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

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Nurse Julie:
That's really interesting Nigel. So, going back to the mess on my pistons / rings on my engine strip thread, it would make sense to me how my engine got in such a state. If a PO had not ridden it as the bike likes to be ridden for optimum air / fuel mixture ie, high revs and the engine 'pulling' a majority of the time then it makes total sense how the damage had occurred. I have said many times that the CB400/4 likes to be ridden like a 2 stroke, high revs and minimal lugging and I can see how riding it for long periods in the wrong gear / too lower revs can start a chain of events leading to damage.
A lot of riders think the CB400/4, being a smallish engines bike, are easy to ride but it's totally the opposite in fact, they are bloody hard work if ridden properly, definitely not a bike for the cruiser or the lazy rider.

Nurse Julie:
All the above makes total sense to me of how, after detonation, the pistons/ rings where damaged to such a great extent on 3 of the pistons/ rings. But, that still does not explain why 1 piston and rings were totally unaffected. I'm now thinking that this was the end result of a chain of events. I think, as Mr Davo and others have commented,  the bike having had been left standing for at least 8 years prior to me owning it, that we know of, rust had started forming between the rings and the bores and someone had unseized it either, forcibly or just by trying to start the engine, the rings sustained damage and the process of destruction had begun. I think the unaffected piston was due to the stroke the piston was on when the bike was laid up.

K2-K6:
It may be linked to storage and stuck rings,  you'd not want to rule out something obvious if you were to recommisiion an old engine. It's possible it could be a reverse of the logic though. If you had the rings stuck on one piston,  then the resulting loss of compression would make that faulty one less likely to detonate. Float height on one carb or leaking float valve would make it run rich,  possibly on one pot. Even something as simple as having one plug with it's gap facing a different way in the chamber will have some influence.

It's something that can easily be done that will completely wreck pistons in even the most pristine condition. I stood in on a dyno run for quite a radical tuned car engine in which they were running it under load to make sure they didn't get it detonating in use. Completing igniting swings ( running at varied static advance to make sure they had the most advance they could use while defining what's too far so they could get ideal setting) it was running close to flat out when it started to exhibit that. It sounds like something from general description to be minor when described as "pinking" but when standing that close to it,  it really does sound like you're listening to the beginning of an explosion! I stood outside the doorway as I imagined being hit by bits of hot engine it sounded so bad. It was only a second or two as they obviously wanted to avoid any damage,  but nevertheless gave an indication of the violence that it can exert if left unchecked.

Nurse Julie:
I re assembled the crankshaft today. Even though initially I had thought there would be signs of damage and wear on the shells due to the break up of the pistons / rings and the debris going around the engine, all the engine shells, to my amazement show no sign of damage, or excessive wear and measure well within the service limit. Therefore I am using all the original shells throughout the re build.

I am so scared of getting anything out of sequence, or position, or put back in the wrong place, I have to keep everything in order, or just 'so', hence the conrods on a handy screwdriver, in order !!!

I blew all the oilways out with the airline to make certain they were thoroughly clean, even though I had given the crank a thorough clean previously. I also cleaned, again, all the bearings, both front and back to make sure there was no contamination when I fitted the shells to the con rods. I then covered the shells with bearing guard, torqued up the conrod bolts and today I am using pink engineers paint as the visual reference to myself that I have torqued the bolts.

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Nurse Julie:
I then set about fitting the top and bottom crank case shells, which I had numbered and bagged up on strip down as top and bottom. Once again, I gave them a thorough clean, again. I fitted the 2 primary chain guards, with the indent facing backwards towards the gearbox end. I also fitted the reduction gear and the photo shows the stopper bolt and the lock washer that holds the gear shaft in place.
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All in, not a bad bit of work today. I'm very slow but want to get it right (obviously!!!) but also want to learn as much as I can throughout the process. I also have to keep thinking what is position No1, for example, as at the moment the top case, being upside down is, in effect, back to front but I'm gradually getting the knack. It's lovely working with everything nice and clean.

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