Topic: New build - cylinder scoring concern

[gp_st3]

Hello all - recently completed rebuilding my K4 engine with CI 836 kit, new bearing shells, valve job etc etc…

Changed oil after 30 miles, have done another 100 miles since then and today had the plugs out to check fuelling.

On impulse, thought I would have a look inside the #1 & 4 cylinders using a cheap USB endoscope that I bought a while back and had never used - pictures are (hopefully) attached below...

So, what’s happening?  Although it doesn't show show very clearly, the scoring is only in the area of the cylinder wall that is actually contacted by the skirt but in any event, I’m surprised that the (aluminium) piston would mark the steel liner so easily.

The rebore was done to normal specs (0.02mm to 0.025mm) by a professional shop with good reputation and while I could not check the actual clearance myself as have no bore gauge, the pistons were a snug but not over-tight fit and once the barrels were on, I was able to turn the crank by hand quite easily gripping the alternator with no apparent scraping or undue resistance so all seemed good.

At the first oil change, there was nothing in the sump apart from a few small bits of non-hardening gasket and while the oil itself did ‘glitter’ a bit under a bright light it was smooth between the fingers.

The bike is running perfectly and had I not looked inside would have no reason to be concerned.

Am planning to carry on running-in and checking every couple of hundred miles to see if anything changes, but in the meantime would be interested to hear the experienced views from the forum ?

Thanks in anticipation.

[Jerry Rxman Griffin aka MuthaF'er]

Rings seating?

[gp_st3]

Rings seating?

don't think so, the scoring appears to be limited to the liner arc at the rear (presumably also at the front) where the piston skirt is tightest and the marks stop approx half way up the barrel.

[uksparky]

The pistons were snug maybe a bit to much....these engines get hot plus only air cooled  so with re bore piston/cylinder will get hotter, at this point i would add marval mystery oil to the gas help with lubing the piston/rings for the first 500 miles

[Tracksnblades1]

I really like your endoscope. Would have thought your hone's cross hatch would be more pronounced with only 130 miles. What was your piston ring end gaps...? Did you do the initial break in like Kenny suggest in his writings at CycleX...? Just curious.

Half way up, crap that sounds like taper. Did you check the  bore job with a taper gauge..?

[gp_st3]

I was conservative on the end gaps and went for 0.3mm throughout

My assumption is that the scoring stops part way up because the 'fat' part of the piston skirt only travels that far?

[Tracksnblades1]

What was your piston to cylinder wall clearance....?
So your thinking your scoring is limited to 63mm and nothing from Top to the oil ring...?   Do you have excessive blow by...?

I’ve been earning brownie points remodeling the master bed and bath...😎
What brand of endoscope did you use for your picture....?

[2wheels]

I have never rebuild a cb750.  My bikes have not needed yet, hopefully never will.

But on other engines the last test to check piston to cylinder wall clearance that I do is using a feeler gauge with the piston in the cylinder (no rings on the piston) .  Kind of a go -- no go  test.
I'm not sure how others feel about this?

[gp_st3]

Personally,
Possible causes of your issue was the cylinders weren’t honed after a rebore, ring end gap is too tight, piston-to-wall clearance is not correct, major oil pressure issue. 3 of these fall to your machinist’s performance, the oil pressure is something you had better investigate promptly.
Pull the plugs. What do they look like? Signs of oil contamination? Don’t keep running that motor- that’s my advice.

Thanks for the input, taking each point in turn - the cylinders were definitely honed after boring, whether enough I cannot judge but the criss-cross hatching is still visible outside and above the scored area (pic below);
The ring end gaps were definitely ok because I had to open every one of them up to 0.3mm (all too small initially which perhaps points to a tight bore but also could be simply that Cruzinimage assumes that you will gap the rings to suit the bore clearance)... also, the fact that the scoring is only around part of the circumference and stops completely half way up the bore suggests that it's not being caused by the rings, rather by the skirt of the piston below the gudgeon pin - yes, both of the bores inspected could be tapered but there was no variation in resistance when pushing the pistons up and down by hand during the build so that seems unlikely.
Oil pressure is absoultely fine, I have an oil gauge mounted and always had good pressure, both before and after the rebuild.
The plugs are clean (a bit lean, so I do need to re-jet) and with no oil deposits; there is no smoking from the exhaust.

