Cosworth Piston Problem - Have U same Problem??

[AC Ace Bristol]

Having detected a water stain on the outside of my Bristol 100D2664 Block, closer examination revealed a 30 to 35 mm Hair crack.
   
   Hence engine out and strip down in order to have the block dry stitched, pressure tested and rebuilt.
   
   Whist dismantling the engine two very deep  Parrallel tram lines     ( vertical grooves ) were found in liner number four. ( the other five were OK )
   
   Closer examination of offending Piston, Circlips and Gudgeon pin, were carried out , concluding that the Gudgeon Pin had travelled through the Circlip which was still in the Piston Circlip groove, allowing the Gudgeon pin to score and totally ruin the liner.
   
   The offending Piston, circlips and Gudgeon pin were sent to Cosworth for inspection , test and evaluation, resulting in Cosworth requesting the remaining set of 5 also be returned.
   
   Cosworth remachined the pistons stating that the circlip grooves were out of tolerance,  My 5 remachined pistons, origonal gudgeon pins and new circlips were returned in due course, in the Meantime Roderick Mac Pherson who had supplied the pistons origonally kindly gave me a free replacement matching piston.
   
   Mr Matthew Bulliman of Cosworth explained in detail that the circlip grooves were out of tolerance on  5 of the pistons,  Now I've been to college to Study Mechanical Engineering, Hydraulics and Thermodynamics and was lead to believe that circlps were a static retaining item and did not wear.
   
   All my engine internals were carefully inspected NO UNDUE Wear was found on any of the pistons, piston ring grooves, Gudgeon pin bosses or the gudgeon pins themselves. There was no wear to be found on other engine components such as Crank shaft Journals, Bearings , Camshaft etc etc.
   
   I have been informed in writing from Mr Roderick Mac Pherson and Mr Grenfell the following :-
   
   Circlips are not a static item, they are not fixed but merely held in the groove by spring pressure, The enormous amount of vibration to which they are subject means some movement in a circlip, causing the circlip grooves to wear.!
   
   Because the crankshaft moves backward and forwards, particularly after hard braking and acceleration and even under clutch depression, pressure will be put on the gudgeon pin and circlip and wear does occur.
   
   Circlip grooves on Cosworth pistons varies from piston to piston as one might expect.
   
   Last but not least it would not be unusual to find bores scored after 21.000 miles......
   
   Please, please can someone out there give me some sound advise, the above  paragraphs contradict with my engineering back ground,  Or was I simply asleep during my three years of futher education ??
   
   Conclusion.......Any ACOC member, thinking of buying COSWORTH PISTONS for their AC, BMW , Frazer Nash or Britsol, please measure, remeasure and treble check all dimensions and compatibility of Circlips and there respective grooves, before fitting into your freshly machined block.! Or Source from alternative supplier & manufacturer.
   
   This story is without prejudice and is based 100% on FACTs.Please  Beware !!
   
   Keith L Lessiter
   AC Ace Bristol

[Robin A Woolmer]

The best Guy to discuss this with is David Fletcher-Jones @Fj-Engineering  'fjengineer@aol.com'(44)1590644644 He has all the tooling from Wellworthy & has the best technical knowledge available.
   He will also make new pistons if required.

[AC Ace Bristol]

Hi  Robin
   
   Thank you for confirming a contact,  I have received numerous telephone calls and a few emails direct reference Cosworth Piston problem I have experienced.
   
   I can assure all readers that My I have 100% faith in my Engine Builder who has rebuilt hundreds of Bristol Engines & BMW engines to various specifications over many years.
   
   He is very meticulous & thorough in the materials used, in his preparation and machining, However We have checked all measurements and examined all components everything is straight, correctly assembled, balanced and put together. Crankshaft end float , shimming etc was and is 100%.
   
   My engine is now running like clockwork, might be a tad noisier at 5500RPM than Big Ben! but all is going well.
   
   The cause of concern and what I wanted members to Know is, how come Retaining Circlips allow Gudgeon pins to pass through and bugger up the lining. If everything else in the engine shows no wear ??
   
   Cosworth Engineering remachined the Circlip Grooves FOC as they were out of tolerence,  How come ??  Why ??  Do Circlips wear ?? Do piston grooves wear and get deeper thus allowing gudgeon pins to travel through into the Bore ? If anyone is using Cosworth pistons thinking of using them, please measure carefully, If they are already being used and have been installed since 2001 check compression just in case there was a batch problem.
   
   If I hadnt taken my engine apart for a totally unrelated reason I wouldnot have known that the Gudgeon pin had travelled through the Cosworth Circlip and was scoring the liner, It is somewhat fortuitous that the problem was found or the Engine could have eventually totally self destroyed.   Who would have to buy, source and build a new 100D2 Bristol engine ?? Try getting Cosworth to contribute towards a £17.000 to £20.000 Bill. You would learn another definition of Tolerence.
   
   Where are the professional engine builders to give us advise when we need them ??  The silence out there says a lot!!
   
   Moral to the Story........LEARN BY OTHERS EXPERIENCE !
   
   AC Ace Bristol