Nikasil coating scratches .. The whole story in one post ..

Could you study the profile of the top ring on the next bikes you look at? See if you can determine if it has any taper on the face. Also compare the top and bottom edges and note the difference between the top chamfer and the bottom sharp corner.
Thanks

Thanks Pete, definitely worth knowing alternate sources. Taffy I intended to do a bunch of measuring on the weekend, but work got in the way. Do any of the Husaberg piston rings from the current manufacturers interchange?

I've done some measuring, but not enough to post anything. Interesting that the second liner you tried was tight. All mine were loose, so my cases must be oversize.

Time to post some measurements.

Liners: All three measured round at the top and slightly oval at the bottom, but still within the tolerances listed in the repair manual (100-100.12mm /3.9370"-3.9375").

Pistons:
All three were indentical, diameters as follows:
Above the top ring 3.918"
Above the oil ring 3.922"
Top of the skirt 3.930"
Bottom of skirt 3.934"
Top ring groove: 3.556"

Rings:
All rings were identical in thickness/width and had good fit in the piston grooves.
The only variations were in the end gaps (measured in the same bore):
Set number 1 with 41hrs. Comp .012", Oil .010"
Set number 2 with 1hr. Comp .009", Oil .009"
Set number 3 with 0hrs. Comp .012", Oil .012"
When number 3 comp ring was machined flat on the face, the end gap became .016"
The repair manual does not indicate minimum end gap, but does show maximums of comp .031", oil .039"

The comp ring section is .162" and this results in a ring OD of 3.880" (3.556+2x.162).
This leaves .0285" play between the base of the ring and the piston groove (3.9370-3.880=.057/2=.0285).
There is no way the ring could bind in the bore based on contact with the groove base. The oil ring also has plenty of clearance. An easy way to see this is to hold either ring to one side and notice how far it sits below the side of the piston.

I can't find any reason for bore scratching other than the interaction between the sharp edged top ring and the bore finish. At this point my plan is to procede with the cast iron liner and machine the number 2 set of piston rings. The number 2 set have the lowest end gap and I expect they will wind up at .013" after machining, well below the maximum liimit.

The combination of "plateau finished" cast iron liner and machined rings should work well. I am not rushing to place the order since the Canadian dollar has dropped greatly relative to the US dollar. Also, winter has just begun here, and I don't want to assemble the engine just to have it sit for 4 months.

After reading this post, one of my riding buddies commented on using cast iron rings in a cast iron bore. He thought there would be problems from having similar materials. I machined an oil ring just to see how it would go and it's definitely cast iron, no chip from the lathe tool, just a fine dust. I can't tell whether it's grey iron or ductile iron. Apparently ductile iron will produce a ringing sound like steel when it's dropped. Grey iron has a dull sound. I tried spinning the rings like you would a coin. Just enough difference in sound that I think the oil ring is grey iron. The top ring is definitely steel, it makes a tiny chip when machined.

From an article on piston rings:

Uncoated gray cast iron is compatible with cast iron cylinder walls and will not gall or scuff, but it is also brittle.

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Ductile iron has been used for years for heavy-duty truck gas and diesel rings because ductile iron is roughly twice as strong as gray cast iron. But ductile iron has two drawbacks. One is that it is more expensive than gray cast iron. The basic material costs more and it is more expensive to machine. The other is that ductile iron is not as compatible with cast iron cylinder walls as gray cast iron. It tends to scuff and gall unless it is faced with chrome or moly.

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Like ductile iron, steel is not compatible with cast iron cylinder walls, so it must be coated with either chrome or moly -- or gas nitrided. After they are heat treated and shaped the sides must be ground flat. The steel ring is then chrome plated or face coated with plasma moly that is inserted into a recess in the face of the ring.

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I need to keep researching the subject.

Well I suppose the oil ring must be ductile iron then if grey won't take much revs. I just want to be sure I don't have a metallurgy issue when trying a cast iron liner. My friend did say every Berg he's seen this year has puffed out a bit of oil smoke from the exhaust. I think I'll experiment with some scotch-brite on one of the old liners to see if it can polish out any scratches.