Dropped Crankshaft

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Mar 22, 2007
All was going smoothly pulling the crankshaft apart, when at the last bolt a hard twist and the wood in the vice mounts ( screwed in ) splits resulting in the crankshaft dropping and hitting the concrete floor. My concern is what should I look out for? Will a crankshaft balance fix any slight bend in the shaft. My other concern is the flyweel, how can it be tested for damage? ie cracks etc. A dark day indeed.
Brian, You can check for any bends in the shaft by putting the crank on two V-blocks and measuring run-out with a dial guage. Balancing will not correct any run-out. If it runs true I wouldn't worry too much about cracks in the flywheel Norton cranks are fairly robust pieces of kit and I would generally be more worried about the concrete floor. If you have the crank out anyway I would recommend having it dynamically balanced as this can improve smoothness, they were only statically balanced at the factory. The balancing guy would also be able to tell you if it is out of true if you don't have the facilities yourself.
Brian, Crankshafts can be straightened, but you would be well advised only to let a specialist do the job. Am I correct in assuming that you have split the crank into it's three component parts? If so make sure that you have marked the crank cheeks in relation to the side of the flywheel that they came off, otherwise it will definitely be out of balance when you reassemble it. The flywheel is symetrical and can be assembled either way. If you have the crank apart then by simply mounting each side by it's shaft in a lathe there will be run-out on the exposed mounting face if there is any bend. Don't panic just yet until you have checked things out and as a worse case scenario you can probably pick up another second hand crank for a reasonable sum, which may be cheaper than commisioning expets to remedy things. Good Luck
In my panic and disbelief, not thinking I did separate the crank and did not pay attention to the way it was assembled. Though I did take photos earlier and the flywheel does have markings on it so I may be able to tell by looking at the photos which way round. I feel really thick. I have seen a crank on e-bay perhaps I should bid for it?
Brian, Don't panic, sit down and have a cup of tea. Did the whole crank fall on the floor, or just the flywheel? First check if the left and right cheeks are out of true, if not then your problems are over. At a pinch you can probably loosely assemble the crank and put it back in the cases if you have not already removed the bearings. You should get a pretty good idea if it runs true or not. If it looks true chances are it probably is. Norton cranks are pretty low-tech devices with lots of end-play and a known propensity for flexing while in operation hence the 'Superblend' barrell-shaped bearings to accomodate this on the early high performance Combat models. Presumably you have split the crank in order to either clean the sludge trap or in preparation for regrinding the crank pins. You would be well-advised to reassemble using new studs and bolts as the old ones are often peened over to prevent loosening and consequently the nuts get damaged upon removal. You should also use new big end nuts on the con-rods as they are a one-time use item. You can re-use the con rod bolts if they have not stretched. Have the crank dynamically balanced anyway, it can make the bike run smoother but be aware that the balance factor on a Commando is 52% dry according to my Norton Commando workshop manual which covers 750 and 850 models. This is low compared to most bikes and is specifically designed to compliment the isolastic mounting arrangement arrangement. Do bring your con-rods and pistons and all fittings with you if you opt to go this route.
Hello Dave,
The whole crank fell unfortunately. I looked at the ends, and the threads look ok on the timing side, despite cleaning out the concrete from the thread ( hence my worry it being out of true). I will assemble the crank using the old bolts and loosely put the case together. I haven't got a lathe or a v-block so I guess this is my only option at the moment.
I was intending to clean the sludge trap as the bike is a basket case and there was a lovely mess in the bottom of the crankcase. I was hoping to get the crank balanced and get the Norvil bottom end kit which looks like a reasonable deal.
Is coffee O.K instead of tea, the nerves are shot already.
Bryan, I have just last night built the bottom end of my 750 S project, so the subject of crankshafts is fresh in my mind.
I would be more concerned with the condition of the threads and the end of the shaft on the timing side. Make sure that the very end is not deformed, mushroomed or peened over as this goes through an oil seal into the timing cover and is where the crank shaft oil-feed comes from. I'm sure you already know this so appologies if I am being too pedantic.
I have just looked at a crank for one of my other projects in the workshop and it occurs to me that in the absence of V-blocks if you simply rolled the crank supporting the shafts on two parrallel pieces of wood, alloy or anything else straight you would be aware if there was any discernable bend, in the same way that one can roll push-rod tubes on a glass surface to check for bends. The Norvil bottom end kit seems to have everything you will need. If you already have superblends fitted you can slightly grind the inside of the inner journal on the old bearing and use it as an easy removable inner for setting the end float on the crank. The shims are meant to be behind the bearing inner on the timing side(most parts books diagrams are incorrect in this) and it is very difficult to remove the new inner to add or remove shims once it is pressed onto the shaft, so this inner bearing race once modified can serve you well for any future Norton rebuilds.
Hello Dave,
I put the crank together, and rested the crank on two straight pieces of wood. I didn't have a dial gauge so tried to make do with digital vernier. I wasn't completely happy with the setup so a dial gauge may be the only answer. But with the verniers I was getting a difference of .02mm. But I believe the vernier was moving ever so slightly so I cant trust this result. What would be an acceptable figure if any? The end on the timing side does have some deformity on the inside but it looks like its been there for a long time.

