CARSONS

Reconditioning Service for Motor Vehicle Engine Components

 

 

 

Connecting Rod

Connecting Rod Servicing


Automotive connecting rods come with a 'big end' and a 'small end'. The small end has pressed into it a bronze bushing that fits onto the 'gudgeon pin' of the piston. The big end is split transversely in the middle and the two parts held together by alloy steel bolts. The semi-circular plain bearing shells are placed in pairs, one in the arm of the connecting rod, the other in the cap and assembled onto the crankpin. The rubbing surfaces should be lightly coated with engine oil before fitting. A torque wrench should be used to tighten the nuts to the specified limit. There will always be some provision for locking the nuts to prevent them from becoming loose during service. The usual arrangement is to have a plastic insert embedded in a groove at the top of the nut, as this will grip the bolt tightly.

Connecting rods do not require much in the way of servicing. The commonest job is to replace worn out bronze bushings. Original equipment sintered bronze bushes are the best choice here because of their porous structure which allows them to retain oil and thereby resist wear, leading to a long service life. As these bushings are designed to be an interference fit inside the small end, they are usually pressed in hydraulically. Therefore, this has to be followed up with a fine boring operation to obtain a precision sliding fit on the gudgeon pin. A special machine operated by a skilled technician is used for this work.

The big end housing should not require any reconditioning operation unless someone has filed the mating surfaces of the arm and the cap to 'adjust' any excess clearance! In this event it would become necessary, after performing some additional filing, to regrind the internal surface of the big end to its original dimensions. Every time an engine is overhauled, the connecting rod bolts and nuts should be replaced with a new set, as the bolts are subject to very high levels of stress during service and therefore, prone to metal fatigue, and this could eventually lead to failure. The nuts tend to lose their gripping power after long and rigorous service due to deformation of the threads and hardening of the plastic inserts.

Connecting rods are usually made of forged alloy steel but titanium connecting rods have been used on some high performance street engines and racing engines. It is usually possible to straighten a bent connecting rod, but if it is a highly tuned engine (i.e. of high specific power output), then the best course of action would be to replace same. The bearing caps should never be interchanged or flipped around, so to prevent this, should be marked clearly before dismantling.

 

 



Ever had a Crush?


Each semi-circular bearing shell actually extends a few hundredths of a millimeter beyond a semi-circle, so that if a pair of them are placed against each other, the resulting shape would not be a perfect circle but a very slightly oval form. This will become evident when a bearing shell is seated in position in the bearing cap. When one end is flush with the top surface, the other end will project slightly and stand proud of it. This is known as 'crush' and is shown in the illustration below.



Bearing crush


How an oval shape is transformed into a cylindrical
shape under uniform radial pressure


When the connecting rod is assembled and the nuts tightened fully, the bearings shells are highly compressed and adopt a cylindrical shape, following the curvature of the big end housing. There is now an interference fit between these components and this is what enables the bearing shells to resist the powerful drag of the rotating crankpin and stay put. This design feature should not be eliminated by filing the ends of the bearing shells. The lugs on the shells are there primarily to aid assembly by centralizing them laterally and they alone cannot prevent loose bearing shells from slipping and rotating. The tight contact between the parts also aids in conducting the heat generated by friction from the shells to the housing.

 

 



Line Boring of Main Bearing Housings



Line Boring

A line boring machine in operation


Another type of reconditioning operation that is performed once in a while is related to the main bearing housings at the bottom of the engine block. One half of each split housing is integral with the block and the other half machined into the main bearing cap. The main bearing shells are fitted to the recess in each half, and after placing the crankshaft in position, the retaining bolts are passed through the caps and fastened securely to the block. A torque wrench is a 'must' for this work.

As in the case of connecting rods, the bearing caps may not be interchanged or turned around. They are usually marked clearly to preserve the order and the orientation. One or more of the main bearing shells may have thrust faces on either side to locate the crankshaft axially.

Line boring of the main bearing housings is not performed routinely. This operation will be necessitated only in the event of one or more of the housings becoming damaged or eroded as a result of some serious malfunctioning. If such a problem ever crops up, we can put it right for you.

 

 

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