Be warned, I'm using plastigage to just double check the machine shops work. Just had the crank polished (wasn't ground). New STD bearings.

#1 = 0.0015"
#2-4 = 0.001"
(Haven't measured #5 yet.
End play (via feeler gauge) =.0025"

These numbers sound really tight to me. . . going for a reliable as hell build on the track. . . Thoughts?

 

I think a lot of people don't understand Plastigage. Plastigage only measures the clearance between the bearing cap and the crank journal....it doesn't measure the total clearance which would include the clearance on the other side of the crank journal. Now if your engine is upside down on an engine stand, you can be pretty assured that the far side of the journal is resting on the bearing in the engine block and that clearance is zero, or close to zero and that the Plastigage when its crushed between the crank journal and the bearing cap will measure the total clearance. Now if your measuring the connecting rod clearances you have to rotate the crank so that the rod journal is pushed up, into the rod bearing before you measure the bearing clearances with Plastigage. Likewise, if your measuring main journal clearance with the engine in the car; use a small jack to push the crank upward into the (engine side) bearing shell.

A number of years ago I read an article in a car mag that compared measurements taken with internal bore micrometers on the bearing i.d. and the diameter of the crank journals measured with an outside micrometer. The mystery was that the Plastigage clearance were on an average, half of what the precision measurements were.

With all that said, my experiences with used but good chevy cranks shaft is that there is some wear already and polishing increases that wear. True, some journals can use a std bearing, but others may require a 0.001 oversize, or a combo of half standard and half 0.001 oversize for a total oversize of 0.0005. Probably as important is to take a series of measurements around the journal to see if there is any "egg" to the journal.

if you're still with me, per my 96 FSM :
crankshaft clearance #1 0.0007 -0.0021 inch; #2,#3, #4 0.0009-0.0024 inch; #5 0.0010 -0.0027 inch
Crankshaft bearing clearance service limits #1 0.0010 - 0.0020 inch; #2, #3, #4 0.0010 -0.025 inch (possible misprint? maybe 0.0025); #5 0.0015 - 0.0030
Crankshaft end play 0.002 - 0.008 inch

 

Yeah, I checked the clearances on my FSM as well; they seem to setup for a tight motor. If plastigage measures half, I’m golden. I found some research by a bearing manufacturer talking about bearing capacity vs clearance, bearing temp vs clearance, and oil flow vs clearance; 0.002” seems to be the money shot. I’m probably going to buy a bore gauge (already have a mic set) just to be on the super safe side.



Next question: I’m assuming that clearance is diametrically, correct?

 

I appreciate the attention you're giving the lower end. Correct bearing clearances are critical, but the best set up bearings will fail if they aren't lubricated properly. Now I know that the "conventional wisdom" is that the stock small block oiling system was designed back in 1955 by engineering genius' , requires no modification and 10 psi for every 1000 rpm will be all the oil your chevy small block will ever need. I beg to differ. Researching back to the days when NASCAR stock cars actually ran stock block engines, the oil passages were all drilled out larger and the oil holes in the upper shells of the main bearings were drilled oversize to match the oil passage in the block. And builders of small block based marine engines for offshore racing still do these mods; goggle it...to quote Sculler (or Mulder ?) of the "X FILES", "the answers are out there".

 

Yeah, I’m an engineer by trade. So a little **** on these things. I grabbed a telescoping gauge set and a better mic from work. Not ideal in the sense of using a bore gauge, but I’ll take a bunch of measurements and average them out to see what I get.

 

 

There is some variation between bearings. It seems it's tight for a polished crank, but i would check on the availability of .001 and .002 under bearings. Don't be afraid to mix and match to get desired clearance. .0025 is a good number.

 

Track engines tend to be on the loose side. More heat more expansion needs more lubrication.

 

Slightly loose, nobody will know.
Too tight, everyone will know.

 

Welp, looks like plastigage was right. Clearances are definitely on the tight side. I should just get some 0.001" undersized bearings and be good to go

 

Not trying to flame you but it doesn't read right. A stock crank polished for reuse should be a ltlitt loose with the stock bearings. And you should be seeing a tiny amount of taper on the journal from side to side. It's just that I don't expect a stock crank to that tight let alone a used then polished crank.

So I would ask if the bearing saddles, bearings, journals were completely dry? Torque wrench calibrated? What was the condition of the original bearings? And even with Plastigauge you want to measure 4 locations 90 degrees apart.

 

What you are measuring is the norm with LTX engines using factory parts. The crank journals are on the big side of the spec and stock LTXs have betwen .0008 and .0012 clearence.

What you need are STD "X" bearings. .001 bearings are for .001 undersized crankshafts. X bearings are for extra clearence. Also King bearings will have a few tenths extra clearence compared to Clevite.
Will

 

x2 they are put together pretty tight as it is, one reason youd hear about a lot of spun bearings in the LTx stuff

 

The "Max Performance LT1/LT4 " book is well illustrated and alot of good information specific to the GEN II LT engine. Its mostly about assemby and somethings that maybe overlooked and cause trouble later. IMO it's worth the few dollars it sells for.