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Old 01-12-2014, 10:32 AM
AL427SBF AL427SBF is offline
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Quote:
Originally Posted by olddog View Post
Ignore this post. Previous post did not show up as a new post.
... here's your hard work anyways, good stuff -

Quote:
Originally Posted by olddog View Post
I decided to calculate the gap changes myself. I got the thermal coefficient of linear expansion from this website.
Coefficients of Linear Thermal Expansion
I expect these numbers to be in the ball park, however alloys and forging can impact these numbers by quite a bit.

Thermal coefficient of expansion in/in F
aluminum is 12.3 x 10^-6
Steel is 7.3 x 10^-6
cast iron gray is 6.0 x 10^-6

Let's say from 70 F room temp, when you assemble, to 220 F for engine running is a 150 F change in temp.

Consider an aluminum block with steel main caps with a 2.75 inch main bearing with a Gap of 0.0015 inches. 2.750 Bearing ID – 2 x 0.0015 gap = 2.747 crank journal OD. Note I used the term "gap" rather than "clearance" --- less typing.

Block - Aluminum grows by 0.0051" from 2.7500 to 2.7551
Main cap - Steel grows by 0.0030" from 2.7500 to 2.7530
Crank shaft - cast iron grows by 0.0025" from 2.7470 to 2.7495

First note that the nice round main bearing is no longer round and has an average ID of 2.754 inches. Now 2.754 – 2.7495 crank = 0.0045 / 2 = 0.0023 gap. So the bearing gap grows by 0.0008 from 0.0015 to 0.0023 inches. On a -5 morning the gap would close to 0.0011 inches.

If the main cap had been aluminum, the gap would have grown by 0.0013 to 0.0028 inches.

If the crank was steel rather than cast iron:
Crank shaft - steel grows by 0.0030" from 2.7470 to 2.7500
So the bearing gap grows by 0.0005 from 0.0015 to 0.0020 inches.
A steel crank reduces the change in bearing gap.

Note that a steel crank in a cast iron block will cause the bearing gap to get tighter not looser, when the engine warms up!
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