engine rebuild issues

[itsacbx]

hey all,
Need some commentary please...
Im rebuilding a 70,000km [42,000mile] 79, and the sludge in the bottom of the engine was 1/4" deep.....
However the pistons are in good shape and within tolerances and the cam bearings arent showing any noticeable wear nor are the mains, although I havnt checked them with plastigauge yet.
But the valves are absolutely horrendous, with 1/8" carbon buildup on the faces, more on the exhaust than intake but the faces are pitted from what appears to be pre-ignition. The pistin faces were pretty heavily coated also, but I'd put diesel down the plug holes so most of the carbon had fallen off by the time I'd dismantled the engine.
My question relates to the valves in that what is a reasonable degree of pitting on the faces before replacement is required???? The lips/edges of the valves are all excellent, and the sealing surface on all valves is also excellent.
Lastly, and this is presuming the mains are worn but not worn out.....given I have the engine apart, should I replace the main bearings with new originals? Meaning replace like with like [brown for brown; yellow for yellow etc as required to exactly match the existing bearings], NOT tighter bearings? Im following this line of enquiry based on the fact that the engine had so much sludge in it and I believe [without checking to date] that the mains must be worn, although the motor hasnt been thrashed, it clearly didnt enjoy regular oil changes.
At the very least Ill be doing a full assessment of the crank and clearances before I put it back together so maybe my questions are all academic, but still worth asking....
Im throwing new everything else in the engine, so the only question is whether I do the mains as well. This bike is a retirement project and likely wont do more than a few thousand kms per year. And I use 20/50 mineral oil changed every 2000km.
Al

[NobleHops]

Plastigauge what you have and go from there. If they aren’t showing smearing or transfer to the crank journals and they are in spec then new bearings are a waste of money and time. Babbitt bearing cranks ride on a film of pressurized oil, not the bearing itself, and so as long as oil pressure wasn't compromised and they are in spec, you’re probably fine.

[CBX-tras]

The concern that I would have, is the sludge buildup that's trapped in the crank journals that, if not cleaned out will clog the oil flow to the mains in short order. It's something that you can't see or check for but, I'll guarantee that it's in there.
The crank acts as a centrifuge and traps the sludge/crud at the bottom end of each passage and can't escape. Over time, it gets packed in and will block the passage.
It's a relatively big job to clean them properly. On each throw you'll see a pressed-up/staked ball bearing that seals the cross drilled journal.
This needs to be removed, drilled out (same size as the hole) to get the crud out of the bottom. Washed out and a new BB installed at each location.

[bobcat]

My question would be what is the make up or content of the sludge ? Is it carbon, metallic,
magnetic or plastic ? Likely all of the above but it would be good to know what it is.
I would strongly recommend having the valve seats cut to a fresh surface. Don't try to lap
out the pitting.

[itsacbx]

thanks for the input. Valve sealing surfaces are all in very good shape but Ill lap them all in no matter what. Ive cleaned the crank diligently and am happy its been cleaned of all sludge as best as its possible with compressed air, a pressure washer and a few cans of carb cleaner......
She all goes back [well the lower end at least] together tomorrow.
Rolling chassis is almost done, should have the front wheel on tomorrow
Al
Al

IMG_7890.JPG

[Larry Zimmer]

Looking good! Just in time for the riding season.

[itsacbx]

IMG_7893.JPG

[itsacbx]

IMG_7926.JPG

getting there

IMG_7920.JPG

IMG_7924.JPG