FE Power Forums
FE Power Forums => FE Technical Forum => Topic started by: hbstang on March 31, 2026, 03:19:40 PM
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ok,for anyone who has actually measured main bearing bore growth how much does it grow at 180 f operating temps?
for a street strip sohc pond block,what would you set the main bearing clearance at?
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They grow about .0007”.
I’d be at around .002-.0022” cold.
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this guy tested on stock ls block and it grew .002 ! wow hope pond/shelby blocks dont do that.
https://www.youtube.com/watch?v=y2wR4MbFk88
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Any metal hole grows with heat, alum or iron, as do the cranks. I can't hear the video where I am watching it, will watch later, but I would not equate a bore size change alone into oil clearance
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We run our aluminum block stuff at around .002 clearance.
I would go tighter, but I get nervous about cold start and modest cost (Scat etc.) parts tolerances.
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Any metal hole grows with heat, alum or iron, as do the cranks. I can't hear the video where I am watching it, will watch later, but I would not equate a bore size change alone into oil clearance
Plus, the metal grows in all directions, not just out away from the bore.
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....and it would be different measuring bearing clearance or measuring housing bore diameter. A steel bearing inside that bore will change how much difference you see at the clearance side of things.
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Here is some math behind this. Aluminum grows at a rate of about (13 X .000001") per inch (of distance) per degree Fahrenheit. Here is where that information comes from:
https://www.engineeringtoolbox.com/thermal-expansion-metals-d_859.html
So, if we assume 100 degree Fahrenheit temperature rise from measurement to running temperature, the aluminum will grown (13 X .000001" X 100) per inch of distance. This is .0013" per inch of distance. With the 2.75" diameter of the FE main bearing, this is actually (13 X .000001" X 100 X 2.75"), or .003575" of growth!
However, the crank grows too. The crank journal, steel or cast iron, will grow about (6 X .000001") per inch of distance per degree Fahrenheit. So an accurate change in bearing clearance must take into account that growth. The calculation then becomes ((13-6) X .000001 X 100 X 2.75, which is 0.001925".
Somebody please prove to me if my calculations are off, because I think this is correct. Having said that, I've run plenty of aluminum blocks at .003" main clearance, and never had a problem - Jay
PS - One thing I didn't mention is that most of our FE blocks, as opposed to big block Mopar blocks, use a steel cap. That cuts the expansion in half.
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Jay, is the expansion rate the same for steel and iron cranks?
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As far as I've been able to determine it is very close. Maybe not exact, though.
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We can get further into the "weeds" on this stuff by adding in the impact of "hoop stress" from the bearing shells.
An installed bearing pair is essentially a press fit, which we observe as "crush" - the amount of bearing shell that sticks out of an unassembled housing bore.
At F-M our engineers (I am not an engineer) recognized the importance of increased press fit on performance bearings. They saw greater dimensional integrity and enhanced heat transfer. To get that value, they increased the amount of crush. When they increased the crush they had to change the type of backing steel used in the bearing shell because the standard material would buckle under the increased load. That stronger steel and higher crush load will impact housing bore integrity. Another reason to directly check bearing clearances as an assembly rather than simply stacking measurements.
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ok,thanks for all the replys on this. i am now educated on main bearing clearance.my engine builder,brian duffe hit it right on with .002 cold after line bore!should be good.
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Many years ago I made inquiries of Donovan and Keith Black on this issue, both responded with approximately the same statement of: In operation it's going to depend on the operational temperature and the loads and resultant heat being dissipated into the block saddles from the bearings, but on engines making 1500 and more horsepower, figure that at the 1000 ft. mark and out the backdoor the main bearing bores are going to grow in excess of .003" and maybe as much a .005"! :o
One consideration is that although one's first reaction might be to close-up the clearances in order to offset this phenomenon, in doing such you're probably increasing the sum of heat transferred into the block saddle and hence exacerbating the growth issue? :-\
Scott.
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You really need to heat a block and check the clearances with the bearings in it. Bore housing diameter is one thing, but bearing ID is another.
I'm kinda skeptical on the comments about the blocks growing .002-.003 on main bearing clearance when hot. If you start out with .002-.003 and it grows to .004-.006, you'd think there'd be a noticeable difference in oil pressure. I never see any oil pressure differences between cast iron blocks and aluminum blocks with the way I set up the bearings.
But then again, I messed up once and put standard main bearings in with a .010" ground crank. Had about .012-.013" of main bearing clearance. Oil pressure at full song was a hair low, but the only way I really knew something was up on the dyno was the oil pressure took a long time to recover after a pull. Brought it home, started pulling it down and decided to pull the pan and check oil flow with a remote pickup. Thing looked like Niagara Falls coming out of the mains.
So I suppose maybe a good pump could account for .004-.005" of main bearing clearance.
I know that I used to set them up at around .0017-.0018 cold and had a change of heart after I pulled an engine down after a dyno session. I increased it a little bit after that.