SOHC.co.uk Forums > CB350/400
CB400/4 ENGINE STRIP, ASSESSMENT AND REBUILD BY JULIE - NOW COMPLETE
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:
--- Quote from: K2-K6 on October 16, 2017, 06:11:38 PM ---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.
--- End quote ---
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:
--- Quote from: mike the bike on October 16, 2017, 07:26:49 PM ---Have you got any theory as to how the pistons got like that?
--- End quote ---
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.
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