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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).
History of modifications to Bentley bearings and their lubrication.

Identifier  ExFiles\Box 115\2\  scan0584
Date  7th August 1937
  
HISTORY OF MODIFICATIONS TO BENTLEY BEARINGS AND THEIR LUBRICATION.
RM.{William Robotham - Chief Engineer}447.

1) ORIGINAL 3 1/2 LITRE BENTLEY.
A.{Mr Adams} AND B. SERIES 25.6.33 TO G.9.34.
BOTH BIG END AND MAIN BEARINGS WHITEMETAL FACINGS ON STEEL SHELLS.
FRONT, CENTRE AND REAR MAIN BEARINGS AS SHOWN ON LEFT HAND SECTION, HAVE ANNULAR GROOVES .050 DEEP x .175 WIDE.
INTERMEDIATE MAIN BEARINGS, SHOWN ON RIGHT HAND SECTION, HAVE NO GROOVES.
OIL FEED TO GUDGEON PIN VIA EXTERNAL PIPE ON CON. ROD.
A SINGLE HOLE IN EACH CRANK JOURNAL AND PIN .2187 DIA.

2) 3 1/2 LITRE BENTLEY.
C. D.{John DeLooze - Company Secretary} E.{Mr Elliott - Chief Engineer} AND F.{Mr Friese} SERIES G.9.34. TO 30-1-36.
DRILLED UP CON. ROD IN PLACE OF EXTERNAL PIPE WHICH GAVE TROUBLE IN BREAKING ADRIFT FROM THE ROD.
OIL JET HOLE .031 DIA. DRILLED IN SIDE OF ROD TO SQUIRT OIL ON TO THE CYLINDER WALLS TO PREVENT PISTON SEIZURE. INCORPORATED ON 193RD IN "D" SERIES ONWARDS. (20-4-35)
BEARINGS AND LUBRICATION OTHERWISE AS ①

3) ORIGINAL 4 1/4 LITRE BENTLEY.
1ST ON "G" SERIES TO 81ST ON "H" SERIES. 30-1-36. TO 22.8.36.
BOTH BIG ENDS AND MAIN BEARINGS IN SOLID SHELLS MADE IN HALL'S ALUMINIUM-TIN ALLOY A.C.7. WHICH BASED ON TEST BED RESULTS WAS MUCH SUPERIOR TO WHITEMETAL.
GROOVES INCORPORATED ON INTERMEDIATE MAIN BEARINGS .025 DEEP x .100 WIDE -ONLY FOR 240° OF CIRCUMFERENCE.
THESE GROOVES INCLUDED TO IMPROVE THE OIL SUPPLY TO NOS 2 & 5. BIG ENDS ON WHICH FAILURES USUALLY OCCURRED.
LUBRICATION OTHERWISE AS ②

4) 4 1/4 LITRE BENTLEY.
82ND ON "H" SERIES TO 192ND ON "J" SERIES 22.8.36 TO 20.2.37.
INTERMEDIATE MAIN BEARING GROOVES MADE CONTINUOUS.
193RD ON "J" SERIES ONWARDS.
SECTION OF INTERMEDIATE MAIN BEARING GROOVES AS DEPICTED IN RIGHT HAND SECTION INCREASED TO .050 DEEP x .125 WIDE IN ORDER TO OVERCOME BIG END FAILURE WHICH STILL PERSISTED MAINLY ON NOS 2 OR 5.
THIS MODIFICATION CURED THE PREVALENT FAILURES FROM NOS 2 & 5 BIG-ENDS, SUBSEQUENT FAILURES NOT CONFINED TO ANY PARTICULAR BIG END NUMBER.

5) 4 1/4 LITRE BENTLEY.
1ST IN "K" SERIES ONWARDS 25.3.37.
INCREASED CAPACITY OIL PUMP FITTED TO OVERCOME LOW PRESSURES AT HIGH OIL TEMPERATURES
33RD IN "K" SERIES ONWARD 17.4.37.
AN EXTRA AND OPPOSITE HOLE DRILLED IN EACH CRANK JOURNAL .2187 DIA, NOW BEING 2 IN EACH.
THIS MODIFICATION TO MAINTAIN SUFFICIENT OIL SUPPLY TO THE BIG END BEARINGS, IN THE EVENT OF EXCESSIVE CLEARANCE DEVELOPING IN MAIN BEARINGS DUE TO HAMMERING OUT, THIS BEING A FAULT WITH THE A.C.7. ALLOY.

6) 4 1/4 LITRE BENTLEY.
170TH IN "K" SERIES ONWARDS 7-8-37.
MAIN BEARING ALLOY, CHANGED FROM A.C.7. (45 BRINELL) TO A.C.2 (75 BRINELL) TO RESIST HAMMERING-OUT. THIS BEING FOUND SATISFACTORY DURING 15,000 MILES TEST ON 3.B.TV.
DESPITE ALL THE PREVIOUS MODIFICATIONS BIG END BEARINGS STILL OCCASIONALLY FAILED AND REPRODUCED ON THE TEST BED BY RUNNING AT HIGH SPEEDS 1/4 LOAD FOR AN AVERAGE OF 5 MINS.
THIS COMPLETELY CURED BY HAVING 2 HOLES .125 DIA IN PLACE OF THE SINGLE .2187 DIA HOLE IN EACH CRANKPIN.
SINGLE HOLE IN BIG END BUSH TO MAINTAIN STD. OIL CONSUMPTION.
SWOL
  
  


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