CARSONS

Reconditioning Service for Motor Vehicle Engine Components

 

 

 

Head Resurfacing

Refacing of Cylinder Heads and Blocks


The mating surfaces of the cylinder block and cylinder head need to be perfectly flat so that the gasket between the two components will be able to perform its role effectively by providing a perfect seal once the bolts are tightened. Reliable sealing is essential to prevent any leakage of the high pressure gases formed during combustion and to prevent the loss of pressurised cooling water circulating through passages connecting the head and block. As the engine block is usually a heavy casting it will possess considerable dimensional stability and so not require refacing routinely. Cylinder heads though are subject to very high temperatures and pressures, which could eventually lead to some distortion. This is especially true of aluminium cylinder heads. Any curvature or unevenness on the surface of the head should be corrected by an abrasive grinding wheel on a machine specially designed and built for this purpose.

When refacing cylinder blocks fitted with wet liners it is important to ensure that the top faces of the liners after insertion are more or less level with the face of the block so that the head gasket will be compressed uniformly over its entire area. Consequently, excessive removal of material from the cylinder block may require machining of the top of the wet liners by a corresponding amount to obtain good sealing.

As in the case of crankshaft regrinding, a stream of liquid cooling medium is directed onto the surface being machined during the refacing operation. This medium serves a threefold purpose; it prevents heat buildup, washes away ground particles plus abrasive grit and reduces friction during grinding by its lubricating action, thereby promoting a good surface finish. It also inhibits rusting for a short period as freshly machined ferrous surfaces are highly prone to oxidation.

 

 



Refacing Valves, Recutting / Refitting Valve Seats


Valve seats

The valves and valve seats need to be in good condition to
prevent any leakage of gases during combustion


The design of the poppet valve and valve seat have hardly changed during the 120 year evolution of the internal combustion engine. The materials, spatial arrangement and timing of valve gear have changed much, but not the basic design principle. It has proved necessary to raise the degree of mechanical precision of the valve gear to new levels in the modern high technology engines, what with higher compression ratios, increased rotational speeds, variable valve timing and so on.

In nearly all engines, the lobes of the camshaft push down on the ends of the valve stems to open them and the spring loaded valves return to the closed position due to the tension in the springs. There are a few engines though that employ a positive valve closing mechanism to close the valves without relying on springs. A separate camshaft is used for this purpose. This type of arrangement is known as the desmodromic valve mechanism.

When overhauling an engine, the vehicle owner cannot afford to cut corners in any area, and that includes the valves and valve seats. The valves should be refaced and those with bent valve stems should be straightened or replaced. Valve seats should be recut to ensure proper seating of the valves. In many engines, the inlet valves and seats are usually made bigger than the exhaust valves and seats. They are designed that way because piston engines have greater difficulty drawing in the fuel / air mixture than in expelling the products of combustion.

If the valve seats are badly worn out they cannot be simply recut. In such an event, recesses should be machined in the cylinder head, new valve seats turned out to size and inserted into them tightly. This is a high precision job involving an element of risk, which needs to be executed with finesse by skilled machinists and mechanics.

The stems of the valves move inside bronze or cast iron valve guides that are pressed in securely into openings in the head. The guides locate and centralise the valves so that their heads are aligned perfectly with respect to the valve seats to provide a tight seal during combustion. Worn guides can allow oil to be drawn into the combustion chamber / exhaust manifold, resulting in excessive oil consumption.

If this happens, the valve guides must be replaced with new ones. This requires the right sized mandrels and pressing equipment. It will also be necessary to ream and hone the bores of the valve guides to obtain exactly the right clearance. Naturally, this is a job for specialists as there is not much scope for improvisation.

For jobs such as those outlined here, you cannot do any better than come to us.

 

 



Refacing Brake Discs / Drums


Brake disc      Brake drum

A brake disc and a brake drum


Whereas most of the work we get is related to engine components, we do attend to machining operations on certain other motor vehicle parts too, such as brake discs and drums. Speaking of same, merely good brakes will not suffice for the modern motor vehicle. What the motorist really needs are excellent brakes, period! This is an area where any compromise is unacceptable. The disc or drum is the place where the friction needed to stop a vehicle in a hurry is generated, so it needs to be perfectly machined and adjusted to function properly.

In the case of a brake drum, concentricity is of paramount importance. In the case of a brake disc, the plane of its faces should be absolutely perpendicular to its axis of rotation; i.e. there should not be the slightest wobble. Any misalignment here can be felt as a vibration or pulsating disturbance in the feel of the brake pedal. (Vibration can also be caused by an out of balance wheel).

When new brake linings or brake pads are fitted they may not seat properly and contact only the high spots on discs or drums that have been subject to wear. This may result in a reduction of the contact area available to generate friction, lowering the stopping power of the brakes. The new brake linings or pads could also possibly wear out faster due to the uneven contact. The remedy for these ills would be to reface the drums or discs so that they present the maximum contact area and also run true.

The main components in the clutch assembly of a vehicle suffer the same problems as brake discs and drums. Therefore, the friction surfaces on the flywheel and the pressure plate may also need to be refaced after long service, typically at the time when a new clutch plate is to be installed. The refacing is usually done on a lathe machine. The clutch plate will now engage with a more positive and progressive action and last longer, as it will be working against flat, even surfaces. Of course, these clutch problems only affect those vehicles equipped with manual gearboxes.

 

 

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