Piston rings.
 

I built a BMW K 100 turbo a few years ago, it's an old skool type draw through set-up, T28 turbo with an SU carburettor, 9 psi of boost.
It runs two grades colder spark plugs and always the best 98 octane pump fuel.
I've now done just over 5000 miles on it and during it's last service, I noticed one of the sparkplugs was a bit oily.
Today I removed the cylinder head and withdrew the pistons and found that the cylinder with the oily plug had a broken top, (compression), ring.
They're standard BMW rings and are cast iron.
They is no evidence of detonation, all the internals are in perfect condition.
I've spent most of this afternoon on the phone trying to source a steel/chrome compression ring, the bore size of 67mm is not a problem but the bike uses a 1.1mm width piston ring rather than the norm of 1.0mm.
This winter I was planning to modify the layout of the cooling system to keep the hot radiator air off the plenom and carburettor which will obviously reduce the chances of detonation which I believe may have contributed to the ring's failure, but I'm reluctant to replace the ring with a new standard cast one and have to rebuild the engine again in a few thousand miles!
Has anyone else used cast rings without problem, or has anyone any other areas I should be looking at?
Thanks in advance.

A lot of the ring manufacturers will let you special order rings to fit if you have a nonstandard size; have you tried Total Seal?

I'm assuming you gapped the rings? I had a friend with a garage help me rebuild a motor few years back, he installed the crank and pistons while I wasn't there and he didn't gap the rings properly and I got a ton of blow by and a corner of one ring broke off.