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