I’m seeing 24–26 µin RA after 220/280 then a quick plateau brush on a small-block with a Sunnen CK10 and plates torqued to 70 ft-lb, and I want 18–22 for moly rings without growing the bore. Anyone have a go-to stone/brush combo or dwell trick that tightens surface without changing size more than 0.0001"?
On a CK10, I swap the 280s for J55 400 vitrified stones and do 6–8 very light strokes at minimal pressure (just enough to keep it tracking), then a quick, light nylon plateau brush pass, which reliably pulls RA to 18–20 without growing more than 0.0001". Keep spindle up and stroker slow so you’re shaving peaks, not size, and don’t over-brush or you’ll kill Rpk. @OP what oil are you running — MB-30 behaves better here than thicker blends.
This drove me nuts until I added a cork-bond plateau pass after the 280 on the CK10: bump RPM one notch, 4–6 strokes at near-zero load to just “kiss” the bore, then a very light 320 SC nylon brush — I see Ra drop from 24–26 to about 19–21 with <0.00005" growth. Clean MB-30 and filtration matter a ton at that stage, and your 70 ft-lb plate torque is fine. Also check Rvk/Rpk, not just Ra; Goodson explains it well: https://goodson.com/blogs/tech-talk/understanding-surface-finish — are your valleys still in the 40–50 uin range?
Try a warm-oil ‘spark-out’ before changing stones: get the hone oil to about 100°F, zero the feed and do 4–6 strokes with the 280s at just tracking pressure; I usually see RA drop 3–5 µin with <0.00005 in change, then a whisper of nylon brush if it looks slick… What oil and temp are you running?