Main Bearing Clearances 1

[DMan1]

Greetings,

How valid are factory bearing clearances when performaing a rebuild?

A turbocharged GM engine I'm working on has an OEM clearance range of .0004 to .0018. Yet, when you look up replacement bearings from someone like Clevite Corp. (Clevite 77 Performance Series), they spec something in the range of .002 to .003 clearance. Why do the engine mfr and the aftermarket bearing mfgr disagree so much?

Denis Kefallinos

 

[MikeHalloran]

Aren't "Performance" engines normally built with a little extra bearing clearance, so they don't need a break-in?



Mike Halloran
Pembroke Pines, FL, USA

 

[tbuelna]

DMan1,

A rule of thumb for journal bearing clearance is about .001 inch per inch of journal diameter.

The actual HD fluid film thickness, under load, is significantly less. A properly designed journal bearing operates with a fluid film thickness of about 60 micro-inch or greater. The additional clearance allows adequate oil flow to cool the bearing and keep it within its operational temperature range- usually about 350 degF max.

 

[DMan1]

Tbuelna,

Yes I have seen this rule. Actually Clevite gives it a range of .00075 to .001 per inch of journal. this engine has a 2.5 inch journal, hence you would think clearances up to .0025 would be ok, per the rule. Its funny the gm clearances are tighter, but for a passenger car I guess you get longer life with a tighter clearance.

Denis Kefallinos

 

[GregLocock]

I don't think it is life related, to be honest. It may be to do with sound quality.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[kenre]

too large a bearing clearance will let a lot of oil pass, meaning more volume is needed in the pump. also could overpower the rings.

Ken

 

[DMan1]

Agreed, but whats too large a clearance?

Denis Kefallinos

 

[Greenlight]

There are several factors to consider when determining bearing clearances. Detailed calculations can fairly accurately determine with the clearances should be.

The following are but a few:

1) The oil viscosity (higher viscosity i.e. 50W requires more clearance, 5W20 requires less clearance). The oil has to escape or it will overheat, but it also has to not squish out easily under the load. The old "rule of thumb" was to run high viscosity oil in hi-perf. engines. This does not hold true anymore with the advent of synthetic oils that can withstand high stresses with low viscosity.

2) The amount of distortion the in the bearing bore during operation (i.e. the rod bearing journal bore will distort more at 8000 rpms than at 4000 rpms). The clearance changes as distortion occurs.

3) The shape/concentricity of the bearing (i.e. a high performance bearing will be thinner at the parting line than a stock OEM bearing). The clearance is usually checked 90 degrees from the parting line. The high perf. bearing can withstand more distortion of the journal.

4) In the case of main bearings, the alignment of the main bearing journals in the block. I have seen ~0.001" misalignment occur from a bare block sitting on the floor to a block with the heads bolted on attached to an engine stand. The more mis-alignment, the more clearance is required.

5) The "twist" and wobble/flex in the crankshaft (usually only a concern with high hp/cid engines with high cylinder pressures).

In summary, if you are going to operate the engine in the same fashion as stock you can use stock clearances and OEM recommended oil. If you are going to use the bearings under extreme rpm/load conditions where bore distortions will occur, you should use more clearance. The Clevite hi-perf. bearings may recommend more clearance than the Clevite OEM replacement bearings.

Also, the OEM's have fractional oversized and undersized bearings and can control bearing clearances to about +/- 0.0003". When you get a crank turned and/or rods reconditioned you are lucky to get +/- 0.001". The replacement bearing manufacturer has to account for this amount of variation and will side on the side of safety (excess bearing clearance, rather than inadequate bearing clearance).

I have personally run main bearing clearances down to ~0.016" and rod bearing clearance (aluminum rods) down to ~0.018" on a 12,000 rpm engine and 0W (zero weight) racing oil with no signs of bearing overheating. I don't recommend that you run clearance quite this tight.

Also, engine bearing do not require a "break-in". The "break-in" only refers to the cylinder bores (which are not round in a new engine, especially an OEM engine) becoming round as the rings (which rotate and are made of much harder material than the cylinders) wear the cylinders round.

I hope this helps.

Greenlight

 

[tbuelna]

DMan1:

Tighter clearances do not necessarily produce longer bearing life. As I mentioned previously, bearing clearances are established to provide the required cooling oil flow across the journal. As long as the engine is operated and maintained properly (ie, clean oil, oil temps not excessive, engine not over revved or run at detonation), the limiting factor for bearing life should be fatigue. Hydrodynamic oil film pressures in a typical automotive engine bearing can easily exceed 50,000 psi, creating high stress at the bearing surface.

Fitting your bearings to achieve the minimum manufacturers recommended clearance will minimize the amount of work required by the oil pump. But unless you modify/optimize your oil pump to match, you will see no gain in efficiency.

 

[Azmio]

There are few tests that are related to the main bearing and conrod bearing clearances. One example is the -40 C extreme temperature maximum rpm test.

The microphones can surely detect strange sound coming from the bearings. Not to mention possible bearing seizures. Bearing clearance for materials with different thermal expansion is an important thing to look for during this test.

 

[evelrod]

Having built a couple race engines over the years, I have drastically reduced bearing clearance from what I used in 1958, 0.003"+....now around 0.0015 for a 2" main and as much as 0.002" for rods using 15W-50 synthetic. Oil temps are kept in the 190f range (temp taken at the oil filter). Line bore and hone is done with the torque plate in place, etc. Engines are road race application and the bearings are usually in "near new" condition at the end of the season (when the engines last that long :-( )
I din't usually do street engines, but one of note was a T Coupe that I built to race specs...it was still pulling even compression and leak down was >5% at 200,000 miles when I sold it in '95 !

Rod