Drilled crankshaft, Honda Africa twin

[chabal]

Hello everyone!

I'm new here, I'm italian, so please forgive me for my bad english.
So, here the situation..
i'm a owner of a Honda Africa twin, which unfortunately has the crank shaft( where the pinion has to be inserted ) that has the teeth a bit worn.
So i opted for a modify :
I drilled the crank shaft , threaded the hole, then I screwed a prisoner m6 steel 8.8 with thread lock strong , so I paired two pinions(to expand the contact surface) and ended it all with a big washer and a simple self-locking nut.
I give you some data:
- Diameter of the shaft ( at the top of the teeth ) = 25mm
- Diameter of the threaded hole = 6mm
- Threaded hole depth = 16.2mm
- Shaft length ( from the point of exit from the crankcase to the end ) = 22,8mm

The question is:
In light of these measures ,is it possible to estimate the safety of such a change ? the crank shaft is potentially subject to breakage ? you think I'm a fool to ride the bike with this change ? The motorbike has more or less 50 horse power.

thank you so much for the help!

 

[chabal]

Maybe the right term is counter shaft or output shaft. Anyway this is a picture of the threaded hole, where then i screwed the prisoner
http://imageshack.com/a/img910/6312/gcsSTn.jpg [Broken]

 

[Dennis C]

where is the location of the drilled hole, on the end of the shaft, or radially into the shaft?
Post a photo

 

[chabal]

Hi!
The drilled hole is on the end of the shaft. I try to post a photo again
Thank you for the help!

 

[Dennis C]

Definitely less off a stress raiser than if it was drilled radially for a torsional load.
The depth of the drilled hole (16.2mm) seems fine as long as thread engagement is at least 1.5 times the diameter (6 x 1.5 = 9)
I have owned 5 motorcross bikes and 2 street bikes in my life, did all my own maint. and rebuilds and there were models that had the ends of the countershafts drilled for certain reasons but not to locate and secure gearing.
I would definitely clean up the mushrooming of the drilled hole, radius the edge. Square edges and drilled holes are stress raisers.
I would like to see the washer locating and securing the gears, it has to be a very large diameter due to the ID of the gear, correct?.
The original gear was secured to the shaft with an external snap ring correct?
Can you slide the gears on and put the washer and bolt on and take a photo? I am wondering where the washer seats on the shaft vs gears.

 

[chabal]

Hi Dennis, this is the job completed, with the prisoners screwed, the second front sprocket mounted, the washer and the self locking bolt. Everything is very solid an well locked. As i said, my only question is if the countershaft/output shaft, with the hole i made in the center, has become weak or "ready to break". I don't understand what do you mean when you say "mushrooming of the drilled hole, radius the edge. Square edges and drilled holes are stress raisers". I have just drilled the hole, cleaned it, and made the threads.
Sorry if i ask you again, but my english is very poor, and i don't understard if you think that the drilled hole has made the countershaft weak or not...That's my problem..because the front sprocket i
'm sure that is well locked..but i can only imagine what could happen if the countershaft breaks during the riding..
Thank you again for the help!

 

[Dennis C]

I would clean the edge of the drilled hole, if you run your finger over the edge is it smooth or is there a lip there? see photo
As far as strength of holes in shafts in torsion, the inside material of a shaft is very low load. When we raced Formula Fords
anti roll bars (torsional loads) were hollow, some 1/2 shafts are now being made hollow because there is not much structural reason for it.
Chabel, your shaft is already drilled, not much you can do now, if you asked if the shaft would be weakened before you drilled it you had the option of not drilling it.
My fear would be the fastener failing / thread stripping etc, not the shaft failing. It looks like a nylon lock nut on there, I would use that or doublenut it or possibly even safety wire it to keep it from loosening.
50 HP, how many CC is the engine I am thinking 750

 

[chabal]

Yes, it' a 50hp 650cc...
Well, if you tell me that my fear for the breaking hasn't a reason, then i will secure it better with a safety wire, or a doublenut, or a threadlocker i thought...and live in peace :)

 

[Dennis C]

Don't use red threadlocker (Loctite) you need heat to remove the fastener (according to manufacturer) where the blue you do not.
Also the critical time will be during acceleration loads, (and deceleration). take it easy for a while. This is for street use correct (normal design loads), your not racing it are you? Also think about failure, what is going to happen to your leg if it fails. do you have a sprocket guard?

