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Crankshaft run out close to main bearing clearance


turbogrill
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Hi,

 

My crank measured 0.003" runout, that is very very very close to my main bearing clearance.

 

I would think that is a problem? Since:

 

real_clearance = measured_clearance - runout?

 

Have another crank, that measured 0.007" runout. That is much much more than what is accepted in terms of clearance, but that engine was running fine. I would think that the center main bearings would be gone. But no main bearing failures.

 

Seems like I am not understanding something here....

 

It's a straight six with seven main caps, it's not uncommon for these to have some runout. The race engine building book says 0.001" is max, OEM rebuild says 0.003" is max.

I measured in the block with a dial indicator and a bearing in only 1 & 7, main caps off.

 

Should add a little clearance on the center caps? Have some extra bearings I can mix and match with.

 

 

Edited by turbogrill
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20 minutes ago, mender said:

T

Press on the crank with your hand while supported by the end caps so you can see how much force it takes to deflect the crank by a few thou, then take it to a crank shop and get it straightened.

 

This is a good point, wouldnt the weight of the crank itself will bend it a little. Or should this measurement be done in outer space where there is no gravity?

 

By just lightly touching it it will bend a thousand. 

To me measuring this way doesnt make sense. I did measure with the middle bearing in and there was no run out.

 

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Assemble crank in block with mains, bearings, and oil bearings. Measure rotating torque with an inch lbs wrench. Compare to book or normal specs, if it spins very easily i bet it is fine. 

 

Totally agree you should not have runout if you can avoid it, but it is also easy to have measurement errors. Ultimate test, is crank "tight" or not when rotating without pistons. I haven't measured runout on a few engines i did, all spun very easy by hand and none had a problem in service. 

Edited by Black Magic
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9 hours ago, turbogrill said:

 

This is a good point, wouldnt the weight of the crank itself will bend it a little. Or should this measurement be done in outer space where there is no gravity?

 

By just lightly touching it it will bend a thousand. 

To me measuring this way doesnt make sense. I did measure with the middle bearing in and there was no run out.

 

That was just for your personal information about the amount of force that will deflect a crank a few thou. When you consider the forces acting on the crank during running you can just imagine the contortions it goes through! No need to be in free fall, the runout measures the difference between one side of the crank and the other while gravity maintains a constant pull.

 

The length of the crank means that it will definitely run with 0.003" bend, but I would get it straightened to increase the main bearing life. But before you do that, do what Black Magic suggests and install the crank and all the main caps to check the torque required to rotate just the crank. If you feel tight and loose spots, you'll need to get the block align honed or bored as well when you get the crank straightened. Any bind means the crank is rubbing a bearing. Off-centre wear on the old main bearings is another indication that your block needs the work.

 

If you don't have tight and loose spots, you could just use everything as is, most of the wear from an out of tolerance crank from a bend will be at low oil pressure (hot idle) and I assume that will be more the exception than the rule.

Edited by mender
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Long ago I hung around a buddy's old school crank grinding shop, the measuring\straightening rig was a lifetime of experience, dial indicator, a set of stands the crank could rotate on, and a hammer/punch to ping it "here and there". He preferred doing it to a freshly ground crank that had a little heat in it, took all of 3 minutes.

 

Im 90% sure this could be duplicated at home with a junk crank to practice, on bending and unbending...There has to be some youtube vids

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