Jump to content
Sign in to follow this  

Crankshaft trueing question about runout.

Recommended Posts

I took apart and pressed together and trued my 1st crank an old 1983 rm250 crankshaft.  I used the k&l jig and trued it up on my lathes live centers.  Manually says .001 runout is acceptable. My question is is that the sum of both sides or is .001 acceptable if each side is out by that much.  1 side has runout of .0004 and the other is .0008.  For a total of .0012 or a little over 1 thousands of an inch in total. 

Edited by CrobarGreg
Mispell

Share this post


Link to post
Share on other sites

I am not a certified technician so I cannot deduce the manufacture's intention of the specification.  However some deductive reasoning might help.  Remeasure the concentricity and clock the position of the high points on each side.  If they align the part is within spec as both side of the crank would move in tandem and you would take the difference of the 2 measurements.  If they clock on opposite sides you could be out of spec as the crank would wobble by the sum of the measurements.  What the manufacture is attempting to control with this specification is the bending of the crank between the main bearings as it turns and is forced to accommodate the lack of concentricity.  The stress will go from max positive to max negative once for each revolution of the crank. The bending from non-concentric centers is additive to the bending from the load induce by the piston.  If the 2 bearing surfaces are perfectly concentric the bending stress in the crank under no piston load would be zero.

Share this post


Link to post
Share on other sites
1 hour ago, arc2arc said:

I am not a certified technician so I cannot deduce the manufacture's intention of the specification.  However some deductive reasoning might help.  Remeasure the concentricity and clock the position of the high points on each side.  If they align the part is within spec as both side of the crank would move in tandem and you would take the difference of the 2 measurements.  If they clock on opposite sides you could be out of spec as the crank would wobble by the sum of the measurements.  What the manufacture is attempting to control with this specification is the bending of the crank between the main bearings as it turns and is forced to accommodate the lack of concentricity.  The stress will go from max positive to max negative once for each revolution of the crank. The bending from non-concentric centers is additive to the bending from the load induce by the piston.  If the 2 bearing surfaces are perfectly concentric the bending stress in the crank under no piston load would be zero.

Cranks is straight not twisted.  Webs are spread a hair.  I know when you rev a 2 stroke at high rpm the crank spreads and returns at low rpm. So even if you ere able to get the crank to zero runout it wouldnt be after its been run. 

Share this post


Link to post
Share on other sites
1 hour ago, mxsteve1968 said:

Call kenoconnorracing.com, he'll explain it to you.

I know ken had him do some machine work for me.  Great guy very talented. 

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Reply with:

Sign in to follow this  

×
×
  • Create New...