Manual - Chapter 1 Engine

22. Examination and renovation: connecting rods and big-end bearings

  1. Examine the connecting rods for signs of cracking or distortion, renewing any rod which is not in perfect condition. Check the connecting rod side clearance, using feeler gauges. If the clearance exceeds the limit specified it will be necessary to renew the connecting rod or the crankshaft assembly. Connecting rod distortion, both bending and twisting, can only be measured using a great deal of special equipment and should therefore be checked only by an expert; otherwise the rods should be renewed if there is any doubt about their condition.
  2. If the necessary equipment is available, the condition of the small-end assemblies can be checked by direct measurement, referring to the tolerances given in the Specifications Section of this Chapter. If the equipment is not available, it will suffice to ensure that the bearing surfaces in the connecting rod small-end bearing, in the piston bosses and over the entire gudgeon pin are smooth and unmarked by wear. The gudgeon pin should be a tight press fit in both connecting rod and piston, and there should be no free play discernible when the components are temporarily reassembled. If any wear is found, the component concerned should be renewed.
  3. If the small-end bearing is to be renewed, it must be pressed out using a drawbolt assembly, as shown in the accompanying illustration (fig. 1.5). On fitting the new bush, align its split joint at 60 to the left or right of the connecting rod centre line and drill and deburr a new oil hole through the bush, using the connecting rod oilway as a guide to the position and size. Finally, ream the new bush to the specified size.
  4. If a connecting rod is renewed at any time, it is essential that it is of the correct weight group to minimise vibration. Rods are supplied in seven weight groups each being identified by having one, two or three spots of yellow, white or blue paint. All rods must have the same paint marking, if this has been erased on all rods they must be individually weighed (less bearing shells but complete with cap, bolts and nuts) so that the weight group can be identified by an authorised BMW dealer when a new rod is ordered. The weight groups vary by 7-8 grams.
  5. The big-end bearing cap retaining bolts and nuts should be renewed whenever they are disturbed. The bolts are of the expansion type which are permanently stretched on tightening and must never be re-used. They are an extremely tight fit in the connecting rods, however, and great care will be required to avoid marking the rods when tapping out the old bolts and fitting new ones. If a rod is clamped in a vice for support. be careful to fit the vice with padded jaws to avoid any risk of marking the rod surfaces.
  6. Examine closely the big-end bearing shells. The bearing surface should be smooth and of even texture, with no sign of scoring or streaking on its surface. If any shell is in less than perfect condition the complete set should be renewed. In practice, it is advisable to renew the bearing shells during a major overhaul as a precautionary measure. They are relatively cheap and it is false economy to re-use part worn components.
  7. The crankshaft journals should be given a close visual examination, paying particular attention where damaged bearing shells were discovered. If the journals are scored or pitted in any way, seek the advice of a good authorised BMW dealer. While undersize shells are listed, making it possible in theory to reclaim wear or damage by regrinding the crankshaft. BMW give very little information to assist the repairer and state that since the crankshaft is surface hardened it cannot be reground by normal methods.
  8. To select new shells, use the manufacturer's size code system. The standard crankpin outside diameter is divided into three size groups, to allow for manufacturing tolerances. The size group of each crankpin is indicated by a dab of white, green or yellow paint on the crankshaft webs immediately adjacent to the crankpin; if the equipment is available, these marks can be checked by direct measurement. Given that the connecting rod big-end eye diameter does not vary significantly in service, the shells, which are identified by a similar colour coding, are selected to match the crankshaft paint mark, ie a green paint mark on the crankshaft means green-ended bearing shells for that crankpin.
  9. If the existing inserts are to be checked for wear with a view to re-using them, use Plastigage to check the clearance as described in Section 14. If the clearance measured is within the specified limits, the existing shells can be re-used. If the clearance is excessive, even with new shells (of the correct size code), the crankpin is worn and expert advice must be obtained.


22.1 Connecting rod assembly - retaining bolts and nuts must be renewed whenever they are disturbed


22.8 Measuring crankpin diameter