BUILDING A 3/4.5 LITRE REAR AXLE NOSEPIECE FROM PARTS
ABOVE IS SHOWN THE BEAUTIFULLY MACHINED 4.5 LITRE REAR AXLE NOSEPIECE. I BOUGHT THE BARE CASTING FROM A WELL KNOWN BENTLEY PARTS SUPPLIER AND THEN HAD IT MACHINED TO THE BENTLEY DRAWINGS BY A JIG BORING SPECIALIST IN COVENTRY. THE REASON THAT I DIDN`T BUY THE NOSEPIECE ALREADY MACHINED WAS THAT I KNEW THAT THE CASTING SUPPLIER DID NOT HAVE THE MACHINERY SUITABLE FOR THE ACCURATE MACHINING OF THE NOSEPIECE. ALSO THE MACHINING COST ME FAR LESS THAN THE SUPPLIERS MACHINING COSTS PLUS HIS PROFIT, SO THAT I SAVED 50% OF WHAT I WOULD HAVE HAD TO PAY IF I BOUGHT IT FINISHED.
ABOVE IS THE UBIQUITOUS FRUIT BOX ALMOST FILLED WITH PARTS FOR THE NOSEPIECE. I BOUGHT MOST OF THIS AS A JOB LOT IN A SALE BY A FAMOUS 3/8 RACER. TO THE WELL HEELED AND THE UNINITIATED THIS WILL PROBABLY LOOK LIKE A BOX OF SCRAP DESTINED FOR THE TIP. BUT FOR A BITSA BUILDER, IT`S MANNA FROM HEAVEN, AS MOST OF THESE PARTS CAN BE USED AGAIN THUS SAVING A SMALL FORTUNE OVER WHAT WOULD HAVE TO BE PAID FOR NEW PARTS.
NEVER FORGET THAT THESE ARE ORIGINAL PARTS THAT WILL FIT TOGETHER, THERE IS NOTHING MORE FRUSTRATING THAN BUYING NEW PARTS THAT DON`T FIT AND EITHER HAVING TO MODIFY THEM TO FIT YOURSELF OR TO SEND THEM BACK TO THE SUPPLIER TO CHANGE FOR ANOTHER PART THAT MAY NOT FIT EITHER. THERE IS ONE SUPPLIER THAT SENDS OUT DODGY PARTS AND WHEN THEY ARE RETURNED JUST SENDS THEM OUT AGAIN TO ANOTHER CUSTOMER UNTIL IT DOESN`T COME BACK!
NEVER FORGET THAT THESE ARE ORIGINAL PARTS THAT WILL FIT TOGETHER, THERE IS NOTHING MORE FRUSTRATING THAN BUYING NEW PARTS THAT DON`T FIT AND EITHER HAVING TO MODIFY THEM TO FIT YOURSELF OR TO SEND THEM BACK TO THE SUPPLIER TO CHANGE FOR ANOTHER PART THAT MAY NOT FIT EITHER. THERE IS ONE SUPPLIER THAT SENDS OUT DODGY PARTS AND WHEN THEY ARE RETURNED JUST SENDS THEM OUT AGAIN TO ANOTHER CUSTOMER UNTIL IT DOESN`T COME BACK!
THE CROWN WHEEL AND PINION
ABOVE IS A NEW STRAIGHT CUT CROWNWHEEL AND PINION THAT I GOT IN A RATHER COMPLEX SWAP. THE BENTLEY DOGMA SAYS THAT STRAIGHT CUT GEARS ARE LOUDER THAN THE USUAL SPIRAL BEVEL TYPE. THIS IS NOT ALWAYS THE CASE AND IF THE GEARS ON THE CWP ABOVE HAVE BEEN CORRECTLY CUT IT SHOULD BE QUIET ENOUGH FOR AN OPEN TOURER. THIS CWP HAS A RATIO OF 3,785 TO 1, A SPEED MODEL RATIO, SO IT SHOULD BE GOOD FOR THE FASTER REVVING 4 LITRE ENGINE.
CHOOSING A DIFFERENTIAL FOR THE NOSEPIECE
IN THE BOX OF PARTS SHOWN ABOVE THERE ARE THREE OPTIONS THAT WILL HAVE TO BE EXAMINED. THERE ARE TWO "TWO STAR" DIFFS, TWO STAR MEANS THAT THERE ARE TWO STAR OR PLANETARY WHEELS IN THE DIFF.THIS WAS THE ORIGINAL DESIGN FOR THE 3 LITRE CARS.
