SOHC.co.uk Forums > CB750
750 K2 Strip & Rebuild - help please
AshimotoK0:
I've talked with a few guys whose opinion I really rate and have hundreds of thousands of miles on Honda 4 engines under their belts. I still think the jury's out on this one but my gut feeling now I know the debris is probably in there but well compacted it to ideally clean it out by removing the balls (or screwed in but still staked plugs on early cranks). However I agree that if you are not going to do that then dont mess and try to soften the debris as you may soften but not remove it ..only for it to become dislodged when you run the engin It's a pity we dont have any close up photos of the peened area post removal of the balls and also some idea of how much force it takes to pull the balls out. Nevertheless, I think I will have a go with my 750 crank simply because I have no idea of its history , other than Bryan kindly sold me it yonks ago and I think it came from a batch of engines he had....In terms of wear it's very nice though.
K2-K6:
I don't think you can dissolve carbon Ash, is that true in chemistry? Only mechanical means I've been aware of. It's essentially, the soft element grease with carbon black in it as far as logic determines.
Ordinarily something like oven cleaner, sodium hydroxide should dissolve the "fats" to release the debris and flow it out. Also in chemistry labs steam is used to remove tarring from glassware, vessels etc. Small bore probe attached to a wallpaper stripper could be used. Maybe, in the interest of science, it'll be worth steaming one or two before then removing the balls to gauge effectiveness of method Ash.
Oddjob:
Well I've managed to remove the ball bearing out of a 500 crank. I'll be really honest here and say I do think this is a storm in a teacup. First off, the buildup is a grey powder, this is on a crank that's sat outside in the rain and such for some years and the buildup was still bone dry. The powder smells exactly like old burnt oil, it's just got that distinctive smell that we all recognise. The hole is 6mm and the buildup was quite solid, I didn't drill it out as that's wouldn't show how deep the powder was, so I used a small straight pick and essentially just pushed it in and turned at the same time. The powder was approx 1" deep, the gallery was completely free of powder after that. So I think what happens is that old oil collects in this pocket below the ball bearing, over time as Nigel has explained due to heat and such it just dissolves leaving the polymers, strands etc behind, this keeps collecting and is compacted due no doubt to the rotation of the crank throwing it together. However it only forms I think because this pocket is a dead end, below the journal the oil doesn't collect so it doesn't dry out and form this powder.
In short I think the powder isn't really a problem, ok if you've got the equipment yeah you can remove it but it's not going anywhere IMO. The powder is extremely fine and doesn't seem abrasive at all.
So let's hear your thoughts on this opinion.
Lobo:
You’re possibly right Ken; I’d submerged my whole crank in a degreaser for 24 hrs - which may be the reason its collection was a paste versus powder. On the other hand, it was an active crank versus a log-time unused… could this be the issue? Given a crank normally lives in a very wet & oily environment how could a dry powder result?
Notably the remnant sludge (ex drill extraction) dissolved easily with multiple squirts of degreaser down the hole - going to almost a grey thick ink. (The crank / holes were then power washed using a high pressure hose, with compressed air blown finally blown through the galleries. You’ll forgive me Nige not suggesting Lindsay that I take it home to steam it through 😂!)
Ash, yea sorry wrt pics. Certainly not much force needed striking the balls out with the cold chisel… I’d describe it as a ‘middling’ strike. But, there was no visible damage to the holes and the new balls had to be lightly punched back in before the OEM peening holes were used a second time buttoning it all up.
The reason the buildup was in there was described due centrifugal forces, ie any small crap migrating there / into the blind alley.
As a final comment to your post Oddjob, if Honda agreed the buildup was of no consequence to the longevity of the big-ends why would they go to the additional expense of modifying the balls to grub screws?
Oddjob:
Who knows why Honda did things. It could be for safety reasons, maybe one of the ball bearings let go and they decided to modify it to stop that in future. You've got to admit it's a cowboy way of doing things, punch holing a ball bearing in order to keep it in place, maybe there was oil leaking past and reducing oil pressure. TBH we'll never know why they changed it, don't forget they probably didn't make the cranks, they'd have sub contracted them out, maybe they specified a grub screw instead of the ball bearing and there was no additional expense for them but for the sub contractor.
As for the paste versus powder, maybe we need another source for comparison, either way I don't think it would ever shift as there is no flow past it to cause it too move. Plus, and think about this. The oil trapped there was already filtered as it had already passed through the filter, so what exactly is the sludge/powder but dried up oil, as I said it felt non abrasive, so if it did let go and move what would if affect and where would it go, it should just dissolve into the new oil and get filtered out by the filter as it passed through. IF it felt abrasive, and I'm not saying it couldn't be, I'd be saying for gods sake remove it before it does let go. However IMO the potential for damage doesn't warrant the effort to remove it.
Just my opinion of course but I won't be doing my crank and that's done MAJOR mileage, well over 100000 miles.
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