Triumph BSA 650 750 Driveside Main Roller Bearing Hepolite MRJA 1-1/8 C3 PN# 70-2879C3 H

Selecting the left side main crankshaft bearing has long been confusing, as there are three different conditions of this bearing, all using the same part number, 70-2879. This Hepolite MRJA 1-1/8 C3 roller bearing features increased diametral clearance over the CN and C2 versions. Due to the behavior of aluminum crankcases that have survived decades of service, sometimes it is necessary to use a bearing with more internal clearance to achieve a smooth operating bottom-end in your BSA or Triumph 650 or 750 engine.

The Hepolite line of reproduction parts for vintage Triumph and BSA motorcycles now includes the MRJA 1-1/8 two-piece roller bearing in C3 clearance. This crankshaft bearing was used in drive-side applications in many Triumph and BSA 500, 650, and 750 engines from 1966 through 1976, although generally in CN clearance. The change to the tighter C2 clearance was made to the Triumph 750 models during the 1977 model year production. The ID is 1.125″, with a 2.8125″ OD, and a width of .8125″. Made in the UK.

The C3 clearance refers to the internal diametral clearance of the bearing and is important to consider when ordering bearings for an engine overhaul. CN clearance is normal clearance, C2 is less clearance than CN, and C3 clearance has more clearance than CN. The MRJA 1-1/8 main bearing supports the crankshaft on the driveside end. While a very robust bearing that offers a long service life, it is usually replaced during an engine overhaul due to the damage inflicted to the bearing during removal from the crankshaft journal.

The inner bearing is typically warmed before being fitted to the crankshaft journal. The outer race can be frozen for several hours and the left crankcase half heated in an oven to assist in the installation of the race. Replacing crankshaft main bearings requires patience and skill and may be best left in the hands of a professional mechanic for best results. Please consult your machinist or engine builder if you have any doubts about which version of the 70-2879 MRJA 1-1/8 bearing to use in your engine build.