Dang Dirt!

[jeff-register]

Hi all,
Have a Mark 20 that has great compression. Checking it over & removed the by-pass covers. FOUND small round indents in one of the compression rings & don’t know the history well. WHAT WOULD cause cavities in the one ring & lucky no damage to the piston? Looks like tiny pea gravel or something that size. I may just Rering or go with .15 over pistons if needed. I have the oversize pistons in the shop now.
WHAT would you do if the bore is OK? Please let me know.
Jeff

[PugetSoundBoater]

Hi Jeff,would you be able to get a photo of the "tiny pea gravel "or the damaged ringing post?
I have found that a magnifying glass in front of my cell phone camera lens works for better closeups. Jim PSB

"Some people want to know how a watch works, others just want to know what time it is"
Robbie Robertson

[Pete]

What is great compression? Should be about 120 psi in both cylinders with throttle open and cold. If you pull the pistons and the block has no or nil scratches and mics ‘OK’ (acceptable out of round and taper) just hone it and new standard rings, check end gap. I would hold on +15 pistons unless the cylinders really mic bad. Post numbers of bore out of round and taper. If cylinder mics ‘OK’ to glaze bust I use a 240 grit silicone carbide ball hone at 400 rpm and 1 stroke per second and about 20 strokes should do it, with lots of oil, gives a correct angle (about 45degree from up to down stroke) cross hatch and does not get hug up in ports. Metal removal is small with ball hone at less than 0.0003". Works well in blind bores if you get the right ball hone. I use Brush Research ball hone of correct size for the bore and buy them thru Summit Auto parts, they had best price when I purchased.

If you decide to bore +15 you will need to get rid if the created sharp edge at all the ports (bad for new rings). I do this with 180 grit wet paper with finger rotation on each port( patience here) some do with a fine file from the out side in (covers removed) then ball hone as noted above. Regardless of just honing or bore and hone thoroughly wash with hot water and dish soap several times (I use a round toilet bristle brush to scrub) until bore wipes absolutely clean with white cloth or paper napkin. Then blow dry and "quickly" oil up with outboard oil to stop rust and bag until ready to assemble. Do not use WD40, mineral spirits, gasoline, or any other stuff etc. just hot water and soap and outboard oil.

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