ACL Race Series Main Bearing Set for Honda/Acura B/K-Series

ACL Race Series Main Bearing Set - Honda/Acura B-Series & K-Series

Main bearings are what stands between your crankshaft and a very bad day. On a built B or K-series spinning past 8,000 rpm, the loads going through those shells are nothing like what a stock engine sees, and stock bearings aren't built for it. ACL Race Series is what most serious Honda builders reach for, and this is the five-main set for B-series and K-series bottom ends.

Which Part Number Do You Need?

This is the whole decision, and getting it wrong means pulling the engine back apart. Here's what each suffix means:

The two STD sets are for a crankshaft with untouched journals. The undersize sets are for a crank that's been reground, and the bearing is thicker to restore proper clearance on the smaller journal. Order the size matching what your machine shop actually cut, and note that .025mm and .25mm are ten times apart, so read carefully.

H-STD Or HX-STD? Measure, Don't Guess

The only difference is wall thickness. ACL machines the HX shell .0005" thinner than the H, and since there are two shells around each journal, that works out to .001" more oil clearance. Everything else about the two sets is identical.

Here's what matters: neither one is an upgrade over the other. The bearing doesn't set your clearance by itself, your crank journal diameter and your housing bore do, and the bearing just fills what's left. So the right answer depends entirely on what your engine measures.

Start with H-STD if your measured clearances already land where you want them. It's the baseline race sizing and it's what most performance street and motorsport builds run.

Go HX-STD if you measure on the tight side and need room, or if the engine sees sustained heat and load where a thicker oil film helps. Track cars, drift cars, and anything living at high oil temperature are the usual candidates.

What you should not do is order HX because it sounds like the better one. Too much clearance gives you low hot oil pressure and accelerated wear, and it'll look perfectly fine on the stand before it bites you. Mic your journals, measure your bores, and pick from there.

A Trick Most People Don't Know

You're allowed to mix grades. ACL says it's common practice to combine adjacent grades like H-STD and HX-STD to dial in a clearance that neither one hits on its own. Two rules if you do it: no more than .0005" difference between shells, and put the thicker shell in the most heavily loaded position. That's a legitimate tuning tool if you're a tenth of a thou off where you want to be.

Why Race Series Instead Of Stock

These are built for load, not for a 100,000-mile warranty. You get a high-strength overlay plate for fatigue resistance and a hardened steel backing supporting the lining, which is what keeps the bearing from failing under the cyclic loading a high-rpm engine generates. ACL also builds in increased crush for better holding power in the housing, and eliminates flash plating so heat transfers out of the bearing more effectively. On an engine making real power, all three of those matter.

Yes, They're Supposed To Look Like That

Worth saying before you open the box. The final step in manufacturing Race Series bearings is electroplating, which leaves them a dark tarnished color that can come out black, brown, blue, or even copper. It varies between sets and sometimes within a set. That coloring is only on the surface atoms and has zero effect on performance. Your bearings aren't used, defective, or contaminated, they just look that way. Plenty of first-time buyers panic about this.

Fitment

This five-main set covers Honda and Acura B-series and K-series engines, including the B18, K20A2, K20A3, and K24A. Both families share the same main journal size, so one part number serves both, which is handy if you've got more than one Honda project on the stand.

As with any bearing purchase, confirm your crank size before ordering. If the crank has been reground, you need the matching undersize set, not the standard.

Who Should Run These

This is for anyone assembling a B or K-series bottom end who's going past stock power or stock rpm. Fresh build, refresh, or a rebuild after a bearing failure, this is the set most Honda engine builders trust. Measure everything first, pick the size that gets you into your target clearance, and put it together right.

Fits: Honda/Acura B-series and K-series, five main bearings. Part numbers: 5M1959H-STD (standard), 5M1959HX-STD (.001" extra clearance), 5M1959H-.025 and 5M1959H-.25 (undersize for reground cranks). Construction: high-strength overlay plate, hardened steel backing, increased crush, no flash plating. Note: verify measured clearances before selecting a size.