ACL Race Series Main Bearing Set (0.025mm Reduced Clearance) - NA/NB Miata (90-05)

ACL Race Series main bearing set in the .025 size for the Mazda B6 1.6L and BP 1.8L engines in the 1990-1997 Mazda Miata (NA) and 1999-2005 Mazda Miata (NB). In ACL's size system the .025 grade is for a standard (not reground) crankshaft: each shell is .0005 in. (0.0125 mm) thicker than the H-STD shell, giving about .001 in. (0.025 mm) less oil clearance than the STD set. It is used to tighten oil clearance on a standard-size crank; it is not an oversize part and not an extra-clearance set. One set covers all five main journals; thrust washers are sold separately.

• ACL Race Series tri-metal construction: high-tensile SAE 1010 steel back, copper-lead-tin lining, nickel barrier and lead-tin-copper overlay; no flash plating, so the shells look dark rather than silver
• Size .025: each shell .0005 in. (0.0125 mm) thicker than H-STD, for about .001 in. (0.025 mm) less oil clearance on a standard crankshaft (ACL's H-.025 / H-001 grade)
• For the 50 mm main journal: ACL lists a standard main journal diameter of 49.938-49.956 mm, a block housing bore of 53.998-54.018 mm and a shell length of 17.63 mm; maximum STD wall 2.013 mm (2.000 mm for the HX extra-clearance set)
• Set for one engine: upper and lower shells for main positions 1-5 (10 shells); thrust washers not included (ACL lists them separately as 1T8353)
• ACL Race Series features: full 180-degree grooved upper shells with partial grooves in the lowers, increased crush for retention and heat transfer, increased eccentricity for high-rpm bore distortion, and enlarged ID chamfers for larger crankshaft fillet radii
• ACL sizes for this application: HX-STD (standard crank, .001 in./0.025 mm extra oil clearance), STD, .025 (standard crank, about .001 in./0.025 mm less clearance), 0.25 mm and 0.50 mm undersize (reground crank). ACL notes grades can be mixed per journal: one HX-STD with one H-STD shell adds .0005 in. (0.013 mm); one H-STD with one H-.025 shell removes .0005 in. Measure journals and assembled bearing ID and choose the size that gives your target clearance

Fitment: Main bearings for the Mazda B6 1.6L and BP 1.8L engines, which share main journal sizes. 1990-1993 Mazda Miata (NA, 1.6L), 1994-1997 Mazda Miata (NA, 1.8L) and 1999-2005 Mazda Miata (NB), including the 2004-2005 Mazdaspeed. Not for the 2006+ MX-5 (NC/ND), which use different engines.

Part number: ACL 5M8353H-.025

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