ACL Race Series Rod Bearings for K Series

ACL Race Series Rod Bearing Set - Honda/Acura K-Series

Ask anyone who's blown up a built K-series what went first, and the answer is usually a rod bearing. They live at the end of a swinging connecting rod taking the full force of combustion thousands of times a minute, and when the oil film breaks down for even a moment, they're the first thing to know about it. On a stock engine that's rarely a problem. On a high-compression, high-rpm, or boosted build, the factory bearings are working well outside what they were designed for. This is the four-rod ACL Race Series set for K20 and K24 bottom ends.

Which Part Number Do You Need?

Get this right before you order, because the wrong size means pulling it back apart:

The two STD sets are for untouched rod journals. The undersize sets are thicker to restore proper clearance on a crank that's been reground, so order the size your machine shop actually cut. Note that .025mm and .25mm are a factor of ten apart, so read carefully before clicking.

H-STD Or HX-STD? Measure First

The only difference between them is wall thickness. ACL machines the HX shell .0005" thinner, and with two shells around each journal that adds up to .001" more oil clearance. Everything else about the sets is identical.

Neither is an upgrade over the other. Your clearance is set by your rod journal diameter and your rod big end bore, and the bearing just fills what's left. So the correct choice depends on what your engine measures, not on which one sounds better.

H-STD is the baseline. Start here if your measured clearances already land in your target window, which is the case for most performance street and motorsport builds.

HX-STD is for when you measure tight and need room, or when the engine runs sustained heat and load where a thicker oil film helps. Track, drag, and high-rpm builds are the usual candidates.

Do not order HX just because it sounds more serious. Excess clearance costs you hot oil pressure and accelerates wear, and it'll look perfectly fine on the stand right up until it doesn't. Mic your journals, measure your bores, then decide.

Enlarged Chamfers For Performance Cranks

This is a detail that matters more on rod bearings than most people realize. Performance and reground crankshafts often have larger fillet radii where the journal meets the cheek, and a bearing with a standard chamfer can ride up on that radius instead of seating flat. That's a fast route to a failure. ACL Race Series bearings use enlarged chamfers specifically to clear those larger fillets, so the bearing sits where it should on a crank that isn't stock.

Built For Load, Not For Mileage

These are engineered around the demands of a race engine. You get a high-strength overlay plate for fatigue resistance and hardened steel backing supporting the lining, which is what survives the cyclic loading a high-rpm K-series produces. ACL builds in increased eccentricity to help oil film formation, and increased crush so the shell holds firmly in the rod. They also skip flash plating, which improves heat transfer out of the bearing and grip in the housing. Wall tolerances are held consistently across the set, which matters when you're chasing a clearance number across four rods.

Yes, They're Supposed To Look Like That

Worth knowing before you open the box. The last step in manufacturing Race Series bearings is electroplating, and it leaves them a dark tarnished color that can come out black, brown, blue, or copper, sometimes varying within a single set. That coloring sits on the surface atoms only and has no effect on performance. Your bearings aren't used or defective, they just look unusual. This catches a lot of first-time buyers off guard.

You Can Mix Grades To Dial It In

Useful if you're a half-thou off your target. ACL confirms it's common practice to combine adjacent grades, like running some H-STD and some HX-STD shells, to land on a clearance neither hits alone. Two rules: keep the difference between shells to .0005" or less, and put the thicker shell in the most heavily loaded position.

Fitment

This four-rod set fits Honda and Acura K-series engines, covering K20 and K24 applications:

• 2013-2015 Acura ILX
• 2007-2012 Acura RDX
• 2002-2006 Acura RSX Type-S
• 2004-2008 Acura TSX
• 2009-2014 Acura TSX
• 2003-2007 Honda Accord
• 2008-2012 Honda Accord
• 2006-2011 Honda Civic Si
• 2012-2015 Honda Civic Si
• 2002-2006 Honda CR-V
• 2003-2011 Honda Element

Confirm your crank size before ordering, and if the journals have been reground, order the matching undersize set rather than the standard.

Who Should Run These

This is for anyone assembling a K-series bottom end that's going to see more than stock power or stock rpm. Fresh build, refresh, or a rebuild after a spun bearing taught you the lesson the expensive way. Measure everything, pick the size that gets you into your target clearance, and assemble it properly. Rod bearings are cheap. Cranks and blocks aren't.

Fits: Honda/Acura K-series (K20, K24), four rod bearings, see the application list above. Part numbers: 4B1972H-STD (standard), 4B1972HX-STD (.001" extra clearance), 4B1972H-.025 and 4B1972H-.25 (undersize for reground cranks). Construction: high-strength overlay plate, hardened steel backing, increased eccentricity and crush, enlarged chamfers, no flash plating. Note: verify measured clearances before selecting a size.