THE SECOND POSSIBILITY IS A CAST IRON DIFF CAGE WITH FOUR STAR WHEELS WHICH WAS THE STANDARD DIFF FOR THE 3 AND 4.5 LITRE CARS AFTER THE TWO STAR DIFF WAS PHASED OUT..
THE THIRD POSSIBILITY IS THE SPECIAL YOKE THAT HAD BEEN USED IN A NON STANDARD SPECIALIST AFTER MARKET DIFFERENTIAL. THIS COULD BE USED IF A HIGH STRENGTH ALUMINIUM SPLIT DIFF CAGE WAS MACHINED UP TO SUIT.
THE SECOND POSSIBILITY IS A CAST IRON DIFF CAGE WITH FOUR STAR WHEELS WHICH WAS THE STANDARD DIFF FOR THE 3 AND 4.5 LITRE CARS AFTER THE TWO STAR DIFF WAS PHASED OUT..
THE THIRD POSSIBILITY IS THE SPECIAL YOKE THAT HAD BEEN USED IN A NON STANDARD SPECIALIST AFTER MARKET DIFFERENTIAL. THIS COULD BE USED IF A HIGH STRENGTH ALUMINIUM SPLIT DIFF CAGE WAS MACHINED UP TO SUIT.
THE TWO STAR DIFFERENTIAL
ABOVE, A CROSS SECTION THROUGH THE TWO STAR DIFF TAKEN FROM THE BOOK PUBLISHED BY THE BENTLEY DRIVERS CLUB "TECHNICAL FACTS OF THE VINTAGE BENTLEY". ESSENTIAL READING FOR ANY POTENTIAL BENTLEY OWNER. JOIN THE CLUB AND BUY IT NOW!
THIS WAS THE FIRST BENTLEY DIFFERENTIAL, PROBABLY DESIGNED BY BURGESS ON HIS KITCHEN TABLE. IT`S A BIT OF A LASH UP REALLY, DESIGNED TO ELIMINATE ANY EXPENSIVE TO PRODUCE STEEL CASTINGS. THE PRINCIPLE IS THAT THE DIE CAST ALUMINIUM CAGE IS SANDWICHED BETWEEN TWO SIDE PLATES AND HELD TOGETHER BY EIGHT HIGH TENSILE BOLTS. THE COMBINED PRESSURE EXERTED BY THESE BOLTS COMPRESSES THE CAGE BY SEVERAL THOUSANTHS OF AN INCH AND THE WAY THAT THE BORES FOR THE STAR PINION SHAFTS WERE FINISHED OFF WAS TO MAKE THE BORES IN THE BUSHES SLIGHTLY UNDERSIZED WITH THE OUTER DIAMETER OF THE BUSHES A PRESS FIT IN THEIR BORES. THEN ASSEMBLE THE CAGE AND SIDE PLATES TOGETHER AND TORQUE UP THE BOLTS. THIS DEFORMS THE BUSH, WHICH HAS TO BE REAMED TO SIZE TO ALLOW THE STAR WHEELS TO BE ASSEMBLED.
THIS WAS THE FIRST BENTLEY DIFFERENTIAL, PROBABLY DESIGNED BY BURGESS ON HIS KITCHEN TABLE. IT`S A BIT OF A LASH UP REALLY, DESIGNED TO ELIMINATE ANY EXPENSIVE TO PRODUCE STEEL CASTINGS. THE PRINCIPLE IS THAT THE DIE CAST ALUMINIUM CAGE IS SANDWICHED BETWEEN TWO SIDE PLATES AND HELD TOGETHER BY EIGHT HIGH TENSILE BOLTS. THE COMBINED PRESSURE EXERTED BY THESE BOLTS COMPRESSES THE CAGE BY SEVERAL THOUSANTHS OF AN INCH AND THE WAY THAT THE BORES FOR THE STAR PINION SHAFTS WERE FINISHED OFF WAS TO MAKE THE BORES IN THE BUSHES SLIGHTLY UNDERSIZED WITH THE OUTER DIAMETER OF THE BUSHES A PRESS FIT IN THEIR BORES. THEN ASSEMBLE THE CAGE AND SIDE PLATES TOGETHER AND TORQUE UP THE BOLTS. THIS DEFORMS THE BUSH, WHICH HAS TO BE REAMED TO SIZE TO ALLOW THE STAR WHEELS TO BE ASSEMBLED.