To me, things point to the re-bore clearance being too small and the tight skirt of the piston marking the cylinder wall - almost as if there was grit trapped between them, however I was very careful cleaning eveything before mounting the barrels and (if it's not dirt/grit) cannot understand how the soft aluminium of the piston would mark the steel liner so easily...

[gp_st3]

Actually I should have said 0.025mm because the machine shop guy said he normally does them to 0.001" for his other customers and I checked that against this forum's recommendations and 0.001" seems to be the consensus (CI does not give any clearance recommendation).
I did fit new wrist pins, which came with the kit and used new genuine Honda clips instead of the CI supplied ones.
Cannot say for certain about worn small ends, but the new pins slid smoothly into position with some oil and there was no discernable rocking movement as do I remember trying that since I could not actually measure the clearance - with hindsight I really should have bought an internal bore gauge since all of this could most likely have been avoided!

[Tracksnblades1]

Did your initial assembly set for a long period of time before you started it...?
What assembly lube did you use on bore/piston and rings....?
Are your Cuizing Image Pistons cast or forged....?

[gp_st3]

Did your initial assembly set for a long period of time before you started it...?
What assembly lube did you use on bore/piston and rings....?
Are your Cuizing Image Pistons cast or forged....?

The engine was started maybe 10 days after the barrels were fitted, engine oil was used to lube the rings and bores (Lucas assembly lube on bearings etc.) - I see what you are suggesting, that perhaps the lube had all drained away before start up, which is possible however I had full oil pressure using the starter motor (with ignition off) in less than 30 seconds and then immediately on actual start-up. Also warmed the engine fully at about 2500 rpm before setting off on the first ride so imagine that would get things sufficiently oiled but perhaps not...

For whatever reason the damage, such as it is, has been done and I don't want to pull apart a well running and leak free engine just yet!

Did a compression check earlier today and the four cylinders are all between 145 and 150 psi, which is promising so my hope is that since the scoring is limited to the lower part of the barrels then as the engine runs in it may diminish or at least not affect performance - wishful thinking on my part perhaps?

[gp_st3]

Will do, moral support appreciated!!

Winter weather is going to limit the amount of running for the next three months but I will get out there when I can and update this thread with photos at 1000 miles or so...

Thanks to all for your imput.

[cb750f-2010656]

Did you have a fan on it while you fully warmed it up?  I'm told these engines don't like to run very long at all standing still

[bryanj]

Just read this, what air cleaners are you running?

[gp_st3]

Did you have a fan on it while you fully warmed it up?  I'm told these engines don't like to run very long at all standing still

The bike was outside and temp was about 8C/46F so didn't overheat as far as am aware.

Just read this, what air cleaners are you running?

Standard air box with K&N filter, the same as for the past couple of years so no change pre/post rebuild.

[Erny]

Intereted pics from inside your cylinders - can you share brand/model of usb camera used? I have also one, but it's unusable crap..

I hope your engine won't suffer from what is seen on pics.

[dave500]

photos often leave out detail and may show more sheen than it really is?youll always have some verticle marks,i cant see any cross hatch hone marks?

[gp_st3]

Intereted pics from inside your cylinders - can you share brand/model of usb camera used? I have also one, but it's unusable crap..

I hope your engine won't suffer from what is seen on pics.

I bought this one https://www.ebay.co.uk/itm/USB-Endoscope-Borescope-Snake-Inspection-Camera-Android-Mobile-Phone-2M-3-5M-5M/263891217519?ssPageName=STRK%3AMEBIDX%3AIT&var=563296200748&_trksid=p2057872.m2749.l2649

All those I found on sale are of chinese origin and pretty much the same so I went for one of the cheapest with in country delivery - it's low quality, especially the LED lighting > to get the pics inside my cylinders I had to shine a powerful torch into the plug hole and hence there is the general sheen and reflections.  To be honest I cannot recommend the particular unit I have due to the poor illumination... also the Android app that they recommend is horrible (however others would probably work with this generic unit).

