strange problem with big end Villiers Mk15

Dear Forum members,

Last weekend I've experienced a weird problem.
After a short run I heard a knocking sound and stopped immediately.

After dismantling the oil sump I've seen that the big end cap had a play of several millimeters.
The locking pins in both nuts were still there.
Thank god there is no serious damage yet.

Has someone had the same experience before?
I have still no idea how this could happen.

Please let me know your thoughts.

Best regards,

Steef Pronk

Just wondered could the heads of the bolts have not been right hard up against the shoulders?  I assume these are "fitted bolts" that hold the cap on.  Ie perhaps the corners of the hex on the head were against the side of the "rod" and running forces have allowed them to turn slightly so they slide right up to the shoulder allowing it to be loose.  I've had bolts get pushed back slightly while pushing the cap, on then rotate to jam against the side while screwing the nut on.

Dear Darryl,

Thanks a lot for your prompt reply.
I will check the bolts again.
Will let you know my findings.

Best regards,

Steef Pronk

Who put the oil in the sump last time & where did they get it from ? Your big end shells sound knackered to me mate , there should be 2 , possibly sharp as razors now so watch your fingers on the bits , also it,s quite possible that your crank,s big end journal is now bashed out of round . measure it all round it,s circumference with a vernier , if not perfect , contact Villiersservices , brierley Hill , ( North end of Birmingham , England ) , they MIGHT be able to regrind it down & supply new thicker big end shells .