Regards Bryan
Bryan, 0.02mm is nothing, the permissible rod end clearance is listed in the manual as less than 0.254mm. I suspect that you would be lucky to get within 1mm with a couple of pieces of wood no matter how well planed they were. My suggestion was really a rough and ready method to see if the crank wobbled visibly. I would be inclined to the view that the crank is not bent and would put it back together and send it to the balancer. If you refrain from peening over the new crank studs that hold the three bits together you can remove them and reuse them again if the crank specialist finds anything untoward and requires to split it apart. If not you can peen them over once it comes back from balancing, give it a good clean as well since the balancer will likely grind and drill sections out of it which produces swarf. The inside of the hole in the end of the timing side of the crank merely has to flow oil without undue restriction it is the outer diameter of the shaft that runs in the oil seal and must be free of burs and other damage to prevent pressure loss to the big ends.

On a slight tangent I have seen a modification suggested to Combat crank cases which is alleged to improve scavenging and hence oil feed. I myself am considering this on a project that I have pending at the moment which has such cases. It might be worth looking into this since you have your cases apart. Unfortunately you will have to search the net as I don't recall where I saw it but it was on some site like "Captain Norton's Notes" or another of these helpful technical pages that some British-parts sellers host.

I am off to the UK tomorrow to drive a 1912 Rolls Royce around Scotland for 2 weeks (it's a dirty job etc. etc.) so I will likely be out of touch for a while, however there are numerous helpful fellow enthusiasts more knowledgeable than I on this forum, although it has probably helped our communication that we are more or less in the same time zone
Good Luck. Dave
Thanks very much Dave,
I will go and get the crank dynamically balanced. My confidence has been somewhat restored. I think I have seen the Combat mod on the Old Britt site. I believe they use arildite to block the hole. I will definitely look into it.
Can the ends be cleaned up with a bit of emery cloth? Seems like you've got the hard life? 1912 Rolls sounds like fun and it will getting warmer over there. Here its just getting cold and bleak. Thank goodness its raining though.
Bryan, You don't mention it so I assume that your crank pins do not require a regrind. This can only be done when the crank is split into it's component parts so don't rush into a reassembly and balance before you have had this done if required. The shafts can generally be cleaned up with some fine emery tape and some lubricant while rotated in a lathe for best results.
I think you are correct about the Combat mods being on 'Old Brits'. One hole requires blocking, one new hole drilled and a piece of the inner case is then ground away to prevent pooling and thus allow the oil in the crank case to be better scavenged back to the oil tank.
I'm glad to hear it's still raining in Australia, you guys have had a very worrying time of it with the prolonged drought.

Ive measured the journals and they are ok. One question though do they need the connecting rods for the balance? If so Should I polish them before I get the crank balanced?
The balancer will need the rods and the pistons to weigh and match up.

Polishing afterwards shouldn't be a problem but if you're planning to lighten components or profile the end caps etc. then that should be done first.
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