Thanks for the info, both of you. If there's no clear advantage, then I'm tempted to stick with the original Lister spec and use the 4 ring piston. It's cheaper too, although that is not so important - I live in Grenada, so by the time I paid shipping and import tax, the original cost price kind of becomes less significant.
Thanks for the tip on checking the head clearance. I've just looked it up - for a 6-1 it should be 45-50 thou and for an 8-1 it's 75-80 thou. I must admit, I don't understand why the 8- runs at lower compression. Any ideas on that? My listeroid was sold as an 8-1, but interestingly it has the change over valve. I suspect they sell the same engine as a 6-1 or an 8-1, just changing the flywheels and governor spring.
I'm pretty good at general DIY mechanics, but I have to admit, I've never undone and reassembled a big end. Are there any obvious pitfalls for me to walk blindly into. Upon reassembly I guess just make sure it goes together as it came apart and tighten the bolts. Should I use a torque wrench or is f...ing tight good enough. I think the nuts are wired on.
Cheers,
Martin