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  • Crankcase differences

    My RCS Silverstone has GPA2 crankcases that are looking a little sad and I decided to use a mint pair of GPA1 (MX1) crankcases I have. But, the GPA1, and GPA5 set that I have appear to slightly different.
    The GPA1 and 5 sets are flush across the main bearing faces but the Silverstone GPA2 cases are machined back approx. 1.5mm across that face leaving a raised boss for the main bearings. There is also a quarter circle machined out below the barrel studs. This would result in a lower primary compression although larger volume compared to the GPA1/5 but why was this done? It appears to be original unless someone knows otherwise.
    Any thoughts?
    Dogsbody

  • #2
    Dogsbody - your cases sound similar to some I fitted to my Silverstone. They were also GPA2 but I have no idea why they were machined in the way they were. It seems these cases were fitted to all RBS and RCS models. I fitted padding discs to bring them back to 'normal'.

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    • #3
      Rob, I was thinking the opposite, ie machining out the MX1 cases to match the Silverstone's, reasoning that the larger carb might need a bigger volume to fill. I have since read this,
      "Referring again to the work of Fujio Nagao (and his results have been verified by other researchers) we find some very interesting conclusions with regard to primary compression ratios: First, Nagao tells us that for given port areas, there is an engine speed at which maximum air delivery to the cylinder occurs, and that this engine speed is inversely proportional to crankcase volume, but that the maximum value changes only slightly with changes in crankcase volume. To put it another way, the crankcase-pump's volumetric efficiency is nearly constant, but the engine speed at which it attains maximum efficiency rises as crankcase clearance volume is reduced. Significantly, too, Nagao goes on to say that any deficiency in air delivery due to a crankcase volume too great for a given engine speed is fairly well compensated by properly tuned intake and exhaust pipes. He says in conclusion that “little advantage is obtained by making the crankcase volume excessively small.”"
      Dogsbody

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      • #4
        Oh well, it didn't seem to mind much (or make any difference). I assume they went back to flat-sided again later? Interesting article though.

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        • #5
          Interesting subject, I didn’t realise there was any difference between Silverstone and Challenger crankcases. After the rod broke and took out the original crankcases of my Silverstone at the last race of 2015 I am rebuilding the motor with a set of GPA5 (653) cases that I got many years ago, fitted with a Silverstone cylinder in a speedway style cycle.
          My original cases had a number stamped by Peter Williams when he built the bike in early 1966 but he started with an RCS as the RDS bikes had not been produced when the season started, so he built a heavily modified frame from this, that was fitted with Reg Orpin’s water cooled cylinder. He restamped the frame with a new number 24RDSSTE1 but you can still see the remains of the bracing tube between the rear motor mounts that had to be removed to fit the RDS CC gearbox. Anyway, these cases did not have the step around the main bearing bores so, as the GPA5 also did not have the steps, I assumed that the cases were common. When I check my RCS that is in boxes awaiting restoration, I see the cases (GPA2 164) for that have the steps and the machining under the cylinder studs.
          On another subject, has anyone compared the port maps of the RDS and RES cylinders? I have been using what I think is an RES part (stamped G1213) and the piston covers 5mm of the bottom of the exhaust port at BDC and 4mm of the bottom of the lowest point of the angled transfers. On the air cooled cylinder that came with the RDS (stamped G1088) about 2mm of the exhaust port remains covered by the piston at BDC but the piston crown is level with the lowest corner of the transfers? I don’t have a drawing of the RDS sleeve but there is a copy of a sleeve drawing in the RES section of Derek Pickard’s book, and this seems to match the cylinder I have, so I don’t think the differences are due to someone’s over-enthusiastic porting!
          Anyone have any information on this?
          Thanks, Stan

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          • #6
            Nobody has any information about Silverstone porting?

