Catastrophic piston gouging

[loxley007]

Hi all, I rebuilt my ‘79 CB750 and have been chasing low compression in cylinders 3&4.

I pulled the engine and discovered catastrophic gouging to pistons 3&4 and the corresponding cylinder walls. All the piston rings are intact. Anyone have any idea what might have caused this?

Pistons/cylinders 1&2 are fine - no wear/no damage/

I’m leaning towards piston slap as I used donor pistons during the rebuild - potentially marginally smaller the bore.

Does anyone have any other theories?

 

[drgnrdr750]

Sure looks like you had some metal shavings in there. Can't really tell by the pic of the piston if the bottom is chewed up, like this might be the cause, or just scraped like the rest of the piston. What ever caused this is most likely going to be a mystery.

 

[dirtdigger]

This is excessive heat and or lack of oil or possibly just too tight of clearance. This was the piston seizing to the cylinder, not excessive clearance or slap.

 

[loxley007]

This is excessive heat and or lack of oil or possibly just too tight of clearance. This was the piston seizing to the cylinder, not excessive clearance or slap.

Thanks for the ideas:

- I’ll double check (what’s left) of the pistons for sizing to check clearance.

- On the oil starvation suggestion, is there a likely culprit for cylinder 3&4 (right side of engine facing forwards)? Is there an oil passage that serves this side that could get blocked? The bike has only really done less than 20 miles since these pistons went in.

- Would valve shim issues (too big) causing valves to hit the pistons be a possibility? Given that gouging on cylinder was at the lower half I kind of discounted this - thoughts?

 

[dirtdigger]

I am thinking more along the lines of extremely lean condition causing excessive heat or extremely tight fit in the bore. How did you lube the piston upon assembly? Piston oiling comes from oil slung from the connecting rod bearings. Valves hitting pistons would do nothing but bend valves would not cause skirt scuffing.

 

[loxley007]

thanks, liners were lubed/oiled up - by hand on assembly.

I’m interested in the lean running idea, would this leave a lot of soot on the surface of the piston?

 

[dirtdigger]

No it would be clean. If there is a ton of soot then it is extremely rich which could in theory wash the oil off the walls and cause damage.

 

[dirtdigger]

Send some more pictures maybe.... What do the plugs look like comparing the good and bad cylinders

 

[loxley007]

So, here are all 4 pistons. They were clean when they went in but 3 & 4 are really covered in soot.

I mixed the plugs up so I can’t tell but a couple are also quite covered in soot.

That suggests the mixture was too rich I guess. Would it really have caused that much damage?

Edit: also, conrod 3 and 4 have a fair amount of left and right play in them. 1 and 2 are rock steady - could that be a factor?

I’ll split the case this weekend and investigate further.

 

[dirtdigger]

Well looks like maybe run rich but also looks like a lot of oil being burnt. Top of piston isnt melted so dont think it was from mixture or combustion issue. Did you maybe let it idle and get hot without any airflow and piston seized from that? the two better pistons also have some scuffing on the skirts that are the beginning of what happend to the two bad ones. They were not far behind at having the same damage as the other two.

 

[loxley007]

thanks, I wouldn’t say that it’s ever been run super hot without airflow. I’ve been running it on post rebuild and chasing low compression/cold pipes for quite a while.

It’s frustrating as I don’t feel like I have a smoking gun on this yet.

 

[Bo82CB]

Why are your 3 and 4 pistons have excessive side play, were thrust washers supposed to be there!? (I'm not sure, look at rebuild manual)
I think you may have piston sizing issue.

 

[loxley007]

Sorry, to be clear when I removed the block, the conrods have some side to side motion when they are on the crankshaft - not much, just a bit but certainly a lot more than 1 and 2. I think it’s probably within tolerance (the manual allows for a bit of movement).

I’m going to split the case and continue to diagnose - watch this space please as all advice is appreciated!!

 

[loxley007]

We have a smoking gun…the oil pump gear on the primary shaft had a rapid unscheduled disassembly.

That would have led to oil starvation and the piston gouging.

I’m going to look at the oil pump to check it is functioning properly - in case somehow that has failed causing the gear to fail. Then a good clean out of the crank case to find the missing parts of the gear!!

Thanks all.

 

[Bo82CB]

Wow, this means yout top end probably sustained damage too!? How your cam lobes / buckets look like, was oil pressure light on!?

 

[dirtdigger]

You need to look at every bearing surface in the engine. It has to have damaged bearings for the pistons to be that bad.

 

[mrtwowheel]

Wow, gotta be honest, that would be my stopping point with that engine.

 

[loxley007]

I’m splitting the case and taking it back to a full rebuild. Will be checking tolerances on every bearing, lobe, moving part etc.

Usefully I have many spare parts from a donor engine so will be replacing any damaged parts.