[gp_st3]

photos often leave out detail and may show more sheen than it really is?youll always have some verticle marks,i cant see any cross hatch hone marks?

correct about the photos, it's hard to hold the cable, angle the torch and then take a snapshot at the right time!
the cross-hatching is definitely there but the strong lighting and reflections makes it less visible.

is this one any clearer?

[Erny]

Intereted pics from inside your cylinders - can you share brand/model of usb camera used? I have also one, but it's unusable crap..

I hope your engine won't suffer from what is seen on pics.

I bought this one https://www.ebay.co.uk/itm/USB-Endoscope-Borescope-Snake-Inspection-Camera-Android-Mobile-Phone-2M-3-5M-5M/263891217519?ssPageName=STRK%3AMEBIDX%3AIT&var=563296200748&_trksid=p2057872.m2749.l2649

All those I found on sale are of chinese origin and pretty much the same so I went for one of the cheapest with in country delivery - it's low quality, especially the LED lighting > to get the pics inside my cylinders I had to shine a powerful torch into the plug hole and hence there is the general sheen and reflections.  To be honest I cannot recommend the particular unit I have due to the poor illumination... also the Android app that they recommend is horrible (however others would probably work with this generic unit).

Believe or not, I have the same! So trick is powerful torch you used... now I understand 
Agree, that built LED illumination is crap, app too.
I tried to use it to inspect my 4into4 stock mufflers from inside but I failed, due to light - nothing was visible...

But I'll try next time to have a look inside my cylinders using your trick, thanks for the tip 

[trigger]

0.02mm to 0.025mm is a little too tight for CI pistons, OK for OEM ones, 1 thou to 1.2 thou is a good clearance on these pistons. Your guy that did the rebore seems to know what he was doing but, did he match each piston to its own bore?
 I have used CI pistons for many years but, have found that they can be out by half a thou and sometimes more.

[dave500]

each piston shouldve been matched to its bore,its the only way.

[PeWe]

Is the shiny spot really a problem? It look like a reflection of the valve pocket.

My old 836 cylinder has shiny spots at front. Probably due to way too much total clearance, 0.2mm.  No oil smoking nor drinking..
(Cyl bore diam - piston diam at skirts (front rear).

Last setup did not run long when the bike got bigger bores which the hone marks show. I gave it a session with flexhone brush before assembly. Ring gaps OK.

Those bores have been honed several times, 3 different pistons used. First time for forged RC 836 that luckily had smaller diam to fit ibore with larger clearance.  Next Action Fours cast that made around 30.000 km. Last pistons old cast RC where i measured around 0.2mm in total clearance. I used a good Mitutoyo bore micrometer, measured 4 different heights in each bore. Pistons with a std quality micrometer. I made several measurements until piston could not fit the tool.
Last setup had around 190 PSI compression with Action Fours SS-1 cam.

This cylinder has a new set of CycleX 65.5mm pistons. Cylinder need to be left at a shop. I'll go for total clearance of 0.04mm (0.015")

I'm convinced it is very important be clear about the clearance when leaving the parts for bore. Total clearance. Different if the shop sell the pistons and do the bore. They should know then.

[gp_st3]

0.02mm to 0.025mm is a little too tight for CI pistons, OK for OEM ones, 1 thou to 1.2 thou is a good clearance on these pistons. Your guy that did the rebore seems to know what he was doing but, did he match each piston to its own bore?
 I have used CI pistons for many years but, have found that they can be out by half a thou and sometimes more.

yes, the pistons were matched - I numbered them before handing them over and similar to your experience, on collection about 3 months ago the machinist mentioned that three of the pistons were very close to spec but one was 'a bit out' (don't recall if he said by how much).. remember trying to measure the them with a micrometer but struggled to get a consistent reading due to my lack of experience/skill and just made sure to fit each in the correct bore!