            Stan

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            • #7
              Silverstone porting

              Stan,
              Apologies for the lack of replies, I was hoping someone would step in, they haven't so I'll hunt out the info. I did the drawings for Derek Pickard's book so I'm sure I have the drawings somewhere.
              Druid

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              • #8
                Originally posted by druid View Post
                Stan,
                Apologies for the lack of replies, I was hoping someone would step in, they haven't so I'll hunt out the info. I did the drawings for Derek Pickard's book so I'm sure I have the drawings somewhere.
                Druid
                If I remember correctly, I attempted to put an MX5 gearbox on the early MX1 cases and the primary chain would not fit. My only guess is the gearbox mount on the early engine cases are slightly longer.

                Kenny Sykes
                USA

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                • #9
                  Here we are, info from the parts books:

                  RBS Cylinder = G1088
                  RCS Cylinder = G1088
                  RDS Cylinder = G1088
                  RES Cylinder = G1213

                  BUT, drawings are as follows:

                  G1069 RBS Cylinder Sleeve Dated 17/1/66

                  G1089 RBS Cylinder Liner Dated 3/6/65

                  G1168 RBS Sleeved Cylinder Dated 17/1/66

                  However, the last RBS left the factory on 2/10/64 and that last RCS seems to have left on the 29/4/65.

                  Therefore, despite claiming to be RBS parts, they weren't drawn up until the RBS and RCS had all left. The first RDS was delivered on the 7/4/66 so these drawings must be specific to that model (I would have thought). G1168 does refer to G1088 as the 'material' so it looks as though the RBS cylinder was modified for the RDS but kept the same part number!

                  This explains why there are no RDS cylinder drawings in the file despite page 102 of the Greeves book stating that 'the RDS had it's port timings altered and was fitted with a suitably modified piston'. The RBS and RCS piston was part number G1005 but the the RDS was 15298/V which looks a bit like a hepolite part number to me.

                  Now, back in the day, or just after to be more precise, it was reported in the press that Greeves were making a batch of Silverstone barrels. After a certain number had been produced, some vital piece of equipment destroyed itself and that was the end of the batch or anymore ever. I have, somewhere in the loft, a barrel allegedly from this batch together with a piston so I must take a look and if I can find it, I will measure the ports and see if it lines up with any of the drawings.

                  Oh well, that's another evening taken care of in the interests of science, attempting to sort fact from fiction and yet more Greeves vagaries uncovered!

                  If you want better copies of the drawings Stan, drop me a PM.
                  Attached Files

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                  • #10
                    Silverstone port timing

                    Rob,
                    I sent Stan a pm yesterday stating exactly the same as you say, together with those drawings although I redrew because of the state of the originals. These were the drawings that Derek Pickard used for his book.
                    The barrel drawing shown was for a barrel that had been sleeved, I believe these were for service exchange barrels.
                    Whilst the port timings were determined by the liner/sleeve there are no drawings showing port dimensions only the interfaces such as inlet and exhaust flanges. These can have quite an effect on the barrel characteristics but unfortunately this info isn't around.
                    As far as the vital piece of equipment being destroyed, this could be that the only guy at the factory who knew how to run the pantograph left the firm.

                    Druid

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                    • #11
                      Originally posted by Bearingman View Post
                      If I remember correctly, I attempted to put an MX5 gearbox on the early MX1 cases and the primary chain would not fit. My only guess is the gearbox mount on the early engine cases are slightly longer.

                      Kenny Sykes
                      USA
                      Silverstone with GPA2 or GPA5 cases with cam barrel box and Griffon clutch takes 72 link chain not 70 as I found out recently. Colin

                      Comment


                      • #12
                        Thank you Druid and Rob.

                        As both the transfers and exhaust port are farther down the bore on the RES vs. the RBS/RCS it does look like the it was intentional that a portion of both ports remain covered by the piston at BDC. Strange! Now all I have to find will be the original overall length of the sleeve so I can determine the port timing measured from piston TDC.