THERE ARE TWO DIE CAST ALUMINIUM TWO STAR DIFF BODIES SHOWN ABOVE WITH THE PINIONS IN PLACE
ABOVE, A CLOSE UP OF A PAIR OF PINIONS FOR THE 2 STAR DIFF. YOU CAN SEE THAT THE THINNER ENDS OF THE TEETH ARE WORN TO A POINT. THIS IS DUE TO PLAY DEVELOPING IN THE BRONZE BEARINGS THAT THESE RUN IN. THIS IS THE WEAK POINT IN THE 2 STAR DIFF SO THAT THE BEARINGS WILL HAVE TO BE CHANGED EVERY 20,000 MILES OR SO. I HAVE ONE IN MY BITSA AND IT HAD NO PROBLEM COPING WITH THE FORCES OF A 4.5 LITRE ENGINE.
HERE ARE TWO BETTER PINIONS THAT CAN BE USED IN THE 2 STAR DIFF
ABOVE LEFT PHOTO, THE LATER TYPE OF DIFF SIDEPLATE IS THE ONE ON THE LEFT. THIS IS SUITABLE FOR ALL 3/4.5 LITRE DIFFERENTIALS. THE ONE SHOWN ON THE RIGHT IS THE EARLIER TYPE. ON THE ABOVE RIGHT HAND PHOTO ARE SHOWN THE REVERSE SIDE OF THE SIDEPLATES. AS THESE SIDEPLATES COST £176 NEW FROM BENCHMARK THEN IT WOULD BE COST EFFECTIVE TO BUILD UP THE WORN SURFACES ON THESE USED SIDE PLATES TO THE LATER PATTERN WITH WELD AND MACHINE THEM BACK TO SIZE. THESE ARE SUITABLE FOR ALL THE OPTIONS. AN UNUSUAL FEATURE OF THE DESIGN IS THAT THE SUNWHEELS SHOWN BELOW RUN IN THE BORE IN THE SIDEPLATE, STEEL TO STEEL. IT WORKS BECAUSE THE SUN WHEEL ONLY ROTATES WITHIN THE SIDE PLATE WHEN THE WHEELS ARE ROTATING AT DIFFERENT SPEEDS SUCH AS WHEN CORNERING, SO THAT THE NUMBER OF REVOLUTIONS PER MINUTE ARE NOT HIGH OR PROLONGED. NONE OF THE SUN WHEELS SHOWN BELOW SHOW SIGNS OF WEAR ON THE DIAMETER THAT REVOLVES IN THE SIDE PLATE.
THERE ARE SEVEN SUN WHEEL PINIONS THAT FIT ON THE END OF THE HALF SHAFTS TO CHOOSE FROM. THESE ARE SUITABLE FOR ALL THE OPTIONS.
THE FOUR STAR DIFFERENTIAL
ABOVE IS A CROSS SECTION THROUGH THE FOUR STAR DIFF, ALSO TAKEN FROM THE TECHNICAL FACTS.
THIS WAS THE STANDARD DIFFERENTIAL CAGE FOR THE 4 CYLINDER BENTLEYS AFTER THE TWO STAR DIFF WAS DISCONTINUED. YOU CAN SEE THAT THE DESIGN WAS DEVELOPED TO INCREASE THE NUMBER OF STAR WHEELS TO FOUR TO SHARE THE LOAD. THE CAGE IS A CAST IRON CASTING AND THE CASE HARDENED STAR WHEEL SHAFTS ARE NOW STATIONARY, CLEVERLY HELD IN PLACE BY THE THROUGH BOLTS THAT TIE THE SIDE PLATES TO THE CAGE. THE STEEL STAR WHEELS ROTATE ON THEIR SHAFTS.
THIS WAS THE STANDARD DIFFERENTIAL CAGE FOR THE 4 CYLINDER BENTLEYS AFTER THE TWO STAR DIFF WAS DISCONTINUED. YOU CAN SEE THAT THE DESIGN WAS DEVELOPED TO INCREASE THE NUMBER OF STAR WHEELS TO FOUR TO SHARE THE LOAD. THE CAGE IS A CAST IRON CASTING AND THE CASE HARDENED STAR WHEEL SHAFTS ARE NOW STATIONARY, CLEVERLY HELD IN PLACE BY THE THROUGH BOLTS THAT TIE THE SIDE PLATES TO THE CAGE. THE STEEL STAR WHEELS ROTATE ON THEIR SHAFTS.
