It's a little hard to tell which piston goes to which picture but it looks like a couple of the cylinder head images show carbon build up. Those sparks you see are "burning brands" (chunks of burning carbon).
You need to look closer into the valve guide conditions of those cylinders although I think that is an unlikely source. I'd be more suspect of the oil rings in the carboned cylinders. How much of a cylinder ridge is there? I see you attempted to show a video but it is not visible with stated "error" message.

Looks like allot of piston/sleeve clearance and glazed sleeves. Can you move the valve stems side to side? Much play? Have the injector capsules been replaced?

Hi Team,
in the video the popping and oil smoke puffs are typical of a piston/cylinder that is getting oil into the combustion chamber and when the oil gets hot enough some of it will burn in an uncontrolled manner thus causing the popping sounds and the blue smoke puffs.

In the piston photos shown one of the pistons has a washed clean appearance around the crown of the piston extending into the combustion crater ringed by carbonized oil, this indicates that that piston is passing oil by the rings in an amount large enough to wash the piston crown edge clean.

These engines are really easy to get out of these machines--remove the radiator assembly by removing the bolts that hold the entire unit to the main chassis--after removing the fan shroud and hoses, cooler lines if fitted, etc.
After removing the belly guards, the drive coupling, Hyd hoses, wiring, governor linkages as needed etc. the engine lifts straight up and out the front.
If you do not have a crane with sufficient lift height to come out over the raised lift arms set the engine on a sheet of steel sled and very carefully drag it out from under the lift arms.
Be sure to keep the shim washers under the engine mounts in their original positions as these set the alignment of the drive coupling, the rubber drive coupling pucks will fail quickly if the engine and transmission are not close to correct alignment. A failed front engine mount rubber and a damaged/bent main chassis can cause misalignment also.
There is an alignment procedure to follow for D4, 941/951, 955K, & D5.
If the puck rubbers are cracked or hard they should be replaced as a failed puck in the set of four can cause crankshaft breakage if left for an extended period of time--vibration is a clue to a failing coupling puck.

As an aside crankshaft failures were seen in 955K's with failing pucks and/or engine transmission misalignment back in the day, it was thought to be due to their higher H/Power 3304 turbo engines.

Cheers,
Eddie B.

Good info there Eddie, I'm sure your new best buddy in Italy appreciates your help.
All the best
Mike

glazed bores means new rings and at minimum a hone job depends on wear if worn more than the liner wear limit that means a set of liners and maybe pistons. that is the idea behind using replaceable liners no need to rebore you just change them and the engine is back to original spec. much faster and cheaper. but you may be able to get exchange parts for cranks, conrods, heads, injectors etc and save money and time on reconditioning your own parts. the cost is in the workshop time to reco the parts. hence why exchange can be cheaper as they are doing multiple of the same part not just a set.

if your not comfortable doing this stuff it may be a workshop job. also check out a exchange engine. if you where in australia id say contact tillys at toowoomba they specialise in cat and komatsu parts. it still might be worth contacting them the freight costs though might be a bit expensive.

Hi guys . I edited the video link
Now it shows

its a rebuild those guides are knackered. they should wiggle a little but not that much. by the way how many hrs has it done. sounds like this might need a full engine rebuild. if your paying someone to do it an exchange engine may be cheaper. and you can get a few different types of exchange motors y can get a short block which is just the block and rotating assembly without heads manifold etc or you can get a full engine minus the manifolds or the whole engine with accessories. it varies depending on their definition of a short or long motor.

i would say those valve guides are all of knackered and plus...wow...more like horrific...

-be careful going forward on what you recondition...depending on what your goals, budget, are...for use, work, play, etc..
-when you repair the head, it shows up other weaknesses such as the bottom end sealing, pistons, rings etc..but it's all an improvement..so consider everything

 
I removed the piston 2. It seems to me in excellent condition. The rings are like new. I have not found any anomalies. I think all that oil comes out of the valve guides .... I think.
 

Agree, there is no physical damage or blockage of the rings.....but, that does not mean they are sealing.
If the cylinder/s are glazed then a proper cross hatch honing and new rings would be the appropriate fix.
You didn't state what the bore wear is at the top of the liners or how much ridge is present.
Yes, no doubt the valve guides and probably valves or a least some of them need replacing if the stems are worn beyond spec.
Be sure to check ring end gap when placed in the bore when replacing rings. Also check ring side clearance in the piston ring grooves. Otherwise the rod bearings and pistons look really good.