<VV> Corvair tight rebuilt crank shaft

[kovacsmj]

OK, Time for the engine builders to put the thinking caps on!

 

 The situation is a LM engine being rebuilt from scratch. Using the
original undamaged crankcase and a Clark's best .010 under crankshaft
(renitrited), .010 over CL44 bearings and a new camshaft. When
rechecking the clearance with plastigage I read approx .0015 to .0018
clearance for the crank journals and approx .002 for the cam. So far so
good! 

 

 However, upon assembling the crankcase halves, the crankshaft has a
tight spot for about 90 degrees. Looking from the rear of the engine the
tight area is between 12 and 3 o'clock. The tight spot remains despite
loosening and retorqing the crank halves and careful adjusting with a
dead blow hammer on the crankcase halves. Defined tight spot = I can
turn it thought the 90  degrees with moderate hand twisting on the
distributor gear end, but if I stop in the middle (45 degrees) it takes
a bit more effort to start it to turn again. It starts with a little
jerk and then loosens up as it goes past 90 degrees. The other 270
degrees are   almost frictionless like past engines I have rebuilt. This
is telling me the journal may be touching a bearing for about 90
degrees.

 

 Based on other experiences it may be telling me that one main journal
is a bit out of concentricity with the others. (Unlikely?). Or perhaps
one of the bearings is not quite seated properly. The worst scenario is
a bent crank shaft. (The plan is to disassemble and examine the bearings
to see if I can determine which journal is touching the bearing. Based
on this I may swap the bearing halves or the 2 and 3 bearing to see if
this brings a better result. I'll also check it without the cam shaft to
eliminate that but I doubt that is the problem as the cam turns at a
different ratio.

 

 I will also replastigage at 6 places on each journal to see if I can
find a tight spot like .0005 in one place and .0015 in another on the
same journal. I don't have equipment to set up and run out the journals
to find an out of round or a concentricity problem. 

 

 Past engine history? PO had holed (# 6 140) piston that liberated a lot
of aluminum into the oil that imbedded that debris in the old bearings
that looked fine other wise. That engine ran for thousands of miles with
a new domed piston (comp 190 psi) and .035 copper head gaskets! 

 

 I will appreciate any and all suggestions. There are no dumb questions.
Ray Sedman still has my heads, so there is no rush to screw it up.

 

Getting all the information is most of the solution.

 

MIKE KOVACS