TECH-fax

Performance
611 - ACL Performance Engine Bearings


ACL Bearing Company manufactures a range of engine bearings for performance engine builders. These bearings have special characteristics to provide enhanced performance for engines used in competition and other highly loaded applications.

Construction

The bearings are manufactured with:
a) Steel backing material.

b) High fatigue strength copper lead lining.

c) Nickel barrier to prevent tin from the overlay migrating to the underlying copper lead lining (tin diffusion reduces the strength of the overlay plate).

d) Lead/Tin/Copper babbitt overlay 0.013mm/0.0005” thick for improved fatigue performance.

Load Carrying Capacity




All ACL Performance bearings are manufactured from F780 Sintered Copper-Lead intermediate layer, the same intermediate layer material as used in ACL Duraglide® (regular aftermarket bearings). This material offers significantly improved fatigue resistance under extreme operating conditions.
Overlay Plate



To improve fatigue strength on performance bearings the overlay plate has been reduced to a thickness of 0.013mm/0.0005” (regular replacement bearings have an overlay plate thickness of 0.020/0.0008 - 0.025mm/0.001”). Due to this reduction in plate thickness to improve fatigue strength, embedability and conformability properties of the overlay are compromised slightly. However the performance engine builder will pay particular attention to:
a) Housing - size and alignment.
b) Crankshaft - journal size and geometry.
c) Cleanliness at the time of assembly.
and therefore these properties (embedability and conformability) are not critical for satisfactory life.
Special Features

Wall Size

Performance bearings have a bright micro-fine machined bearing surface to give ! 0.002mm (! 0.000075”) wall size tolerance to ensure consistent oil clearances when matched to a precision ground journal and correctly sized housing.
Eccentricity

Wall eccentricity has been increased to compensate for rod bore and block distortion (which occur due to high inertia loads at high RPM) and to assist in the formation of an adequate oil film between the bearing and crankshaft journal.
Crush

Crush has been increased over normal aftermarket bearings to provide secure placement in the housing thereby ensuring full back contact for efficient heat transfer.
Partial (¾) Grooving

Some main bearing sets feature ¾ grooving (or the addition of a partial groove on the lower half). This allows for an increased oil supply to the highly loaded connecting rod bearings and also carries pressurized oil closer to the highest load area on the lower main bearings. The partially grooved lower main bearings do not suffer from the increased unit loading which occurs in the crown of the bearing shells with the fully grooved lower main bearing design (due to the reduction in cross sectional area caused by the addition of the groove).


Camshaft Bearings

For some applications, cam bearings in heavy duty aluminium material are available. Whilst these bearings will take much heavier loading than the lead based babbitt materials commonly used for most cam bearing applications, correct alignment of housings and correct assembly are important to ensure satisfactory performance. The camshaft must rotate freely after assembly; a minimum clearance of 0.038mm (0.0015”) should apply to these heavy duty cam bearings and a good lubricant supply (and pre-lubrication at assembly) is very important.

Installation and Assembly

1) Bearing Clearances

Most High Performance Engine Builders will have their own specification for vertical oil clearance i.e. measured at 90° to the parting lines of the bearings), which they determined from past experience, however, the following information will give some guidelines as to what clearances should be attained for satisfactory bearing performance.
(a) Determining Oil Clearances

The best method of determining oil clearances, if suitable measuring equipment is available to the engine builder, is to measure the ID of the bearings installed in their housings with the bolts torqued to assembly specifications. A dial bore gauge is suitable to measure the bearing ID at 90° to the bearing parting lines, then subtract the crankshaft journal size from the bearing ID to determine the oil clearance. This is repeated for each bearing position in the engine. Many High Performance Engine Builders will perform the above measurement of bearing ID’s prior to having a crankshaft ground, this will give the crankshaft grinder exact journal sizes to work to in order for the required vertical oil clearances to be attained.
If suitable measuring equipment is not available, then the next best alternative is to use ACL Flexigauge, a system using a thin precision plastic strip which is assembled between the bearing and journal surfaces, with bolts being torqued up to the correct tension. The bearing cap is then removed and the width of the flattened plastic strip is measured against a scale (included with the product) which shows the clearance (the wider the flattened plastic strip, the less clearance). Full directions for the use of this product are enclosed in the ACL Flexigauge box.


