Disassembling Crankshaft?

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Dan1950

Dan1950

1974 MK II Roadster
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I have the crankshaft removed from the cases of my '74 MK II 850. The crank journals seem to be okay. Plastigauge had the clearance with the old bearing shells at between .0015" and .002". That is with well used inserts. I expect new inserts to bring those clearances down.

One journal mic'd out at 1.7498", the other at 1.750" and appear to be in good shape.

I plan to just polish the journals and recheck clearances with new inserts.

Since I am the type that believes in leaving well enough alone, I'm wondering if it is really necessary to disassemble the crankshaft for a thorough cleaning?
 
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I am of your type.
The only reason to disassemble the crankshaft at this point, is to clean the sludge trap.
If someone can tell us the volume of the cavity, then a measure of how clean/
Impacted it is can be determined by filling the crankshaft with water, and measuring its volume.
Should the cavity be clogged, a caustic soda solution can be used to clean the inside.

Slick
 
kommando said:
If you do then mark the flywheel first so you know which way round it goes, the sludge trap normally needs cleaning out but the filtering on the 850's seems to reduce the amount that collects.
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The engine seems to have been well maintained. I don't see any evidence of sludge anywhere inside the engine..
 
I did a Combat crank that had bad rod journals.
The engine internals were clean and so was the sludge trap.
Not so sure I would have split the crank had it not been for the rod journals.
Then I would have wondered about the trap.......
 
elefantrider said:
I would not put any water in a blind ferrous cavity. Maybe gasoline or solvent, but not water.

Has it ever been taken apart before?
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Not 100% sure, but I don't think so. The cylinders are std with no evidence of taper, just glazed.

I can get huge cans of spray brake cleaner and will concoct a sealed tube to spray into the crankshaft oil feed. By covering the individual journal oil holes, I think I can get a thorough flush job accomplished.
 
You and everyone else has probably already thought about this:

Is there scar evidence from grit in the oil on the bearing shells? If not, you could probably get away with leaving it alone. Whatever is in there is probably going to stay put. If you do decide to flush it, you could make it worse by loosening up something that won't come out until the crank is spinning with hot oil in it. Taking it apart and cleaning the sludge trap and everything else out is the best solution, but a PITA.

Take my word for it. Cutting corners with the crank is a bad idea when you least expect it to be. Even a new crank should be flushed out. I have a brand new assembled crank and I am flushing it out. Never know what was left inside from the machine shop and assembler.
 
Some sluge is pretty thick. Some is more fluid.
Usually requires some scrubbing out in solvent with a small nylon brush.
IMHO, I'd take it apart if there was no evidence of it being apart before.
 
If the bike was complete and fairly original when you bought it, you might be able to decide whether to disassemble the crank by looking at the miles which have been clocked-up on the speedo.
 
Dan1950 said:
The engine seems to have been well maintained. I don't see any evidence of sludge anywhere inside the engine.
Click to expand...
Its not the sludge you see in a sump, its the fine grains in the oil that get centrifuged out of the oil by the forces from the spinning crank. Before paper type cartridge filters were introduced it was recommended that every time the engine was dismantled enough so the crank was free then the trap was cleaned. Much less likely to be needing doing with a filter plus the Norton Twin trap is bigger than most anyway.

Either take the crank apart to clean it out or do nothing at all, if you loosen it and some stays in there but is free to move, its only way out is via the big ends which it will destroy. I have cleaned out a few traps over the years, its like solidified valve grinding paste.
 
Have seen only one sludge trap on a 750. Packed solid. Very difficult to dig out.
 
kommando said:
Its not the sludge you see in a sump, its the fine grains in the oil that get centrifuged out of the oil by the forces from the spinning crank. Before paper type cartridge filters were introduced it was recommended that every time the engine was dismantled enough so the crank was free then the trap was cleaned. Much less likely to be needing doing with a filter plus the Norton Twin trap is bigger than most anyway.

Either take the crank apart to clean it out or do nothing at all, if you loosen it and some stays in there but is free to move, its only way out is via the big ends which it will destroy. I have cleaned out a few traps over the years, its like solidified valve grinding paste.
Click to expand...
Exactly.
 
Dan1950 said:
I have the crankshaft removed from the cases of my '74 MK II 850. The crank journals seem to be okay. Plastigauge had the clearance with the old bearing shells at between .0015" and .002". That is with well used inserts. I expect new inserts to bring those clearances down.

One journal mic'd out at 1.7498", the other at 1.750" and appear to be in good shape.

I plan to just polish the journals and recheck clearances with new inserts.

Since I am the type that believes in leaving well enough alone, I'm wondering if it is really necessary to disassemble the crankshaft for a thorough cleaning?
Click to expand...
I always take them apart and usually find that there was no reason to. Generally, I've found that Commandos with an oil filter usually have no sludge in the crank. Those without, have a little. I always put them back together with a new fastener set.

I DO NOT recommend flushing them instead. I tried that once with a crank for a 70 and after a LONG time of trying the sludge was getting looser and coming out faster. I finally split it as it didn't look like I would ever have confidence that is was clean and I didn't want new shells immediately "attacked" by sludge.
 
If you are this close you should go ahead, but that's just me and I lack the experience of most all the responders.

My whoa! moment was cleaning the notorious oil slingers on my R69 - I foolishly thought I'd save the slingers themselves - abandoned the first one halfway through. The sludge is a microfine hard clay that is brittle and breaks in chunks as you dig deeper. The engine overall seemed faultless.
 
As a test, if you plan to dismantle anyway, you could try your flushing process, then see how effective it may have been. But if it turns out whistle clean within you'll not know if the flushing did it all or if it was already clean.
 
Sorry to make this even more complicated but based on personal experience I would dismantle and also get it crack tested. Especially at the drive side section change to the crank porkchop. My 1974 Mk 11 had cracked there. A bike I had owned over 40 years and never abused or over revved. Not a race bike. There are threads on here about how to radius the section change. Jim at JS motorsport said that radiused all his cranks

Thread 'Seeking any experience on crankshaft cracking prevention.' https://www.accessnorton.com/Norton...ence-on-crankshaft-cracking-prevention.28658/
 
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