                        Stan

                        Comment


                        • #13
                          Thanks to the information from Druid and Rob, plus the stuff in Derek Pickard’s book, I have put together what I think is the history of the Silverstone cylinders from the RBS onwards although, as an ex jig & tool draughtsman and after a 36 year career as a production engineer, the lack of any revision control on these drawings does make me wonder how they controlled drawings and the subsequent parts manufactured from them:

                          The name Liner seems to be used as the term for the heavy wall, cast in liner and the drawings do show pictorially a thicker wall than those for the Sleeve titled drawings, used to reclaim cylinders that have reached the max oversize of + .010” before exceeding the 250cc capacity limit for racing.

                          G1089 (dated 9/5/64) Titled RBS Liner: This is probably the original porting for the RBS as the first RBS left the factory in May, 64 according to Derek.

                          G1089 (dated 3/6/65) Titled RBS Liner: Exhaust and inlet port sizes increased. According to the available drawings these sizes remain constant from this point through the RES. Both G1089 drawings call up the same H&G sleeve part number as material. I checked the cylinder from my RCS and that, as far as I know, is original and it has the same porting as the earlier revision of the drawing. So it would seem that the RCS had the same porting as the RBS. As this version of the liner drawing is dated after the last RCS left the factory that would make sense, although why Greeves would produce revised porting in June 65 and not introduce it until the 1966 models is a mystery. The cylinder from my RDS matches the porting sizes and positions shown on this 3/6/65 version of the drawing.

                          G1169 (dated 17/1/66) Titled RBS Sleeve: This drawing is for the thin wall sleeve used to reclaim oversize cylinders and carries the same porting dimensions and positions as the 3/6/65 version of drawing G1089. This would make this sleeve suitable for use with RDS cylinders.

                          G1168 (dated 17/1/66) Titled RBS Sleeved cylinder: Companion drawing to G1169 showing the machining dimensions for fitting the G1169 sleeve into RDS cylinders.

                          G1235 (dated 3 Feb 67) Titled RES Cylinder Sleeve: Pictorially this drawing shows a thin wall sleeve and the title, sleeve, indicates this was for reclaiming oversize cylinders if the same drawing convention was being followed. However, the material call up is the same thick wall H&G sleeve as used for the cast in liners, G1089. Either this was an error or the same drawing was used for both the cast in and thin wall sleeves but using the different H&G sleeves, as the RES cylinder carried this part number. While the port dimensions are the same as those of the 3/6/65 version of G1089 the ports were all moved down in the bore by 1.75 mm (or the imperial equivalent as the transfers were dimensioned in inches), including the transfer windows at the bottom of the bore. My RES cylinder stamped G1235 conforms to the porting shown in this drawing.

                          According to Derek Pickard, the RDS was considered by many to be the fastest of all the Silverstones and this would make sense as the cylinder had the larger ports without them being moved lower in the bore as per the RES, which resulted in less duration for the exhaust and transfers while giving increased duration for the inlet but leaves the exhaust and transfer ports partially covered by the piston at BDC. I have used all 3 versions of cylinders over the past 33 years I have been racing my Silverstone but have never run it on a dyno to see what power it developed. I changed cylinders as each was damaged, was awaiting reboring or had reached the max size and still needed further boring to remove ovality, etc. When I was running with the original Hepolite pistons with the Dyke top ring, any tightening up would usually result in a broken top ring and subsequent damage to the top of the exhaust port. I went through a lot of pistons and rebores over the years. Luckily we are allowed up to 5% oversize engines in classic racing here so 2mm oversize is just within the limits. I have just had a new sleeve fitted to the RES cylinder to repair the damage caused by the broken rod and need to get my RDS cylinder resleeved again (it was done here in Canada per drawing G1168 many years ago) and has now reached the point where the inner sleeve is perilously thin. As Greeves never expected these sleeved cylinders to be bored out more than + .010” I was not sure how far to push my luck.

                          Stan

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                          • #14
                            Crankcase differerences

                            I was just about to post a photo showing different challanger type crankcases when I stumbled across this thread which answered my question, but I will post them any way as a picture speaks a thousand words.
                            Cheers Pete from the Antipodes:
                            Attached Files

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