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

[Nurse Julie]

Most of it goes down the camchain tunnel.

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Ah yes Hairy, I didn't give any thought to that bloody great big hole in the middle 

[Johnwebley]

John,
There is an oil bath of sorts in the 400 head directly below the camshaft. From memory I think only the cam lobes themselves dip into the oil in the bath as the cam spins.
As the oil sprays out of the drillings from the spray bar over the cam it collects in the oil bath to a certain level. When the level is reached the oil runs back down into the engine. Maybe Julie has a photo of the top of the cylinder head to demonstrate the theory?

I have just put the camshaft back in place to see if I can get any further insight into this. It was impossible to photograph the underside of the lobes in their 'fully down' position and impossible to measure also. To me it looks like there is about 10mm clearance under the lobe to the bottom of the 'bath'. What I did see though is a definite 'tide mark' around the edge of the bath which would suggest that what Laverda120 says is correct, that the cam lobes dip in the oil bath on each revolution and possibly the tip of some of the lobes may sit in oil when the engine is not running (obviously depending on where the cam stops) . What I can't find though is any drain holes out of the baths, so where would the excess oil that is sprayed from the oil arms go when the bath is full?.

Tide mark
(Attachment Link)

And again
(Attachment Link)

Like the 500.there is no drain. It is a mini sump to provide oil to the cam after standing. During start up before the oil pressure builds up.a simple method to protect lobes and followers.
During running the oil gets splashed
Arround and lubricants valves. Chain. Before returning to the sump


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[Nurse Julie]

John,
There is an oil bath of sorts in the 400 head directly below the camshaft. From memory I think only the cam lobes themselves dip into the oil in the bath as the cam spins.
As the oil sprays out of the drillings from the spray bar over the cam it collects in the oil bath to a certain level. When the level is reached the oil runs back down into the engine. Maybe Julie has a photo of the top of the cylinder head to demonstrate the theory?

I have just put the camshaft back in place to see if I can get any further insight into this. It was impossible to photograph the underside of the lobes in their 'fully down' position and impossible to measure also. To me it looks like there is about 10mm clearance under the lobe to the bottom of the 'bath'. What I did see though is a definite 'tide mark' around the edge of the bath which would suggest that what Laverda120 says is correct, that the cam lobes dip in the oil bath on each revolution and possibly the tip of some of the lobes may sit in oil when the engine is not running (obviously depending on where the cam stops) . What I can't find though is any drain holes out of the baths, so where would the excess oil that is sprayed from the oil arms go when the bath is full?.

Tide mark
(Attachment Link)

And again
(Attachment Link)

Like the 500.there is no drain. It is a mini sump to provide oil to the cam after standing. During start up before the oil pressure builds up.a simple method to protect lobes and followers.
During running the oil gets splashed
Arround and lubricants valves. Chain. Before returning to the sump


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Thanks John. I have just found this oil flow chart in the Workshop Manual which helps identify the paths

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[Dave487]

Don't do what I did and fit the restrictors wrong way up, they get bent and when you try to straighten them they break.
They have to go with stem downward.
Luckily I was just doing a dry assembly so spotted it.

[Nurse Julie]

Don't do what I did and fit the restrictors wrong way up, they get bent and when you try to straighten them they break.
They have to go with stem downward.
Luckily I was just doing a dry assembly so spotted it.

Thanks, I will remember that (could someone remind me nearer the time please  )

[Nurse Julie]

A little more progress today. I have cleaned all the nuts, bolts, washers etc that will be going for re zincing. We use the ‘cement mixer method’. Fill cement mixer with items to be cleaned, add a load of traffic film remove and boiling water and switch machine on. This cleans off all the old oil, grease and dirt. We used to put sharp sand in as well but found it cleaned just as well without.
Then every single item has to be cleaned fully by hand on the electric wire wheel machine. This removes the remainder of the dirt, grease and rust from in the nooks and crannies, including the threads. Dry off on a big tray in the oven for a few minutes. Whatever dirt you leave on will still be there after zincing but hidden temporarily under a nice new layer of zinc, until it starts falling off with the dirt. I only had a fairly small amount to clean so threw them in the mixer with the huge amount Trigger was cleaning and now they have all been thrown in big buckets together ready for taking to the platers. The fun will start when we collect them from the platers as they all have to be sorted out and there are 100’s and 100’s of them !!!

Another thing I did was take a good look at the rocker arms. On the CB400/4 you remove the rocker arm shafts by removing the cap nut and screwing a 10mm (1.25 pitch) bolt in the end of the shaft and pull. The rocker arms come off the shaft with the springs.
All it says in the Haynes Manual and the workshop manual is ‘Examine the ends of the rocker arms for damage and examine the tappet’. No real explanation of what ‘damage’ you are looking for. I have found some rocker arms in the workshop from a CB750 and compared mine with those. Mine look a lot better !!. There are no ridges or deep scratches, therefore I will be using mine again when the engine is rebuilt.

