ACL H and HX: extra clearance is not an undersize journal

Bearings work within an oil-film system

A plain bearing is not intended to carry continuous dry metal contact between journal and shell. Motion, oil and geometry must establish the required lubrication conditions. Oil clearance is a design parameter, not a simple quality ranking.

More clearance is not automatically safer; less does not automatically create a stronger oil film. Temperature, oil behaviour, speed, load, supply and the actual shape of journal and housing belong in the assessment.

Distinguish H, HX and repair sizes

For relevant Race Series applications, ACL identifies HX as an extra-clearance version relative to the corresponding H bearing. It describes approximately 0.001 inch additional diametral clearance, around 0.025 mm. Check the exact part number and availability for the application. H/HX source

This differs from a bearing intended for a reground undersize journal. A repair size works with a changed journal diameter. HX does not mean the crankshaft has been ground smaller.

STD also does not guarantee correct assembled clearance. It identifies a size variant; the actual crankshaft and housing contribute to the result.

Keep units explicit

One inch is exactly 25.4 mm, so 0.001 inch is exactly 0.0254 mm. Writing 0.025 mm is a rounded expression, not another size class.

Original arithmetic example, not a clearance recommendation: adding exactly 0.0254 mm to a hypothetical 0.0400 mm clearance gives 0.0654 mm. This does not prove that an arbitrary HX set will produce that result in an engine. Initial dimensions, tolerances and assembly still need measurement.

Retain the source unit beside any conversion. “One thousandth” without inch or millimetre creates an avoidable factor-of-25.4 ambiguity.

Measure the assembled fit

Compare the assembled bearing diameter at the specified checking position with the measured journal. Cap, fasteners and shells must be assembled under the appropriate conditions. Housing size and shape also matter. ACL's bearing guide treats these as connected mechanical requirements. Bearing-guide source

Another position around the circumference is not automatically equivalent. Bearing geometry can deliberately vary, including near the parting line. Use the applicable checking method.

Inspect journals at the relevant positions and directions. An average can conceal a local defect; a straight, round journal differs from one whose average diameter merely matches.

Mixed shells are a separate specification

Combining different shell variants is sometimes discussed as a way to obtain intermediate clearance. This is not general permission to mix arbitrary halves. Compatibility, orientation, grooves, oil holes and application instructions still apply.

Record any approved mixed arrangement as its own assembly. Half a catalogue clearance difference is at most a geometric expectation, not a measured release. Without an appropriate specification for the combination and positions, do not improvise a mixed set.

Do not conceal a defect with a different bearing

If clearance is unexpected, first check instruments, clean contact surfaces, cap identity and assembly procedure. Then evaluate journal size, housing geometry and bearing identity.

An extra-clearance shell cannot repair an incorrectly seated cap, dirt behind a shell or an out-of-round housing. A thicker shell does not replace assessment of a damaged journal. Changing the reading is not necessarily fixing its cause.

A package choice should keep base bearing, size grade, coating and application distinct. Record each position's part number, journal size, bearing measurement and fastening condition. H or HX is a way to achieve a supported assembly specification, not a universal answer to which bearing is best.