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StanG wrote: One 'STD', ...which I think would be standard, and the other 'L O/S' - I don't know what it stands for, but my feeling is this is the 20 hole piston.

My bet is the 'L O/S" is an OverSized piston for when 'boring out' is required.

My bet is the 'L O/S" is an OverSized piston for when 'boring out' is required.

That might be as well. I wonder what that would mean - a thicker more robust connecting rod? Larger diameter piston pin? I'd imagine the stroke wouldn't change and the crank shaft stay the same, so the big end would stay the same. If the pin stays the same, then what the difference would be.

Or - oversize meaning sideways play beyond the basic service limit? They don't mention it with switching to thicker bottom end connecting rods in the manual as far as I know.

There must be a logical code that corresponds to the right part number, so I'm still wondering what the codes on the rods and the bearings mean. I can't imagine a pallet of brand new connecting rods being tossed in the bin just because a paper label on the box fell off. Same with the journal bearings, if a machine ran out of ink.

He stated that at a minimum he would like to see 20 lbs force. He has seen 11 lbs force and the end result was oil consumption. He feels that oil scraper ring pressure is paramount to successful oil control.

Yeah, no surprise there - that's why the braking in period to seal rings and cylinders. That 20 lbs. number is with new rings, right? If I remember correctly, a 10 kg put on a piston would make the piston fly through it like through butter, haha. That's quite heavy. It might be also different from engine to engine.

I think it's a combination of things. I hope we can decipher it eventually. It would be good to get our hands on real Kawasaki engineering documentation. I wonder if that's now in the public domain, or a corporate secret. I've looked for a contact on Japanese pages with no luck so far.

StanG wrote:

My bet is the 'L O/S" is an OverSized piston for when 'boring out' is required.

Or - oversize meaning sideways play beyond the basic service limit? They don't mention it with switching to thicker bottom end connecting rods in the manual as far as I know.
.

Yes, 'oversize' as in larger diameter. It wasn't uncommon for MFR's to supply 1st size over pistons. However, typically, if required, people buy lower price, arguably better made and stronger oversized pistons from the likes of Wiseco. I'm sure the only oversize pistons sold by Kawasaki, or another other Japanese bike MFR, was for warranty work.

Kawboy wrote: He feels that oil scraper ring pressure is paramount to successful oil control.

That does re-enforce my guy's view that the extra holes do little for oil consumption, but that the oil scraper rings' condition is key to that issue.

But I can also see Stan's POV that more holes means easier for oil to go back into the crank.

On balance: if my guy is happy drilling the holes and the bevel, I'll do it, but if that is either expensive and/or risky, then I'll stick with the 10-hole version.

Paul

I think I'm getting mixed up a little bit talking about pistons and connecting rods at the same time, lol

That does re-enforce my guy's view that the extra holes do little for oil consumption, but that the oil scraper rings' condition is key to that issue.

Actually, my view on this is quite different. But that's of course my speculation. The piston rings in any of these bikes and pistons were the same, as far as I know.
Looking at the parts list, they were offered in STD and L, which goes along with what Bucko said - standard and oversized, for standard and oversized pistons respectively.
But rings themselves were the same - it shows they are good for all KZ and ZN models from 1979 to 1986. So what has changed? - the piston design, including these holes.
So, in my humble opinion, the machined grove and extra holes placed strategically there have a lot to do with how the oil flows. Rings scrapes the oil and directs it through those holes - where else would it go? And that's totally besides the natural wear of the rings over time, which if broken in well, should last for a very long time without much adverse effects. Rings fitting tight or worn, the holes would works the same regardless.

I am sure you will keep updates coming Paul! Curious about the cost of fabricating the pistons. I think there is enough meat on them to do all that - comparing both here they look the same in terms of metal volume when looking on the inside.

StanG wrote: I think if your machinist is willing to drill those holes, that's awesome, but I think he should also machine that groove. I have no idea how the engine would behave having holes and no groove.

Just had a closer look before taking them to the machinist and there is already a groove on the 10-holes ones, but it is sloped rather than stepped.

Machinist says no problem to drill another set of holes into the bevelled edge, will take about an hour or so. He reckons about 50 quid... so I have asked him to do it, and I will report back once they have been done.

Anyone want to buy a set of Injection model pistons? If not, they will go back to the original vendor.

Paul