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CB400/4 ENGINE STRIP, ASSESSMENT AND REBUILD BY JULIE - NOW COMPLETE
Nurse Julie:
--- Quote from: Oddjob on October 10, 2017, 03:14:27 AM ---I had to look again at the first set of pics you posted with the pistons. Now you look closely you can see the damage on the crown and the one pic you showed of the side where I commented that it was showing signs of compression loss past the rings as the skirt was burnt is No4 piston. I'm wondering though how you missed the gaping holes in the rest of them Julie, no amount of carbon would hide those holes. I've seen worse but not for many many years, damn that's a mess.
--- End quote ---
I missed the damage Ken because I wasn't looking for it at that stage. I was concentrating on the head that day and I think the pic I put on of the top of the pistons was just a random photo showing the carbon build up. It wasn't until i took the pistons out yesterday and started cleaning them up to have a closer look that i found the damage. The holes on the pistons were full of carbon and melted piston rings.
K2-K6:
I believe the two stroke melted piston problem is pre-ignition which is usually, as you say Bryan, related to the plugs. It's normally the plug getting too hot which is what the NGK guy was looking for and it's different to mixture. With a magnifying glass you can see if you have got any electrode melt on the plug, this shows the plug is too low a heat range ( low number in NGK) but the mixture and colour can be correct.
With road going two stroke it's plug fowling that you're trying to avoid to make them run reliably, so you use a plug that's going to run hotter than a race motor. The risk is if you get alot of flat out running then the plug tip overheats and starts to glow red, it's this that sets off the new mixture well before the desired timing event "pre-ignition" and causes such catastrophic heat build in the piston crown.
It's usually the opposite in a two stroke race motor. They use a plug higher up the NGK heat range which will withstand the flat out combustion heat during a race, but of course you get that classic racer's problem of fouled plugs as they won't self clean when run at less demanding speeds.
Just broken rings in an engine leaves a characteristic imprint in the head of very sharp and defined marks, these examples have very clear aluminium melt than only comes from detonation. As opposed to pre-ignition, this follows the start of the engine spark being timed and they are very different causes.
MrDavo:
It shows what tough little buggers these engines are that it could bob along quite happily in the state, albeit with a slight rattle.
Way back when we first saw the bores there seemed to be signs of water damage, I was wondering if this is one of those engines that has seized over time as the rings have rusted into the bores. Along comes Bubba with his trusty breaker bar, and without pausing to wonder why it's called that, manages to free the engine and get it running, quickly flogging it on. What he couldn't see was the carnage caused to the stuck rings, ripping them out of their lands, and the debris damaging the head before eventually exiting through the exhaust port.
It could even explain the mystery of why #4 pot is ok, if that cylinder was lucky enough to be on compression, with both valves shut during the long stand.
K2-K6:
I think that's an entirely plausible route to damage an engine and really don't think any theory should be discontinued when looking at this type of subject.
But, I would offer to counter that in the melted parts of the alloy it shows very high heat was present. If you just physically break the rings and piston lands you just don't get melting. The little craters, like moon examples, show straight up detonation of uncontrolled fuel burn and you can see the piston structure melted and smeared both up and down around that top ring area.
It's interesting that a few of these are appearing on here with this one, matt mouse, John webbley, the twin cylinder heads on another thread all showing what seems to be the same. We've not got pictures from some of those mentioned though.
One of the main influencers of detonation is fuel octane performance. We don't usually have anything below 95 oct now, I think. But these bikes cover the period of two star which was 92oct with the attendant risk going up for that fuel in these engines.
Picking up on trigger saying this happened a long time ago, I'd hold with that too judging by how much carbon was it the ring spaces. It could even have happened in the 1980s.
MrDavo:
Could little broken bits of ring be tempoaraily embedded and glowing brightly, causing melting? Some of the head damage looks to me like physical impact rather than heat, but I agree about the piston craters looking like they were caised by melting
It would logically be harder for the heat to soak away from the piston crowns than for the head, plus its going to get hotter with blowby caused by the missing top ring heating the piston rather than leaving through the exhaust. Normally, heat applied by normal running to the piston crown would dissapate through the top ring to the cylinder bore, without that there, where would it go? Would it build up until things got so hot the piston top began to melt? It doesnt do so catastrophically because there is still a path for the heat through the surviving ring and can soak into the piston and wristpin / conrod, but could cause the damage we can see.
I agree there seems a lot of commonality with the damaged twin head we saw the other week.
What makes me dubious about the low octane or bad timing explanation is why would #4 not be just as bad?
I'm not trying to start an argument, just to make sense of a mystery. 8)
ETA: Mine was running hot on one pot due to a rock hard inlet rubber that wouldn't seal properly, I guess if number 4 sealed Ok and the rest didn't, then that would do it just with heat. Were the pipes blued?
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