Topic: Piston sliding/scraping sound on reassembly

[zabzob]

Hi all,

Just put some new cruzinimage overbore pistons into my CB750. Fed the rings in using fingernails and wiggling, and everything fit together nicely. When I rotate the engine with the alternator, there isn't a tonne of resistance, but I do hear a sliding/scraping sounds. Is this just because it's new rings on newly honed bores, or is something potentially amiss with the rings?

[Tracksnblades1]

New rings in a new bore and hone can tell you they are doing their job.
What ring end clearances do cruzin images recommend?
What piston to cylinder wall clearance do they recommend with their pistons.?

[robvangulik]

Did you put some oil on the pistons?

[calj737]

Did you put some oil on the pistons?

Amen.

[PeWe]

Scraping sound when rotate the crank with new pistons and rings is normal. Most important it moves with constant drag when rotating the crank with no plugs in.

You measured the ring gaps with feeler gauge before mounting the rings on pistons? Rings in bore, min distance 0.2-0.3mm, Oil rings around 0.25mm. Top ring has the wider gap. It should not be too much either...

If head is not on... check bores for scrape marks. I always check this before mounting the head. Pistons at bottom, make sure bore has a thin layer of oil. Rotate until the other 2 at bottom, oil the bores.
You'll see the oil film when moving and if it scratch somewhere.

I had to grind off the ring edges on my Cruzinimage stock pistons. Even my JP 71.25mm pistons had some sharp edges I smoothed out with a grit 400 paper or maybe 240 .
I felt the hard burrs with my nail. I did not want any scratches in bores. Some pistons must be sanded all over to remove burrs, hard edges. 400-800 paper.

[zabzob]

Thanks for all the advice!

@PeWe I had a motorcycle machine shop check bore and hone the cylinders to my specs, and had them gap the rings as well. I have not checked this work as I don't have the tools to do so. There is consistent drag with no catching. Head is not on so when I head out to the garage next I will inspect the bores well to look for scoring.

@robvangulik I oiled the rings and the pistons when I put them in, but didn't put any oil in the cylinders themselves. I can certainly oil the bores, but will it affect ring seating?