<VV> checking bearings
djtcz at comcast.net
djtcz at comcast.net
Mon Dec 17 07:06:41 EST 2007
The rod big end bore size directly effects the bearing clearance.
Were the rod big ends reconditioned? Regardless, their cleaned, torqued, finished bore sizes (and roundness and taper) should be checked.
So, in any case 0.008" is way too much.
And as Bob H mentioned, 0.008" is "out of range" for green Plastigage, which is the correct product to use. http://www.hastingsmfg.com/Tools/plastigage.htm
If accidently reading the green PG's metric scale at 0.08 mm the clearance is more like 0.003 inch. That is still on the loose side, and would benefit from 0.001" undersized bearing inserts, but I would confirm the crank journals have less than 0.0005" taper and out-of-round. 0.002 in the middle of a tapered journal is a lot less at the edges if the rod bore is perfect. If the rod was reconditioned carelessly or unknowingly the bore can be barrel shaped and tighter/smaller at the edges. How tight? That's what bore gages are for. But one of my well meaning but inexperienced co-workers finished seveal sets of Chevy 350 rods that measured very round and right to size in thecenter, but 0.001 inch small at the edges.
When polishing with one of those powered polishers
( http://www.farrisequipment.com/nss-folder/pictures/DSC01752.JPG ) it is possible, if not easy, to polish measureable amounts of taper into a journal. Those journals sure "Look good," that is until the micrometer results are in. When I polished cranks I kept micrometers and cleaning rags close at hand, and I used them to look for trouble on every journal before, during and after the polishing process.
And polishing, especially on a freshly ground or Tufftrided crank must be done in the correct direction.
As others have suggested, measuring the crank and rods is required to solve this mystery and confirm the actual clearance. A good method is to gently measure the bearing bore as installed in its con-rod, and compare that figure to the crank measurement to determine the actual clearance.
Outside and inside Micrometers and bore gages are the proper tools for this kind of measuring.
Vernier calipers, even fancy electronic ones, are not. http://www.triginstruments.co.nz/catalog/images/caliper_digvernier.jpg
-------------- Original message --------------
Date: Sun, 16 Dec 2007 23:20:41 -0600
From: Jason Morri <neosore at gmail.com>
Even though the machine shop told me my crank was in spec
for STD bearings, I still used some plastigage to make sure. I checked to
make sure the rod bolts were tight and torqued them to 30 lbs. Well long
story short after checking different rods multiple times, I'm showing a .008
clearance. Which means, according to the assembly manual, I'm .005 over the
My question is, is this something that I should worry about? Should I trust
the machine shop or trust the plastigage (which by the way, is the first
time I've ever used it). I checked Clarks and they only have rod bearings
that are .000, .001, .002, .010. How does one get to .000 with a .008?
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