ABOVE YOU CAN SEE THE DIS-ASSEMBLED CAGE AND PINION ASSEMBLY. THE SHAFTS HAVE BEEN SCORED BY LACK OF LUBRICATION BETWEEN THEM AND THE GEAR BORES. THEY COULD BE SAVED BY GENTLE HONING TO ELIMINATE BURRS. THE GEARS ARE USEABLE BUT WILL HAVE TO HAVE THEIR BORES HONED TO A SMOOTH FINISH. THE ACHILLIES HEEL OF THIS DIFF IS THAT IT HAS BEEN FOUND THAT CRACKS CAN OCCUR AT THE INNER ENDS OF THE SHAFT LOCATING BORES, SO THAT THIS CAGE WILL HAVE TO BE TESTED FOR CRACKS BEFORE CONSIDERING USING IT.
THE MYSTERY AFTER MARKET DIFFERENTIAL PARTS
ABOVE LEFT THE PARTS THAT WERE IN THE JOB LOT THAT I BOUGHT ALL THOSE YEARS AGO, COMPRISING THE YOKE AND THE STANDARD STAR PINION THAT HAS BEEN BUSHED WITH A BRONZE BUSH DOWN TO 3/4" FROM 23mm. THE YOKE IS A CLEVER DEVELOPMENT OF THE 4 STAR DIFF AS NOW THE SHAFTS FOR THE STAR WHEELS ARE SUPPORTED AT BOTH ENDS INSTEAD OF BEING SUPPORTED AT ONE END ONLY. ON THE UPPER RIGHT PHOTO I`VE PLACED THE YOKE AND STAR WHEEL ON THE TWO STAR DIFF CAGE TO SHOW HOW IT WILL BE ENGINEERED. THE CAGE WILL BE MADE FROM HIGH TENSILE ALUMINIUM AND BE SPLIT SO THAT THE YOKE CAN BE ASSEMBLED IN IT. THE EXISTING STAR WHEELS CAN BE BUSHED TO SUIT THE YOKE WHICH HAVE ACCURATELY GROUND 3/4" DIAMETER SHAFTS.
THERE WE ARE, THREE DIFFERENT POSSIBILITIES BUT WHICH ONE WILL I CHOOSE? IT`S GOOD TO HAVE A CHOICE AND LOOKING AT THE THREE SOLUTIONS I SUPPOSE THAT THE TWO STAR DIFF IS THE LEAST FAVOURITE. THE STANDARD FOUR STAR, IF THE CAGE IS GOOD, IS THE CHEAPEST OPTION BUT THE MYSTERY PARTS ARE PROBABLY THE WAY THAT I WILL GO. IT WILL MEAN THAT I`LL HAVE SOURCE THE MATERIAL AND MACHINE IT. GREAT ACCURACY IS REQUIRED TO GET THE 4 SHAFTS SATISFACTORILY SEATED IN THE CAGE AND AS THE PINIONS ARE GOING TO BE SLEEVED, THEY WILL BE MACHINED TO FIT PERFECTLY ONTO THE GROUND SHAFTS.
THERE WE ARE, THREE DIFFERENT POSSIBILITIES BUT WHICH ONE WILL I CHOOSE? IT`S GOOD TO HAVE A CHOICE AND LOOKING AT THE THREE SOLUTIONS I SUPPOSE THAT THE TWO STAR DIFF IS THE LEAST FAVOURITE. THE STANDARD FOUR STAR, IF THE CAGE IS GOOD, IS THE CHEAPEST OPTION BUT THE MYSTERY PARTS ARE PROBABLY THE WAY THAT I WILL GO. IT WILL MEAN THAT I`LL HAVE SOURCE THE MATERIAL AND MACHINE IT. GREAT ACCURACY IS REQUIRED TO GET THE 4 SHAFTS SATISFACTORILY SEATED IN THE CAGE AND AS THE PINIONS ARE GOING TO BE SLEEVED, THEY WILL BE MACHINED TO FIT PERFECTLY ONTO THE GROUND SHAFTS.
TO BE CONTINUED