(b) Typical Clearances

As general rule, vertical oil clearances should be approximately 0.025mm (0.001”) per 25mm (1”) of shaft diameter, plus an additional 0.013mm (0.0005”) for high performance applications. For most automotive applications with journal sizes approximately 50-60mm (2-2 ½”) in diameter, a main bearing minimum clearance of 0.05mm (0.002”) and a maximum clearance of 0.075mm (0.003”) will be acceptable. The clearances for the connecting rod bearings within this size range can be slightly tighter, with a minimum of about 0.045mm (0.0018”) and a maximum of approximately 0.070mm (0.0028”). In general, clearances 0.005 to 0.008mm (0.0002 to 0.0003”) under and 0.013mm (0.0005”) over these recommendations will not make a significant difference. For some high RPM applications such as those found in drag racing, clearances can be 0.013mm (0.0005”) higher than those recommended above, with 0.09mm (0.0035”) being about the limit. Engines using aluminium connecting rods should have clearances reduced by 0.013mm (0.0005”) to compensate for increased expansion in operation as compared to a steel or cast connecting rod.
(c) End Float or End Play on Thrust Bearings

0.12 - 0.15mm (0.005 - 0.006”) will generally be adequate for most applications, particularly where the thrust bearing is located at the rear of the engine. An increase to about 0.18mm (0.007”) applies to engines where the end play is controlled at the centre main bearing, due to the increased effect of crankshaft flex. An absolute maximum of 0.25mm (0.010”) should apply to high performance applications.
2) Bearing Housings

The fatigue life of a bearing is greatly effected by the conformance of the housing to manufacturing specifications. The size and shape of the bearings is determined by the dimensional accuracy / geometry of the housings and the intimacy of the contact between the bearing and the housing has a great effect on heat transfer. A number of features must be checked to ensure adequate bearing service life. Before machining a bearing housing, the parting line surfaces should be flat and parallel. They should also match evenly when subjected to the correct bolt torque and all subsequent machining operations to be conducted on housings with bolts torqued to the correct value.
(a) Bearing Crush and Back Contact

The back contact of the new bearings in their housings and also the crush fit should be checked, particularly if old bearings removed from the engine have any bright or shiny areas on the shell backs which would indicate that there has been movement of the shells in their housings. The procedure of this operation is:

(b) Connecting Rods

(c) Crankcase 3) Crankshaft Tolerances

(a) Main bearing and crankpin journals

(b) Thrust Faces Crankshaft Fillet Radii

Some aftermarket crankshafts are available with large journal fillets in order to reduce the risk of crankshaft breakage. The edges of the bearings already have a minimum I.D. chamfer, however if you suspect that the crankshaft you are using has non standard enlarged journal fillets it is important to check to make sure that this enlarged radius does not contact the edges of the bearing and cause “fillet ride”. If there appears to be only a slight interference, the chamfered edge of the bearing can be increased by had with a bearing knife. In extreme cases where the crank has been reground or has been especially ground with very large fillet radii (for added strength) the bearing chamfer may have to be substantially increased by machining them in a lathe. This should be avoided unless necessary as this decreases the cross sectional area and hence the load carrying capacity of the bearing.

Final Assembly




At the time of assembly of the engine it is imperative that all engine components and lubrication galleries are thoroughly cleaned before assembly begins. Before fitment of the bearings to the engine, it is important to examine the bearings and their features to ensure they are correct for the engine application. All bearings should be assembled with the shell backs and the housings perfectly dry and scrupulously clean, but use a liberal supply of a suitable assembly lubricant between the bearing surfaces and the crankshaft journals. Bolt boots should be used on rod bolts to prevent knicking or damage to the crank journal at the time of fitting connecting rod assemblies. Any damage of this nature will most likely cause bearing failure.
Priming

After installing and prior to starting a rebuilt engine, it is mandatory to prime the oiling system (i.e. oil pump, oil galleries, oil filter/filters and oil cooler if applicable) to ensure instant oil pressure upon startup. The best way to prime the oiling system is to remove to oil pressure switch and use a “pressure primer” whereby engine oil is fed into the oil gallery. The “pressure primer” will not necessarily fill the oil filter/filters and oil cooler (if applicable) and therefore they must be filled with oil prior to fitting or connecting to the engine. Failure to adhere to the above could result in a dry start and bearing failure.
A dry start can occur even though the oil pump is in first class condition because the oil pump will aerate. This does not necessarily mean that the oil pump is sucking air, but aerating in its own housing. The reason for this aeration is that by the time the engine is assembled and installed in the vehicle, the oil may drain out of the oil pump, therefore the pump becomes partly dry and unable to pick up the oil.
Priming the oiling system starts with the oil pump after being checked and/or reconditioned at the time the oil pump is being assembled. It is imperative after checking the oil pump clearances, to oil the pump and then put a small smear of light grease (Castrol LM grease or similar) in the pump and rotate the pump as this will make the pump gears tacky and wipe any excess grease off. Too much grease will cause extra load in the pump, therefore as a rule of thumb, put no more than ½ a teaspoon full. This quantity applies to both large and small oil pumps and it is only there to make the gears tacky. When the engine is finally installed and started, instant oil pickup will occur.
The Importance of Checking Micrometers

