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Crankshaft probs. Help!
https://ausmini.com/forums/viewtopic.php?f=5&t=28856		 Page 1 of 1
Author:  Minimad [ Mon Mar 05, 2007 11:10 pm ]
Post subject:  Crankshaft probs. Help!
i have an 1100s large jurnal crank that I have had stored for a while and the tail where the primary gear and flywheel live has got some rust pitting on it that will not be ground off without making the whole crank useless.

Has anybody had any success in having them built up with some sort of "spray transfer" welding???? Where I might have it done??? Cost??? Might be cheaper to just track down another crank!!

OR any problem just running a well fitting primary gear on the pitted crank???

Minimad
Author:  drmini in aust [ Tue Mar 06, 2007 6:47 pm ]
Post subject: 
just run it, mine's been like that for years... :lol:
GR agrees, the pits just hold a bit more oil film. :wink:

<edit> don't make the bush fit too tight, .003" is minimum or it'll pick up.
Author:  Minimad [ Tue Mar 06, 2007 9:25 pm ]
Post subject: 
As you can see from the attached photos...its not good. The primary gear was loose on this when I stripped the motor (I didn't biuld it so it could have had that much play in it when it was put together!!)
Anyway, the hard casing is now gone off the primary gear teeth and the whole gear will need replacing.
The temp plan at the moment is to get the tail cleaned up and maybe hybred a small bore primary gear onto it just to get me going.....
Image
Image

Whats your thoughts....

Minimad
Author:  drmini in aust [ Tue Mar 06, 2007 9:34 pm ]
Post subject: 
Yes it's quite OK to grind the front bit down, and then use a smallbore gear.
Been done lots of times, GR told me when I did Cody's primary gear. :wink:
If you are going to further undersize it, I wouldn't try and get ALL the pits out.
Author:  Minimad [ Tue Mar 06, 2007 9:41 pm ]
Post subject: 
No...not all of them..I want some of the crank left :shock:

Just a tidy up! Wish me luck :roll:

Minimad
Author:  drmini in aust [ Tue Mar 06, 2007 9:42 pm ]
Post subject: 
Minimad wrote:
No...not all of them..I want some of the crank left :shock:

Just a tidy up! Wish me luck :roll:

Minimad

Break a leg. :wink:
Author:  DOZ [ Tue Mar 06, 2007 10:09 pm ]
Post subject: 
If nothing else, remove the pits at the root of the radius where the diameters change. This area is a stress raiser due to the design anyway, so try to remove the additional stress created by the corrosion pits.

From aviation experience, corroded bolts always fail at the root, even if there are much bigger pits only a short distance away, and the crack (leading to failure) always starts at a pit.

Regards

Daniel
Author:  Minimad [ Tue Mar 06, 2007 10:44 pm ]
Post subject: 
Now just so I know what I'm talking about when I lob up at the engine shop....its where the red arrows are...right :?:

Image

Minimad
Author:  drmini in aust [ Wed Mar 07, 2007 5:46 am ]
Post subject: 
Yes.
I'd be inclined to machine the big step off, and take off maybe another .005" to clean the rest up a bit. Then use a 998/1098 primary gear with undersize bushes fitted. :wink:
Author:  Minimad [ Wed Mar 07, 2007 7:36 am ]
Post subject: 
How much under are undersize bearings???

And where do you get them. I only seem to be able to find the 1275 ones

Minimad
Author:  drmini in aust [ Wed Mar 07, 2007 6:17 pm ]
Post subject: 
No, you can't buy `undersize' bushes, you would have to get them turned up, as I said previously.
Refer to your primary gear thread for clearances.

But... if you buy standard bushes, they are a few thou undersize, and need to be reamed or turned to size after press fitting to gear..
Give them .003" clearance on shaft, minimum. .004" is safer. :wink:
Author:  Minimad [ Thu Mar 08, 2007 7:31 am ]
Post subject: 
Thanks again,

I am inclined at the moment to fit a new set of primary gear bushes and ream them the absolute minimum and have the crank ground to suit....then have a look and see how much it cleaned up.

Will keep you posted

minimad
Author:  DOZ [ Thu Mar 08, 2007 9:41 am ]
Post subject: 
Why don't you get the bushes fitted, check for ovality on the internal diameter, if they are ok then get the crank ground clearanced to that size?

I don't think it will matter much if you blend out the radius (as per your red arrows) to a slightly smaller diameter than the parallel bearing surface. So long as the most of the bearing is supported and the pits are gone, you only need to blend a couple of mm away from the corner to remove the stress.

The radius on the stepped part can be blended even further as there is no bearing on this surface for about 1-2inches.

And when I say blend, I mean blend, no steps allowed. This is usually done by adding a taper and radius to the grinding wheel or can be done on the lathe.

regards

Daniel
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