Aluminium Block hot bearing clearances.

[Ant]

Dart aluminium engine has 0.00275" main bearing clearances cold (2.75thou)
I am interested to know if the main bearing clearances increase HOT possibly 0.001" more or less??

Going by some oil companies oil viscosity recommendation of over 0.003" main bearing clearances use a 50wt oil and under 0.003" use a 40wt, that may put me in the 50wt oil category.

[Rick Parker]

Precision Products Performance Center...building the future of performance racing!

Read the Elliot Bearing Report.

[olddog]

This entire thread has a lot of good info.

On page 5 post 62 I did calculations on this very subject.

oil pressure for 427 aluminum block


Note that a steel verses a cast iron crank makes quite a bit of difference.

[olddog]

Ignore this post. Previous post did not show up as a new post.

[blykins]

It's generally accepted that the clearance will grow about 3/4 of a thou to one thou.

I've never set the bearing clearances in an all-aluminum engine that loose. The Dart blocks and the aluminum FE blocks share the same size main journal diameter. I set them at around .0018"-.002".

[AL427SBF]

Originally Posted by olddog

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... here's your hard work anyways, good stuff -

Originally Posted by olddog

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!

[Ant]

Thats really good information which answers my question.

This engine has a main bearing bore of 2.250" clearance of 0.00275"
Rod Journals bearing bore is 2.001 with clearance of 0.002"

I guess that gives me on the mains approx 0.0035" when hot which would indicate at least a 40wt oil. I assume if you wanted to use a really thin oil for less friction and more hp you could go to an oversize bearing shells and reduce clearance.....

[Bernica]

I'm at .0022-.0025 on mains with an aluminum Shelby block. Running Gibbs XP3 oil. No issues so far.

[RICK LAKE]

Ant What is your oil pressure at idle and hot. Aluminum blocks depending on material breakdown can expand from .0015"- .0023". Oil weight and pump pressure is more important. Your motors are different, I run a shelby block with 100# spring in the oil pump and 15W-40 rotella diesel oil. 1 bottle of Lucas oil suppliment. Over fill oilpan to bottom of scraper plate. Oil pressure cold is 125-128 psi motor runs at 1,100 rpms until water temp and oil temp warm up. No RACING the motor to warm. My rods and mains are both under .002" I have been told that the high pressure is wearing out my bearings. 7 years on 452 bottom end an 8 years with 482 kit in same block and no wear to either. This is a motor that is raced 98% of the time. Limit of 6,000 rpms helps. I also have a accusump 3 quart tank to help control oil going to motor on high "G" turns. Idle is 35-38 psi and running is between 75-80 psi on the track hot. Old days rods and mains where .003" and a little larger. As long as the motor doesn't idle, see no problem. IMO idle is the worst thing for any motor that is even with extra oiling to mains and rods. Tests have showed a 10-30 psi drop of pressure from front of oiling system to back of motor and rods. Gauge may read 30 psi but back is 5-10 in most motors. JMO. Good luck Rick L Ps use accusump as a pre oiler to start motor. get about 35 psi before starting. No noise, no wear.