CB400/4 ENGINE STRIP, ASSESSMENT AND REBUILD BY JULIE - NOW COMPLETE

[Nurse Julie]

You been using Triggers camera on those pictures of the bores 

No Paul, that was using my mobile phone camera which is what I have taken all these pics with. Just got too close I suppose. I will try harder next time 

[paul G]

You been using Triggers camera on those pictures of the bores 

No Paul, that was using my mobile phone camera which is what I have taken all these pics with. Just got too close I suppose. I will try harder next time 

Surprised Trigger hasn't mentioned it then after the stick you have given him

[Nurse Julie]

You been using Triggers camera on those pictures of the bores 

No Paul, that was using my mobile phone camera which is what I have taken all these pics with. Just got too close I suppose. I will try harder next time 

Surprised Trigger hasn't mentioned it then after the stick you have given him

Oh he's mentioned it Paul !!!!!

[paul G]

You been using Triggers camera on those pictures of the bores 

No Paul, that was using my mobile phone camera which is what I have taken all these pics with. Just got too close I suppose. I will try harder next time 

Surprised Trigger hasn't mentioned it then after the stick you have given him

Oh he's mentioned it Paul !!!!!

lol what goes around comes around😀😀😉

[Nurse Julie]

Spent some time this morning stripping the head and cleaning and generally having a good inspection of the parts. The valves are all straight, very minor pitting so will just need re facing. Valve springs all OK and the observant ones of you will notice I have lost a washer in the process. It’s here somewhere !!!.
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Now, for those of you of a nervous disposition, you may want to sit down for the next bit.........

I cleaned the head fully and once all the old carbon and crud was off, look what I found !!!. No.s 1,2 and 3 totally bashed in. No.4 is ok. On the pistons the top rings are missing completely, there is a serious amount of damage to the top sides of the pistons and the edges of the top of the pistons look like the mice have been chewing at them. No.4 is fine, no damage whatsoever.

No 4 and 3
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No 2 and 1
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Pistons hanging off the head studs showing the damage to the tops
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[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.

[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.

[hairygit]

Oddly enough, when I bought my only brand new Honda, a CB125T2 back in 1981, the fitter said to me, always let it idle or as close as you can until the rocker cover is too hot to keep your hand on, then ride it, and that way it should last for ages. I've always done that with all the other Hondas I've ever owned, and never had a piston or rings give out on one. So worth bearing in mind once it's back together!

[Nurse Julie]

Thanks for your input chaps, I really do appreciate it very much. I will add here that Trigger is saying exactly the same things and I believe him 100% but it's just nice to hear the same things from other such knowledgeable people who have had hands on practical experience of these things.

[Seabeowner]

Amazing amount of damage for an engine that still ran reasonably well and the top end looked so clean. Presumably its had a few oil changes in the 500 miles and something would have shown. Are you sure the rattling noise was not detonation? Sure sign if opened throttle up from fairly low revs under heavy acceleration.
I'd put those pistons up somewhere as trophies.

[Nurse Julie]

Oil changed every 1000 miles and she was performing fantastically. Are you calling 'detonation' noise what I call 'pinking' if so, definitely none of that.

[hairygit]

It could be that the previous owner had run it weak (pod filters and or non standard pipe?)  Probably couldn't get it to run right and went back to stock, but the damage was already done. The carbon on the pistons and head hid the damage initially until you got the barrels off and started cleaning bits up. That's why I cringe when people on here  ask about pod filters on cafe racers, I always advise against it, and kinda look forward to the inevitable posting of pictures like yours, while thinking to myself "told you so" Obviously once damage of that nature has occurred, even running properly it will only get worse, as you have sadly found. I worked on many two strokes with pods over the years, usually with a hole the size of a 10p piece dead centre of the pistons, they are totally bad news.

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