Micrometers used in any aspect of engine reconditioning must be regularly checked and calibrated.
This is especially important if the micrometer is in constant use, for example when it is used to check piston skirt sizes to get the bore sizes. A 4" micrometer used for this purpose may be used on hundreds of occasions in a given month, and if it gets a lot of use around a specific, its thread may wear in that section.
This will mean that if a frequently used 3" to 4" micrometer is checked when closed at its 3" position with a 3" slips it may appear to be OK. It may also be OK if checked at its fully open position with 4" slips. But if checked with slips at the most frequently used position it may give an incorrect reading. This is because the threads will be worn at this point.
This recently happened when an apparent piston sizing error was suspected. It was believed the piston was 0.0007" oversize. Consequently the bores were honed in accordance with this. When the same piston was checked with another, less worn, micrometer, the dimension was correct and this meant that the bore ended up 0.0007" too large! The consequence of this would be cold engine piston rattle which was attributed to use of the worn micrometer.
Similar errors can occur with micrometers used for measuring crankshaft journals. Such micrometers are frequently used for measuring journals of similar size and they can wear at one position.
Conclusion

All workshop micrometers must be periodically calibrated against slips, and at more than one position in their range.
If frequently used at one position then calibration must be included at this position.
Every micrometer should be maintained in a clean condition and should preferably be stored in a wooden or plastic container. Micrometers are delicate instruments and should never be placed on metal benches, lathe tops etc., unless on a clean soft surface such as rubber or cork.
Micrometers in frequent use cannot be expected to last forever! Nor is adjustment easy or possibly in all cases. Replacement at sensible intervals, or relegation to non critical use is advised.
Finally, remember that inside micrometers are seldom very reliable for absolute measurements. If using an inside micrometer to measure a bore size, do not rely on its reading, rather measure it with the same outside micrometer which is to be used for the bore measurements. The same applies when measuring bearing tunnels. It is better to use spring loaded telescopic gauges for any internal diameter measurement.

Spring loaded telescopic gauge for
tunnel/bore measurement using an
outside micrometer


Part Number Information: ACL Performance Engine Bearings
Application
Big End
Main
Cam
Chevrolet Small Block V8 (262, 267, 302, 305, 307, 327, 350, 400 c.i.)8B91324 -
STD/001/010/020
5M91338 -
STD/001/010/020
5C3349C-STD
Ford L6 (250 c.i./4.1L)6BHP2150 -
STD/001/010/020
7MHP2158 -
STD/001/010/020
N/A
Ford V8 Windsor (302 c.i.)8B91092 -
STD/001/010/020
5M91093 -
STD/001/010/020
N/A
Ford V8 Cleveland (302, 351 c.i.)8B91469 -
STD/001/010/020
5M91470 -
STD/001/010/020
N/A
Holden L6 (138, 149, 161, 173, 179, 186 c.i.)6BHP2380 -
STD/001/010/020
7MHP2384 -
STD/001/010/020
N/A
Holden L6 Red/Blue/Black/XU1 (202 c.i.)6BHP2380 -
STD/001/010/020
7MHP2398 -
STD/001/010/020
N/A
Holden V8 Red/Blue/Black (253 c.i./4.2L, 302 c.i./4.9L, 308 c.i., 5.7L)8BHP2356 -
STD/001/010/020
5MHP2357 -
STD/001/010/020
5C5146C -
STD/002/010

Warranty: This range of Performance parts is made to assist engine builders when the engine is expected to operate in special situations that are subject to higher speeds and loads than in the standard road applications.
The user must accept responsibility when these parts are used in non standard applications. ACL cannot accept any form of liability when these products are used in engines over whose design it has no knowledge of control.



Author: Kathy Dimovski 08/10/98 13:38:19
Last changed by: Kathy Dimovski 11/11/1999 10:47:43 AM

© Automotive Components Limited
Reproduction in whole or in part is prohibited without written approval.