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CRANKSHAFTS & their failures

13K views 23 replies 10 participants last post by  old jupiter 
#1 ·
CRANKSHAFTS & their failures.

I posted this somewhere else so I am posting it here. This is only a start in the explanation of crankshafts and their failures. Many more subjects to be covered.



Hey Guys . I am not on the forums very much as I am always busy.

I have seen thousands of crank failures over the years. Every different type of failure has a specific sign as to what happened. If you have multiple failures and the same type of failed parts something is being missed.

In diagnosing a crankshaft failure I look at the evidence, (what has failed), and work backwards to find the problem. I started rebuilding crankshafts in 1973 and to this day have rebuilt, (my company & workers) close to 50,000 crankshafts. All 2 strokes and a very small amount of 4 strokes.



Right crank bearing outer race failure: The cases are matched sets and CAN NOT BE MIXED . They are line bored from the factory. If you mix cases you will have bearing failure.

Bearing clearance is by spec clearance scale. If the crankcase area where the bearing ride is out of round by even a few 10,000 of an inch it will put pressure on the outer race and this pressure will create friction in the ball or roller area. This friction will create heat that will further close the clearance resulting in catastrophic bearing failure shown in some of the previous pictures. Once a bearing fails or spins in the race the crankcase race is generally out of spec and cases are now trashed.

Crank separation. Crank web separation has a few different causes. First, pin to web clearance is critical. The interference fit or press fit needs to be within spec. If it is excessive or too tight the web will crack and the material will roll and distort. This results in never being able to true correctly.

If it is too loose it will separate. It is up to the crankshaft assembler to be aware of the amount of pressure being applied on assembly. Once a crankshaft has been assembled with the correct pressure and it separates there as other reasons for that failure.



Over-rev: Over-reving is another reason for crank separation. An engine makes horsepower at a specific RPM peak and looses horsepower beyond that. A CPI exhaust has a power peak at 9800 rpm. We have installed data acquisition on many motors. Ones that are prone to crankshaft failure are ones that are continually over-revved. Over-revving creates a harmonic vibration that shows up in different ways.

One is crank separation, Crankcase bearing surface chattering marks, flywheel separation, lower rod bearing cage failure. Over-revving cases a large percent of the failures followed by improper installation or not being aware of any crankcase imperfections.

Also keep in mind you are revving the engine at the start and dropping the clutch. The tires try to stop the engine or crankshaft rotation on the right side while the heavy flywheel is still moving on the left side. Crankshaft twist applies here. Also press fit also applies in this case. The looser the press fit the faster the crank will twist. This twisting creates pressure again on the bearing race and creates heat and eventually failure.



There are 2 different lower rod bearing cage styles used in most of the Banshee cranks. The more expensive cranks use the better bearing. Again over-revving will cause the lower cage to break apart and the small pieces will be evident in the top of the pistons. The small pieces will contact the rollers and make them skid causing lower rod meltdown.



Out of balance flywheel and welding of the flywheel onto the crank web.

If a flywheel is pried on to remove it the flywheel shell will be out of alignment with the inner solid support. The only way to remove a flywheel is to use the correct puller that pulls directly in the center. Any prying or tapping on the outer shell will cause damage. This misalignment will again cause a vibration and make the crankshaft separate and chatter.



Misaligned flywheel key: When installing a flywheel key you must make sure it is seated correctly or the flywheel will be off-center and cause the same separation and vibration. Offset keyways are especially prone to this. I do prefer a good adjustable timing plate.



OIL: Outboard motor oil is not a high-performance oil. Not that you have taken your 33 horsepower from the factory Banshee motor and are producing more that 100 hp the same oil does not sufficiently lubricate the critical parts. Especially if you are using alcohol. Alcohol is a solvent. Solvents dissolve and wash down oil from the moving parts. Special oils are required for alcohol engines. If this oil is not used the crankshaft moving parts will fail quickly. Lack of lubrication is evident when the disassembled parts are inspected for excessive wear or general meltdown of the lower rod parts.



And I can keep going. The crankshaft leads a very hard life and needs proper assembly and proper oil to run a long time. Even if installed perfect with all new parts, over-revving and rider mis-use will destroy it in only a few runs. Just because a crankshaft fails it is not necessarily the crankshafts fault.

All 3 components combined, proper build, proper oil, and proper rider input, will result in a long running motor. If this is not your case carefully read the above.
 
