +0.25 piston is 0.25 mm larger than STD, a +0.50 piston is 0.50 mm larger. The piston must match the machined bore, and the rings must match the piston.

Reboring removes scoring, ovality and taper from a worn bore. The machine shop bores and hones the cylinder to the next repair size and records that size on its measurement sheet. That entry decides the piston you order. On imperial engines the same logic applies in inches: +0.010, +0.020, +0.030.

Rings follow the piston size. A +0.50 piston takes +0.50 rings; mixed sizes leave the ring gap wrong and the cylinder unsealed. On engines with replaceable liners, a new liner returns the bore to STD, so STD pistons and rings apply again.

Undersize: bearings at -0.25 and -0.50

An undersize bearing fits a crankshaft journal ground below its original diameter. The -0.25 refers to the journal, not the bearing. The shell is thicker by the same amount, so the running clearance between shaft and bearing stays at the factory value.

The label points at the journal. Grinding takes metal off the journal, so the journal gets smaller. A standard shell on that smaller journal would leave the oil clearance too wide: oil pressure drops and the bearing hammers. The thicker undersize shell closes the clearance back to specification.

Main bearings and connecting rod bearings follow the same system. The machine shop grinds the main and rod journals to whatever size cleans them up and notes each size on the measurement sheet.

How to order the right size

Order by the size the machine shop recorded on its measurement sheet, or send us the marking on the old part. Pistons carry the size on the crown; bearing shells carry it on the steel back. With that marking or the engine number, the Agroparts engine specialists confirm the exact part before you order.

Send the OEM code or engine number to comenzi@agroparts.ro, or on WhatsApp at +40 758 366 275. For enquiries in English, write to export@agroparts.ro. We confirm pistons, rings, liners and bearings in the size you need.

You can also check the catalog yourself: browse the pistons and rings or the crankshafts and bearings categories, or paste an OEM code into the parts finder and it resolves the Emmark part. We hold 7,600+ engine parts in stock in Oradea; anything not on the shelf we order from Emmark UK.

Frequently asked questions

Can I fit STD pistons after a rebore?

No. A rebored cylinder is larger than STD, so an STD piston would sit loose in the bore, seal nothing and burn oil. After a rebore you fit the oversize that matches the machined diameter, +0.25 or +0.50 mm, with rings of the same size.

What does an undersize bearing mean if the shell is thicker?

Undersize refers to the crankshaft journal after grinding, not to the bearing. A -0.25 journal is 0.25 mm smaller than original and the shell is thicker by the same amount, so the oil clearance stays at the factory value. Order bearings by the ground size of the journal.

Can one crankshaft carry different bearing sizes on different journals?

Yes. The machine shop grinds each journal only as far as it needs to clean up, so main and rod journals can end at different repair sizes. The machine shop's measurement sheet decides; we supply bearings per the sizes on it.

How do I find out which size my engine already has?

Read the marking on the old parts: pistons carry it on the crown (STD, 0.25, 0.50), bearing shells on the steel back. If the marking is unreadable, have the machine shop measure the bore or journal, then send us the measured size and the engine number and we confirm the part.