Crankshaft!

[icanfixall]

For me I will only go to a 10-10 undersize bearing on either the mains or the rods. Some cranks can be ground on only the rods or the mains. I even saw a 6 cylinder Reo truck engine get ground on one rod while the crank was still in the truck. That hardness is only so thick on the bearing journals. It possible to grind and weld up the bearings too. Thats common in a stroker crank. Still, nobody makes a 70 under bearing.

 

[paygie94]

Okay, bare with me here because this is my first time ever getting this far into an engine and I get confused with over and undersize bearings and all that stuff, haha.

This is my broken crank out of my '91 250.

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This is the crank that came out of the engine I got to replace mine; '94 250.

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Now if I am understanding this right.. when I took the crank pictured above to the machine guy to see if he could fix it, he got this little clamp thing out with a digital number display and and it read .070...? That's how much it's been worn down and therefore unsaveable? I haven't gotten it machined or turned or whatever the correct term is for it.

Searching around on the net the "largest" (I'm not sure if I am explaining this part right...) bearing I could find is for .040.

 

[icanfixall]

Thanks for the pics. When a crank is damaged it ground undersize on the bearing journels to get to smooth fresh bearing material. Most sizes are 10 under then 20 under and 30 under and finally 40 under. I don't recommend grinding or using a crank thats 20 under or more. Whats happening is your grinding material off the bearing surface making the crank smaller. That weakens the crank and cuts thru the hard facing on the bearing journal. Not a good thing. In the 1st pic I see the crank broken. Whats the second pic. If that another crank you wanted ground so you could use it or what. It can be welded up and reground to standard size but its expensive to do.

 

[OLDBULL8]

Okay, bare with me here because this is my first time ever getting this far into an engine and I get confused with over and undersize bearings and all that stuff, haha.

This is my broken crank out of my '91 250.

You must be registered for see images attach

You must be registered for see images attach

You must be registered for see images attach

This is the crank that came out of the engine I got to replace mine; '94 250.

You must be registered for see images attach

Now if I am understanding this right.. when I took the crank pictured above to the machine guy to see if he could fix it, he got this little clamp thing out with a digital number display and and it read .070...? That's how much it's been worn down and therefore unsaveable? I haven't gotten it machined or turned or whatever the correct term is for it.

Searching around on the net the "largest" (I'm not sure if I am explaining this part right...) bearing I could find is for .040.

That crank don't look all that bad, the main's look real good so nothing would have to be done to them. Grinding the crank throws to 0.030 thousands under is not unusual. Take it to a machine shop that grinds cranks, see what the cost would be. A good shop will grind it and tell you what bearings to buy, they will grind it the least amount.

Here is the crank I have that your interested in, sent you a PM but no pics are allowed in the PM's. That is all four throws.

 

[jaluhn83]

I've got to ask... how did you break the first crank?

2nd crank seems pretty worn, but 0.070 is pretty crazy. Sounds like the '0.070' was a micrometer reading, but exactly what the measurement was is hard to say. Could have been 3.070" on the mains, but that's still ~0.050" under spec...

It's not just finding undersize bearings, but also the shaft strength - taking too much metal off is going to have a measurable impact on the overall strength of the shaft. Not good. It may not seem like much, but shaft strength is at least a squared function of radius, so a small change has larger effects than it seems. Exactly how much impact a certain reduction has is hard to say, but 0.030" is the general max that's accepted as safe.

I also believe you can build up bearing surfaces. Probably not cheap, but I'm pretty sure I've heard of it being done. Typically using a flame spray process from what I understand, followed by grinding back to size and possible heat treatment.

Ironically I'm actually working on a process to restore worn journals like that in a single automated operation... if it works the way i hope.

 

[FarmerFrank]

You can get a crank spray welded and ground back down to size but you better have a deep pocket.

I had one worn down and sent it I Pittsburgh crack to get this done. It was for an old tractor and those cranks are hard to find. Anyway they called and said it would be $1200. Then they asked if it was for a farm tractor or a puller. They said if it was a constant working tractor then the spray weld would not hold up.

 

[icanfixall]

Heres a note about any ground crank. No matter what the size under its ground to the most important thing to check is the bearing to crank throws radius. Thats the curve surface between the bearing journal and the wall of the crank. Make that a square or tighter radius and the stress builds up till the crank cracks. Without knowing if these cranks that broke were ever ground undersize makes me think maybe they were and the radius was missed. I know its very difficult to check the radius if you don't have the radius tools. Even if you had them knowing how to check them is another deal all in itself. Your trusting the grinder doing the work to know what he or she is doing. Then hoping you don't have a trainee on your crank that day...

 

[OLDBULL8]

The Crank is boxed and ready to ship. I left the IP drive gear on and woodruff key in the damper shaft.

Need any info installing it just PM me. The crank is damn heavy, might need someone to help unload it.

PM sent.