From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Analysis of connecting rod failures in 4¼-litre engines, discussing contributing factors like oil pressure and over-revving.
| Identifier | ExFiles\Box 131\2\ scan0064 | |
| Date | 30th September 1937 guessed | |
| - 2 - 1. B.85.GP 4¼-litre con.rod failure. If 5lbs idling pressure and 25lbs maximum hot is considered dangerous with the absence of any abnormal engine features (bearing thump, roughness etc) there are a large number of 4¼-litre engines in service which under Continental conditions will shew a much lower oil pressure and are still carrying on. In substantiation of this comment of mine, I quote to you the following case:- ø 90-92 m.p.h. 4,500 r.p.m. dropped to 11-lbs square inch. A recent new 4¼-litre, B.169.JY for which the Firm have received a glowing tribute from the owner, and which is still running satisfactorily after climbing the Oberlap, Julier, Mayale and high speeds on the Italian Varse Brescia Autostrada for 83 miles, gave the following oil pressures. (Owner's report). 75 to 80 m.p.h. oil pressures dropped to 22-19lbs. Air temp, 75° F.{Mr Friese} 80° F.{Mr Friese} With normal running afterwards the oil pressure regained its standard pressure of 30-35lbs. (This with the small oil pump). This owner, who although he queried the drop of pressure, has not made a complaint to the Depot. It must be remembered, though, that in the case of 85.GP, the outgoing oil pressure prior to the failure was 7lbs to 9 idling, 28 to 30lbs maximum speed hot, rectified by lapping in the release valve of the pump. This fact, coupled with the examination carried out at Derby when I was present, clearly indicates to me that there are circumstances other than low oil pressures which are either a contributory to, or may be the actual cause of the failure. I am given to understand that the failure of this rod occurred on a route very similar to that which we experience abroad. The Stamford to Grantham straight. A very likely road where an owner may easily over-rev, and honestly thinks he was keeping within the max.r.p.m. of the engine. There is a great possibility that unwittingly he over-revved in third. I have been guilty of this myself. One conclusion that I have come to is that over-revving plus the tightening up of a big end bearing due to abnormal expansion and the trapping of hot oil in the bearing must not be precluded from being a possible solution to the failure of con. rod bearings. Arising from observations made in No.1, will By issue a ruling to the Depots on the range within which oil pressures will be safe - that is to say, Idling 30,60,90.m.p.h. 2. Arising from this, there has been a considerable controversy between myself and the Depot concerned on the question of the amount of clearance given to big end -contd- | ||