Cylinder Wall Finish

[townhouse]

Would anyone like to comment on new trends in cylinder honing of automotive engines ? Diamond stones have been around for awhile, but I hear the trend in the performance industry is turning back to the conventional, silicon carbide stones.

 

[tbuelna]

The hone material depends upon the liner surface material. SiC will work fine on ductile iron. But a SiC hone will not last long honing a SiC surface finish like Nikasil. Nikasil (and similar bore coatings) require a diamond hone.

Regardless of the hone material, the typical procedure is a "plateau" honing technique. Using a coarse stone first, and finishing with a finer stone. Thus leaving little "valleys" surrounded by "plateaus".

 

[HiLift]

It would appear that if the cylinders material is of sufficient hardness then coatings don't offer any significant benefit. That being the case, a clean "base metal" finish would be desirable, one that is the optimal roughness and texture for the ring design and piston skirt being used.

Steve

 

[Majik]

last I've read is that some OEM's have been using laser dots engraved into Nikasil type bore finishes to generate different areas of oil retention and vary the ammounts retained over the length of the cylinder. this was considered a very low wear cylinder at one point. Diamond hones produce to rough a finish for many performance applications from what I understand from the sunnen rep. A torqued hot plateu hone is about as good as it gets right now. The new low tension rings and single semi gapless oil ring are the latest upgrades.

 

[townhouse]

The top rings seem to be getting thinner and harder as the ring manufacturers move forward. The problem is that the package includes the hardness of the block, design of the piston and how far down the top ring groove is from the top. Also gas ports come into play and how many. With all this being said, the rule of thumb for surface finishes, that is RA-Rpk-Rvk-Rk, the numbers seem to be getting smoother and smoother as technology of the top rings moves ahead.

 

[HiLift]

I think, for racing applications, Nikasil is probably yesterday’s news, at least for some applications. As cylinder wall hardness approaches 270-280 Bha on some compacted graphite blocks there just doesn't seem to be any use for it.

Better control of ring land size and finish are also major factors when it comes to ring seal and related bore finish. With the very high vacuum utilized in certain engines I, personally, don't think plateau finishing is so important either, at least not in those engines. The high vacuum levels are probably the main reason relatively low tension rings and very smooth bore finishes can be used and that windage can be brought to such low levels. The usefulness of coatings, like Casidiam, really shine in this type of environment.

Steve