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#2 ·
Very good writeup!!



If you own a Banshee (or 2, 3,etc lol) you know the cranks are limited life span. I like the guys who use regular silicone to put case halves together,have the bearing locator shoved into the case half somehwere, forget the retaining clip, or put the RH seal in backwards. Seen all of those and guess what?? They all had crank failures LOL.
 
#3 ·
Good info, thank you for adding things I haven't heard before. Might I also add that detonation will take out a lower rod bearing very quickly as well. Honda 250R cranks I've seen go very quickly from detonation.
 
#5 ·
If unsure about motor cases, just look at the oem fische. They will show either a set or individual. I have not seen a motor that splits top and bottom yet that you can buy a single half for, it just cant really happen that way.
 
#9 ·
In the end we know that ALL banshee cranks are junk and fail quickly..... LOL!!!




The one thing that I can tell (and do tell) people is that a high performance or race motor is a bomb. If you use good quality parts and do good work, you simply make the fuse longer, but sooner or later the bomb goes off. You have to learn how to change or lengthen the fuse before that happens.
 
#13 ·
Sorry, I meant that as a general statement for 2 or 4-strokes. Just that they will show cases as complete or halves.
 
#15 ·
Cam, I'm sure it's whether or not there's a gasket between the halves. Look at the old LT quadracer 250 Suzuki. It splits horizontally and you cannot buy just one half, and they seal with a sealer instead of a gasket. Just a thought????
 
#18 ·
Don't know about the smart part ... but the way I view it is ... the pressure levels during a detonation cycle are much higher & over a much shorter time frame than that of a comparatively gentle & slow,controlled expansion of fuel.



Can be compared to say ... placing your hand on someones chest, while your elbow is bent, then straightening your arm pushing the dood vs. firing a 120mm tank round at him... lol
 
#19 ·
okbeast, detonation is pre-ignition. When there is too much top end heat or too lean, it can cause the mixture to self ignite before it can be fired by the ignition. That does the same as having too much ignition timing as well. It basically acts like a hammer trying to drive the piston back down as the engine is trying to push up against it. Hence in automotive terms "pinging" because that is the noise it sounds like.



The piston edge will be the first area to show detonation damage because it happens to be before the head design is able to "squish" it in towards the middle of the dome area. Bad enough detomation can show almost everywhere, but a person usually knows something's wrong by then




Now if a crank has a lot of run time on it and just happens to be ready to fail, the lower rod bearing or other material from the bottom end can get through the transfers and onto the top of the piston. That can sometimes resemble detonation to a certain extent, but if you study things closely there is a difference to be found. Hope that helps, Bubba



PS: If there is any pecking, tiny spots, or any kind of material damage on the piston top, I usually will check the crank first, because I run my motors on the rich side to help them live.
 
#20 ·
I'm definately not smart but just like they said its the hammering on the bearing.

I'm sure its a LOT more pressure on the bearing than the power stroke.

Detonation can also break pistons and skirts or in some cases hammer and slightly flatten a pistons crown and try to make the piston larger than the bore size and sieze. All of that would be tough on a crank.
 
#22 ·
Bump.
This is a good read. Tony, can you or someone else comment on the best bearing choice for the cranks? Referring to 7 mil and 10 mil cub motors so 100-120 hp range. I have read where some say to avoid the TZ due to crank walk. They say to use 10 ball max loads. Others have said the 10 ball max load crank bearings are too rigid and are rated for lower rpm. Also, are straight cuts necessary on these cub motors?
 
#23 · (Edited)
"Detonation is pre-ignition"?? Well, that's the first time I've ever seen that assertion, maybe because it's wrong. Bubba, you obviously know a lot, including a lot I don't know, but you should see two pistons I have somewhere in the shop (L-16 Datsun 510) that came out of two engines. One has a hole in it with smooth round edges where the aluminum got so hot it fell away, melted, and one edge of the crown also is melted away; that's pre-ignition. The other piston also has a hole, about in the middle of the crown, with sharp broken edges around it, like you hit it as hard as you could with a ball-peen hammer; that's detonation. Different phenomena, different effects. These two pistons constitute such perfect textbook examples of failure by pre-ignition and by detonation that I have been thinking of giving them to the auto shop at my old high school (in fact I might have already done it; I've given the school some other stuff, and being an old guy I might have forgot about this, LOL).

Pre-ignition can frequently lead to detonation. But you can get detonation without preignition.
 
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