Crank journal tolerances?

tri 6's fanatic · Apr 6, 2014

I have a SBC crank that the 1, 2, and 4 mains are .0012" smaller than the advertised twenty under. Main number 3 is .0019" smaller than the twenty under. Main number 5 (which to me is the rear wider main) is .0022" smaller than the twenty under. The rods are all .001" to .0012" smaller than the twenty under. I was told oil generally fills .001".

I searched in vain for a GM spec sheet publication I read a couple years ago. This GM spec sheet had the tolerances for every component inside the SBC engine. It seems to me that the GM cranks had a window of plus .0000" and minus .0003".

That .0003" is a lot tighter than the machine shops (I have contacted) seem to produce and the above mentioned crank. One shop stated that his crankshafts are ground to within bearing manufacturer's specifications (generally .0007" to .0017" under) what the original goal is (ten under, twenty under, etc.). One shop also stated that the #5 journal on SBC's is usually is .005" smaller than the rest of the journals for proper oiling. Another shop says they can turn the crank to any exact measurement I want (within a tenth, .0001").

I don't want to spin a bearing. I also don't want the loose tolerances drag racers use who tear down their engines between runs. Are the tolerances in paragraph one usable or should I seek a more uniform grind of say .0005" to .001" under the specified grind? Are bearings manufactured to automatically have the .001" oil film? Thanks!
　

Steve
The difference between a hobby and insanity is .0000001"

Lawrence454 · Apr 7, 2014

I like .002 to .0025 on the rods and .002 to .003 on the mains.

The engine books probably allow a bigger variance, but that's what I've always shot for. If #5 main has .003 clearance and the others are a minimum of .002 you should be fine.

350sshugger · Apr 7, 2014

X2 what Anne said. Your aiming for .0025" clearance between crank and bearing. That GM spec is a manufacturing tolerance, not a bearing clearance tolerance. You are more worried about bearing clearance, so if I were you I would have a machine shop polish the crank so that you can use a set of undersized bearings that are all the same undersized to get your clearance within that .0025" window.

tri 6's fanatic · Apr 7, 2014

Anne and Matt,
Thanks for your knowledge and taking time to answer. I like your suggestion of having the crank polished to achieve the 2-1/2 thou! Thanks again.

Steve
The difference between a hobby and insanity is .0000001"

Lawrence454 · Apr 8, 2014

tri 6's fanatic said:
Anne and Matt,
I like your suggestion of having the crank polished to achieve the 2-1/2 thou! Thanks again.

Assemble what you have with plastigauge and see what you're clearances are.
If it's out of spec, they might make a bearing to get it where you want.

I bet you're ok now.

tri 6's fanatic · Apr 11, 2014

Anne,
I will definitely use plastic gauge. Would you happen to know if bearing manufacturers allow for the oil film between bearing and journal? Say an exactly ground and polished crank (.010" under) was mated with ten under bearings, would there still be 1 or 1-1/2 thousands for the oil.

Thanks!

Steve
The difference between a hobby and insanity is .0000001"

Lawrence454 · Apr 11, 2014

tri 6's fanatic said:
Would you happen to know if bearing manufacturers allow for the oil film between bearing and journal?

The correct clearance maintains the correct pressure and adequate oil film thickness.

Some people like to run larger clearances and thicker oil and vice versa.

Steptoe · Apr 12, 2014

A lot ppl are anti plasti gauge these days... think it has something to do with marketting propaganda BS..We always plastigauge
For a low rpm street car we shoot for around 1.5 to 2 /1000.. something a bit more camed/ street cruiser around the 2 to 2.2 /1000 and high end track 3 to 4/1000
These are on dynamically balanced engines.. which even a std street engine highly recommend

almano · Aug 1, 2015

I believe the anti plastigauge thing is a BS.
I working on race bikes with lot more precise and accurate clearances running over 15000 rpm and we use plastigauge all the time. Even the Yamaha, Kawasaki, Honda, Suzuki recommend to use plastigauge to check the recommended tolerances.:thumbsup:

hhott71 · Aug 3, 2015

A set of 2-3 mics and a either a dial bore gauge or telescoping gauge will tell you the exact tolerances.
You also need to check roundness and straightness of the crank using a dial indicator.

almano · Aug 3, 2015

I usually let my guy at the machine shop take care of the roundness, straightness, that's their job to do the work right. I use the plasigauge to select the right size main/rod bearings, after i get the ready crank back from the machine shop. They never failed on me. :thumbsup:

Crank journal tolerances?

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