From the CB400

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From a CB750

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I won't be posting much on this thread for quite a few weeks. The casings go to the acid strippers in about 2 weeks and will be there at least another 2 weeks and I can't do anything else until I get them back and prepared and painted. I will be ordering parts though. I'm off on my travels with my Nursey girlfriends to Seville in about 3 1/2 weeks, so at least Trigger will get his workshops back for a while !!!.

[K2-K6]

Apart from the obvious problem you have with the engine,  and as you feel it's not been rebuilt prior to this, it appears that it has been run on oil of decent condition as all the tell tale wear points seem in very good condition.

[Nurse Julie]

Apart from the obvious problem you have with the engine,  and as you feel it's not been rebuilt prior to this, it appears that it has been run on oil of decent condition as all the tell tale wear points seem in very good condition.

Yes, it's a shame that the pistons are so bad as they really aren't representative of the rest of the engine which is looking really well maintained. The engine has definitely never been apart before. I'm hoping that all the shells, bearings etc measure up ok because they are showing no signs of damage at all.

[mike the bike]

Have you got any theory as to how the pistons got like that?

[Nurse Julie]

Have you got any theory as to how the pistons got like that?

Well Mike, the pistons got in that state due to uncontrolled detonation with no rings. The rings had obviously started breaking up and the damage to the pistons and the rings just escalated from there. The burnt areas down the side of the pistons show that combustion was getting down the side of the pistons as the rings (or lack of them!!!) were not containing everything in the combustion chamber. Having looked very closely at the bores again, I can see a few vertical score marks near the top but, I think this is where the broken piston rings, still in the lands, were scoring. I have looked very, very carefully and can see no evidence of water damage. There is none of the tell tale half moon water marks that you should see if the bike was stored on the side stand for years. There is also no water marks in the same places on #1 and #4 or #2 and #3, which would rule out water ingress whilst on the centre stand. None of this explains why #4 piston/ rings has been totally unaffected, as they move in pairs as we know but this has also happened to 3 pistons on Craig400's bike. Now, if as was suggested, the bike may have run very lean on pods in the past, then reverted to a standard airbox setup, but no adjustment made to the mixture, why still no damage to #4?.
The consensus is that this damage occurred a very long time ago, as seen by the amount of carbon where the rings should have been and one day it was all going rear it's ugly head. The problem is, there is no evidence left to analyse as the main damage occurred so long ago. Yes, we can say exactly what has happened to the pistons because they are there in front of us but what the original cause of the damage was, I don't think we will ever know.

[Green1]

I was thinking about the damaged pistons today and remembered my 2stroke Go kart had almost identical damage except the rings stayed in one piece. It was running far to lean due to a damaged diaphragm.
My brothers KH125 did exactly the same thing when his mates fiddled with the carb aswell 

[K2-K6]

Now there's a bunch of people that work more closely with detonation Mick. They often use different shaped combustion head shapes with differing squish bands to control it.

It's often seen on isolated or any number of cylinders on V8 cars running high power setups. It's by no means odd to get some cylinders to detonate and some not even when all running through one carb.
Those applications they try and get all the spark plug electrodes facing the same way in the heads to avoid any masking of the spark if not facing the flame advance direction of combustion.

[mike the bike]

How did the piston rings get like that I wonder.  I can understand them snapping when installed,  or when the barrels are fitted, but once fitted, how would they break?

[K2-K6]

I think you're entirely right Mike.
If you look at the pistons and the witness marks that have been carved out of them by the rings then you can see in some of them where the piston land material has let go,  this has resulted in the part of the ring that's still attached to bend and flex in an arc which has cut a corresponding hole at its free end that looks like a metronome.
To me this illustrates that the piston material went soft through heat and prior to the ring being broken. Under extreme heat exposure the rings can anneal as well. Spring steel is annealed by heating up over critical temperature,  but not melting,  then leaving to cool gently over a period of time. As opposed to hardening via heat and sharp quenching.

[Nurse Julie]

Thanks for answering Mikes question K2-K6. I have been thinking all day about the reasons for the broken rings. I was swaying between the damage to the top of the pistons causing the rings to break or the rings breaking first and leading to the damage to the top of the pistons. But, my extremely limited knowledge of these things, which quite honestly, would fit on the back of a postage stamp, meant I did not know the answer for certain. I was hoping someone like you would come along to explain....and you did. Thank you.