Big End Bearing Failure

RM engines other than those in the late RMFs and RMHs (Pathfinders) have white metal bearings instead of more modern replaceable shell bearings. As many owners have found over the years these can fail and the first thing the driver notices is a loud knocking from the engine.  In almost all cases where an engine has covered a high mileage this is caused by a lubrication failure due to blocked oil ways within the crankshaft.  In some cases it can be due to a previous poor repair leading to failure of the white metal to adhere properly to the connecting rod.   If this happens the failure may look like the one shown below where part of the white metal has pulled away from the connecting rod cap.  If this happens in a recently rebuilt engine take it back to whoever did it and complain.  It was probably due to either the connecting rod not being preheated correctly before the white metal was poured or the surface of the connecting rod not being cleaned properly.

The Driver's Handbook says under routine maintenance that the sump should be removed for cleaning every 10,000 to 12,000 miles but how many owners actually do this?  Most owners change the oil and oil filter believing this is all that is really necessary.  Unfortunately neither the Driver's Handbook nor the Workshop Manual makes any mention of the need to clean out the sludge traps in the crankshaft so even those owners who do clean the sump are not immune to disaster.  

oil system

The Workshop Manual shows the lubrication system for the 2½ litre engine.  Unfortunately it is not a very clear diagram but it can be seen that the big ends are fed with oil from the main bearings.  Oil from the main bearings passes through drillings in the crankshaft into a chamber within the crankpin journal which is the sludge trap.  From here it is fed to the big end bearing surfaces.

When the Riley 9 was introduced in 1927 it did not have an oil filter.  To keep the oil clean it was centrifuged by the crankshaft and the heavy particles were retained in the sludge traps in the big end journals.  Cleaning out these traps was part of the maintenance schedule. Later engines based on the Riley 9 design such as the RM engines were fitted with oil filters but the sludge traps remained although no mention was made in the documentation of the need to clean them out regularly.  Presumably it was thought that the oil filter would trap all the sludge so cleaning out the sludge traps would not be necessary - how wrong this has proven to be.  As the RM crankshaft spins the sludge within the sludge traps gets compacted into a solid lump.  Eventually there is so much compacted sludge in the trap that it covers the feed hole to the bearing and starves it of oil leading to big end bearing failure and the dreaded big end knock.

RMB crankshaft

The diagram of the RMB crankshaft shows the big end plug as fitted to No. 1 big end journal.  It can be unscrewed with a large hexagon key and the trap behind it can be cleaned out.  After being undisturbed for many years it is often found impossible to unscrew the plug and it has to be drilled out.  This cannot be done with the crankshaft in place so it has to be removed from the engine which in turn means that the engine has to be removed from the chassis.  Although this can involve a lot of work the need to clean out these sludge traps cannot be over stressed and there is no point in repairing the bearings if this is not done as the new bearings will fail very quickly for the same reason that the previous ones did.

Blocked crankshaft

In this picture we can see No. 4 big end journal with the big end plug removed and resting on the crankshaft.  Next to it is a slug of compacted sludge (arrowed) removed from one of the other journals.  The compacted sludge remains within No. 4 journal and is compacted sufficiently to support a length of welding rod pushed into it.  This is an RMF crankshaft so the claim that the problem was overcome by a modification to the to the big end oil ways in the RMF crankshaft is clearly not true.

So how can the big end bearings be repaired?  There are two possible solutions.  The first is to have the connecting rods remetalled by a specialist workshop.  The workshop will need to regrind the crankshaft journals to remove any scoring and to ensure that they circular.  Wear to the journals will result in them being slightly oval.  The remetalled connecting rods will have to be machined to fit the reground journals.  An advantage of this method is that very little metal will need to be removed from the crankshaft.  Finding a workshop which can recast white metal bearings is becoming increasingly difficult as it is a skilled operation.  An Internet search is a good place to look for a workshop near to you but be sure to check their reputation and how long they have been in business.  If possible talk to someone who has had work done there.  In the past there have been workshops who have produced poor quality work resulting in the new bearings breaking up quickly.

The second solution is to convert the connecting rods to accept shell bearings and regrinding the crankpins to suit the new shell.  Before doing this check that shells will be available to fit the reground crankshaft.  A disadvantage of this method is that each time new shells are fitted and the crankshaft is reground, 10 thou. will need to be removed from the journals and there are a limited number of shell sizes available.  A workshop repairing modern engines should be able to do this repair as no white metal casting is needed.

For more information on this subject and for details of the repair procedures please see our Maintenance Notes CD  This gives a full description of the lubrication system including the oil pump, details of the crankshaft and its suitability to accept shell bearings, the bearings themselves, engine removal, engine dismantling, checking for wear and damage, journal and bearing repair and refurbishment, how to convert to shell bearings and finally how to prevent the sludge traps filling up in future.