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!

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[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

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

... 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.

[blykins]

Originally Posted by 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.....

Way, way, way too much clearance for a 302 sized main. I set bearing clearance at .0015 on aluminum 302 blocks. With .0035 clearance you will have a considerable oil bleed there, which can cause cavitation and other issues. I don't even run .0035 on windsors and 460's.

If you have the engine apart, I would put some .001 bearings in it at least.

Rods are right on the money...

[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.

[Ant]

Originally Posted by blykins

Way, way, way too much clearance for a 302 sized main. I set bearing clearance at .0015 on aluminum 302 blocks. With .0035 clearance you will have a considerable oil bleed there, which can cause cavitation and other issues. I don't even run .0035 on windsors and 460's.

If you have the engine apart, I would put some .001 bearings in it at least.

Rods are right on the money...

Engine hasn't been on road yet.
we ran it on the dyno with Delo 400 15w40 60Ib oil pressure, I could've screwed the relief valve up we were happy with that pressure, the engine has spray oilers. That was on a std pressure Peterson dry sump pump, since then I have purchased a HV Peterson Pro Rotor with single scavenge return in order to reduce plumbing and I may be able to slow the new pump down for less shafted horsepower loss.

My US engine builder has built a lot of race engines and has a good name, so I will contact him and ask him about the clearances. Its not hard to remove sump and change shells.

[Ant]

Rick,

Not sure what hot idle oil pressure is, think it was about 25 - 30psi

Regarding pre oiling engine can remove pump drive belt to prime engine but that is going to be a hassle as I can't get a drill onto the drive bolt due to radiator etc, could turn over by hand. I am quite fussy about only racing when engine is fully warm. Running an in tank oil heater.

[blykins]

Certainly don't want to get between you and the engine builder. If it's been dyno'd and ran, and you're happy with it, give it a shot. I may have over-reacted yesterday when I posted, but in my mind I'm seeing a 302 with close to .004" main clearance.

I don't think I would swap bearings without a good way to mic the journals and get a bearing clearance with a bore mic.

[Ant]

No problem appreciate the input.

It had occurred to me that fitting oversize 0.001" bearings without measuring clearances may cause some tightness or clearance issues. that might only happen if the Block wasn't straight...

The engine has sat for a few years since it was dynoed in NZ, then sealed with no pushrods, plenty of oil etc in the bore, I have even primed the oil system on one occasion. Just trying to protect my investment until I get the rest of the car finished.

[RICK LAKE]

Ant I stored a 390 AMX motor in the garage on an engine stand for 5 years and the back 2 cylinders where .0013" out. Over time the core moves. Some of the best engine builders don't have engine stand that work off the bellhousing end in their shops. Rotators bolted to the motor mount pads. I have engine roller stands that the motor sits on the oil pan rails all the way around.
As for preoiling, get a 2 quart accusump and hook it up if you can't reach the front of the dry sump. I know it's done with heaters for oil tanks but what if any down side for putting warm to hot oil on outside temp parts?? years ago we had problems of adding room temp oil to hot motors and got piston and rod knocking from bottom end. Thermo shock to the bearings or crankshaft. Rick L.

[Ant]

Rick,

I have had the engine in the car on its urethane bush mounts attached to a thick piece of angle steel bolted to the block mounts, and bolted to the transmission for a few years, I would hope it should be ok. With no pushrods and plugs out, the engine still turns over nicely..

To be honest over all the years I have raced cars etc I have never bothered to preheat oil, I think it might be a good idea to warm the oil up during the winter but that also creates a differential in temperatures with warm to hot oil hitting the cold engine. I guess if you don't go overboard and just warm the oil it maybe of value if you are using a 20w-50 or 15w-50 oil.
I have had cars that have done 310,000miles sitting outside covered in frost etc and still going strong without oil preheaters and oil primers. We don't worry about our road cars as they are easily replaced.

I will consider an 2 qt Accusump for priming as it can protect my investment, not sure how it works with my dry sump system as there might have to be a one way valve so it doesn't back feed through the pump etc. Peterson Fluid make a remote filter mount primer pump incorporated that you use a drill on that makes 20 psi, that could be an easy installation..

ps My engine stand bolts to the engine rails and bell housing bolts.