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CB400/4 ENGINE STRIP, ASSESSMENT AND REBUILD BY JULIE - NOW COMPLETE
Nurse Julie:
And the damage to the rings / sides of pistons
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The sump
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So, it seems that the intermittent rattle noise, once last year and twice this year was the rings breaking up and being bashed as the pistons came up and possibly bits of the rings going into the exhaust. This would tie in with me saying originally that the rattle was at the top left of the engine. Why it hasn’t damaged No 4, I don’t know. Also, at this stage I have no idea what caused this damage and if the damage has occurred whilst I have owned the bike or if it was already there to some extent and has just got a lot worse. I got Trigger to give the bores a thorough clean and a bit of a hone just to make sure there was no damage that I had not seen the other day and they are fine, no different than what I said last week.
We removed the engine from the frame today as well. Took the sump off, still no big bits of metal debris but when I ran my fingers over the bottom of the sump, it has a layer of very fine grey metal dust mixed with the oil, therefore…. grinding paste.
This means I now have to actually measure the cam shaft to see if it’s still within spec, as although it looks good, it may be beyond the service limit due to effect of grinding paste wear. It also makes me wonder what I am going to find in the bottom end of the engine, wear wise, for the same reason. I was going to split the cases today but need to digest and analyse the findings of this morning first I think.
So, questions I need to answer are, 1) What caused this damage, where should I start looking for the cause?. 2) Has all this damage occurred in my ownership (just under 5,000 miles) or is it old damage that has progressively got worse?. 3) Why is No.4 not showing any signs of damage?.
What I have learnt this morning is that when Trigger says you can never really know what you’re looking at until every single part of an engine is fully stripped and fully cleaned, how right he is !!!
Trigger is not concerned about the damage, he says it is all repairable but to me it looks bloody awful and I am really quite concerned about it.
Rozabikes Tim:
Ouch - like cheeeesse Gromit! Hope no more nasties further on.
hairygit:
Ooh, that looks soooo expensive! I think you can discount ignition system fault, as you would have damage to number 4 as well as number 2. Possibly a weak mixture on 3 cyls, number 4 running richer than the others possibly. The worrying thing is that it only had a minor rattle as you described it. Some I've heard out on the road positively knock pop and bang, what the hell must they be like inside???? I'd love to be able to blame a female rider for causing the damage, but really can't say for sure! It might be worth sending the pistons to Ash or one of the metallurgy guys on here to see if they have any idea, possibly even a duff batch of pistons, although I very much doubt that of Honda. One more thing that springs to mind is the fact that 400/4s carbs have a fast idle when the choke is on, and remembering what Bryan J said about some Honda twins destroying pistons/rings if revved before warming up properly, maybe that has happened here, but why not number 4????? So my first idea seems most likely to me, weak mixture. :o :o
Nurse Julie:
I will let you blame it on a female rider Hairy, only because it's you though and I wouldn't expect anything different from you :-* :-*
It really was only a very minor rattle but I'm pleased I took it off the road when I did, I dread to think what would have happened if I had just ignored it and kept riding her !!! Graham has also mentioned what you said in your last point, hard riding on a cold engine. I don't always warm her up fully but I never thrash her when she's cold, only when hot, and like you say, it doesn't explain why No.4 is still OK. It's amazing seeing the damage as the bike was running and performing very, very well. If it hadn't have been for the rattle, I wouldn't have even considered taking her off the road, let alone taking the engine apart
K2-K6:
That looks like classic detonation, always the rings get blamed as it ultimately takes them apart in the end but it starts with combustion conditions, most likely as hairy says, too lean, and if ignored will get to the rings by melting the edge of the pistons.
I bet if you took those parts to the airfield near you with the old planes flying over you, then those engine guys will give you a guide as to what went on.
Don't know if you looked in on the twin cylinder head thread recently as it's very similar to that one.
Does look a bit scary though.
Probably the source of extra air when you were struggling with tickover.
Nigel.
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