Topic: CB750 scratches on cylinder walls after top end rebuild

[aejmeja1]

I had my cylinders bored, I fit new +0,5 pistons and piston rings. Today I started putting the top end back together. I got the pistons into the cylinders...poured some oil over the piston tops and hand turned the engine a few rotations. What this resulted in was scratches on the cylinder walls. Is this normal and what could cause it? How do I remedy the situation... hone the cylinders? Thanks in advance. This is incredibly frustrating 

[drumstyx]

I'm not an expert, but if those scratches go all the way around, doesn't it make sense that they came from the rings pressing tightly, as they should?

[pjlogue]

Check your ring gap and be sure your ring groves on the pistons are clean.

-P.

[aejmeja1]

I'll check the grooves. Do you recall what the ring gaps should be? i checked the top rings and got 0.2-0.25 and figured they were in spec.

What are your thoughts on what to do with the cylinders? Hone or leave as is? I was afraid the scratches would require a rebore or something 

[markreimer]

Can you feel them with your fingernail? That's what my cb350F cylinders looked like before I had them honed. I highly doubt a boring job is needed. if you only pushed the pistons through a few times by hand, and there wasn't something stuck between the piston/bore that scratched in deep, you could probably hone them away easily. Or who knows, maybe those with more experience will chime in and say ride it.

[pjlogue]

I don't recall the gap off the top of my head.  Do a search and you should find it.  When you gap the rings be sure to measure the gap in 3 positions in the bore.  Close to top, mid way down and towards the bottom.  From looking at your photos the sidewall scrape stops well below the top of the cylinder which would suggest the walls may have a taper to them.

-P.

[scottly]

I would be much more concerned about the huge craters in the wall of the cylinder at the bottom of the first picture!!??

[Nic]

I'm back after a long hiatus, 1st post but anyhoo, to me it looks like they weren't bored enough and I'm not totally beautified with that hone job either let alone as Scot says re the craters 

[drumstyx]

I would be much more concerned about the huge craters in the wall of the cylinder at the bottom of the first picture!!??

Yeah I thought that was just oil, but if that's pitting, that's bad...like, new sleeve/set of cylinders bad.

[ekpent]

 Looks like oil instead of pitting. Make sure you did not damage one of the thin bottom oil rings if you have the 3 piece set-up. They can fold and get pinched and with your bore you probably lost some chamfer on the bottom of the sleeve,harder to install.

[scottly]

It doesn't look like oil to me...

[ekpent]

If its pitting then the bore job guy should have advised not using it one would think. Hopefully for the posters sake it isn't 

[scottly]

Aejmeja1, are there also scuff marks on the sides of the pistons?

[aejmeja1]

Those are oil drops. There is no cratering. Cylinders looked good to my novice eyes when I got them.

It is possible that the bore job guy didn't bore the cylinders enough. I asked if I should hone the cylinders after the bore and he said it is not necessary.

If the bore is a tad too tight, would honing remedy the situation?

I'll check the piston sides when i get to the garage.

[aejmeja1]

I looked at Hondaman's manual before going to the shop and gave the bore guy the following specs for boring:

"piston to bore clearance 0.01 - 0.04 mm"

[drumstyx]

I looked at Hondaman's manual before going to the shop and gave the bore guy the following specs for boring:

"piston to bore clearance 0.01 - 0.04 mm"

Does he really say 0.01mm?? That's 4/10 of a thou, too tight for what I've heard. I had my guy go 0.0009

[kmb69]

I looked at Hondaman's manual before going to the shop and gave the bore guy the following specs for boring:

"piston to bore clearance 0.01 - 0.04 mm"

Does he really say 0.01mm?? That's 4/10 of a thou, too tight for what I've heard. I had my guy go 0.0009

Straight out of the Honda Shop Manual for "cast" pistons.

[markreimer]

I looked at Hondaman's manual before going to the shop and gave the bore guy the following specs for boring:

"piston to bore clearance 0.01 - 0.04 mm"

Does he really say 0.01mm?? That's 4/10 of a thou, too tight for what I've heard. I had my guy go 0.0009

Straight out of the Honda Shop Manual for "cast" pistons.

Yup. That's what I had mine bored too as well as per Hondaman's book. My motor is loving it now


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[PeWe]

Have you checked all rings inthe corresponding  bores and measured ring gap?

Before doing that, deburr the edges of each ring end since it can be sharp edges scratching the bores. I used a fine grit grinding paper last time, before and after filing the ends to correct gap. Some of my new rings had sharp edges.

[BPellerine]

Have you checked all rings inthe corresponding  bores and measured ring gap?

Before doing that, deburr the edges of each ring end since it can be sharp edges scratching the bores. I used a fine grit grinding paper last time, before and after filing the ends to correct gap. Some of my new rings had sharp edges.

that is my thought as well,nothing too serious there.bill