I have had two sets of ACL pistons arrive with incorrect rings. 1st set had a foreign ring that was completely different to all the others, never did find out what it was. And the 2nd set had compression rings that were too thin for the ring lands. So be vigilant when using them, apart from that they are a good piston and a great ring when u get the correct ones :-)
R33 racer - what bearing clearences were you running on your mains and big ends ?