 

[chabal]

Of course, a blue one!
I do not racing, i just use my motorbike as a dual sport bike, for mid and long trip..and now I'm planning a 7 days trip in north africa, with a passenger too. Because of this I'm trying to understand if i can stay "safe" or not.
Yes, i have a sprocket guard too that has to be mounted.
When you say failure during acceleration loads, are you referring to the prisoners/threads/nut failure, correct?

 

[Dennis C]

yes

 

[chabal]

Hi !

I'm still riding my bike in the condition i mentioned, and having no problem at all.
Now, always trying to make the system "safer" I was wondering about changing the actual prisoner that is made of 8.8 steel, with a new prisoner made of 12.9 steel to avoid any kind of fastener failing / thread stripping etc..of course using a better lock nut at the end.
I was also wondering about screwing the prisoner inside the crankshaft with a liquid steel epoxy or something like this, instead of the threadlocker, to partially reconstitute the integrity of the crankshaft. I know that in this case i will not be able to change the prisoner no more, but i think i will never need to change it...
what do you think about my ideas?

Thank you!

 

[Dennis C]

Hey Chabel,
Glad all is well with you and the bike.
To be honest, I wouldn't worry about the bolt / thread.
The bolt has a rolled thread (Forged) and is very strong, and the crank has cut threads which are no where as strong.
An 8.8 bolt is medium carbon steel where a 12.9 is alloy steel. Both are quenched and tempered. The 12.9 proof load, yield strength and tensile strength is definitely higher and can be loaded more.
If a thread is to strip, it will probably be the crank because the threads are cut with a tap.
However, if you want to change, you definitely aren't going backwards. And if you want to do it for piece of mind, absolutely do it.

Just watch when torqueing the bolts, you don't strip the cut thread in the crank.
Did you use thread locker on the install? Certain compounds need heat for removal or you could possibly damage the threads.

Just wondering, what kind of revs are you pulling on this crank?



Dennis

 

[chabal]

Hi Dennis!
So, do you think it would be a good idea using liquid metal epoxy to fix well the prisoner inside the crankshaft ? If i use this kind of epoxy i will prevent the crank's thread from stripping(at least better than threadlocker)? Anyway, I'm quite sure that the thread stripping is a remote problem..the nut stay in contact with a washer, with nothing behind it, so i suppose that, in case of a consistent longitudinal load, the washer will bend, preventing the thread stripping.
Anyway, i don't understand your last sentence, either with the tranlater..sorry for my english

 

[Dennis C]

Chabal,
Did you use a Bolt and cut the head off, or use "all thread"?

What I would be afraid of is removing the existing bolt / all thread. If the thread locker is bonded tight enough, you may pull the threads out when loosening the bolt / all thread. If that happens you will have to increase the diameter of the new bolt and tap new threads.
Personally, I would not use epoxy, I would clean it (threads, inside and the bolt) with brake clean or acetone before applying the threadlocker.
Make sure all the oils etc. are cleaned off the threads.

My last sentence meant, What are the max RPM's of the engine / crankshaft, and do you know the gear reduction from crankshaft to countershaft that the bolt is used in? What is the max RPM of the countershaft?

 

[chabal]

Hi Dennis,

That is a bolt with the head cutted of..
The max rpm reached from the engine is, realistically, 6.000. The front sprocket is 16 teeth and the rear sprocket is 48 teeth.
Other specifications:

-primary reduction: 1.8888 (68/36)
-final reduction: 3.0625 (49/16)
-torque: 51 nm at 6.000 rpm