Rotating assembly shuddering.

[TommyP]

I just installed a new Scat Crank and Scat rods in a block that had been align-honed by a very good shop, using Federal Mogul 125M bearings and Clevite assembly lube.  The clearances were mic'ed at .0020 for all the rods and .0024 for all the mains.

With the pistons and new rings in oiled cylinders.....the assembly "shudders" when turned VERY, VERY slowly by the damper bolt.  If turned a bit faster it smooths right out.
I haven't built enough engines to be familiar with the "shuddering".  Is this something to be concerned about?

[machoneman]

Pop the rear main seal (both pieces) re-install the cap and see if your shudder goes away.

[Barry_R]

Probably the rings.
Pop a couple out & see/feel for any change

[blykins]

I've seen/felt that before and as Barry said, it's probably the rings.  If you're not seeing any abnormal marks on the cylinder walls, I wouldn't give it too much thought. 

[Lenz]

Had the same concern when I assembled mine last spring, turn it over a little faster and it smooths out.  I had been methodical about assembly so I just forged ahead and decided it was "normal" with all the new pieces.  Engine runs perfectly with no issues.

[KMcCullah]

Make sure your crank has good end play. No less than
.003 or.004 thou.

[Joe-JDC]

I agree that the rings are catching on the cross hatch at slow speed turning.   Ring tension, cross hatch angle, not enough oil on assembly, all could be a cause.  If you see metal on the cylinder wall after a complete revolution where the oil left a track at top, then the finish was not fine enough for the rings used, or bores were not washed enough before assembly.  Joe-JDC

[TommyP]

You guys are the best......thanks, I won't worry about it.   
(Part of it might have been the fact that I was using a bar and pointer type torque-wrench to turn it over just because it was handy)

[shady]

the torque wrench is like a spring. you end up with oscillations (chatter). fire that puppy up & give it the bananas.

[Bolted to Floor]

What goes fast doesn't go fast long'
What goes fast takes your money with it.
So I'm slow & broke, what went wrong?


Shady, great quote. LOL!!