Cylinder Head Machining
|Cylinder Head Resurfacing and Valve Seat & Guide Machining
|Cylinder head resurfacing is just one of the jobs that are usually required when rebuilding an engine or reconditioning a cylinder head. It is an extremely important job because the surface finish that is put on the head (and engine block) affects not only the head gasket’s ability to cold seal fluids and combustion gases, but also its long term durability.|
|Surface Finish Specification|
|For years, most aftermarket gasket manufacturers have said surface finishes with a ‘Roughness Average’ (Ra) of anywhere from 55 to 110 micro inches are acceptable. The preferred Range they have recommended is from 80 to 100 Ra. Even so, as long as the surface finish on the head and block end up somewhere between the minimum smoothness and maximum roughness numbers, there shouldn’t be any cold sealing or durability problems with the head gasket (assuming everything is assembled correctly and the head bolts are torqued in the proper sequence and to the specified torque).But like everything else, these numbers have been changing. These recommendations were primarily for older cast iron heads on cast iron blocks.
As castings have become lighter and less rigid, the need for smoother, flatter surfaces has become more important. Consequently, some aftermarket gasket manufacturers now recommend a surface finish of 30 to 110 Ra for cast iron head and block combinations, with a preferred Range of 60 to 100 Ra for best results.
|Aluminium Cylinder Heads|
|For aluminium heads, the numbers are even lower. The typical recommendation today for an aluminium head on an OHC bimetal engine is a surface finish of 30 to 60 Ra, with the preferred Range being from 50 to 60 Ra. Smoothness has become a major issue with bimetal engines because the difference in thermal expansion rates between an aluminium head and cast iron block creates a tremendous amount of sideways shearing force and scrubbing action on the head gasket. If the surface finish is too rough (more than about 60 Ra), the metal will bite into the gasket and pull it sideways as the head expands and contracts. The cumulative effect over time can cause a delaminating effect in the gasket, literally tearing it apart causing it to leak and fail.Even lower numbers may be required for certain engine applications. General Motors, for example, specifies a surface finish of 27 to 47 Ra for its 2.3L Quad Four engine when the OEM replacement gasket is used. Some aftermarket gaskets can handle a rougher finish on these engines, but it all depends on the design of the gasket. Even so, smoother is definitely better on these engines.|
|Ford specifies an unusually smooth surface finish for its 4.6L V8 engine. This engine, like most late model engines, uses a Multi Layer Steel (MLS) head gasket. This type of laminated steel gasket is extremely durable because the multiple layers of prevents the gasket from losing torque over time.|
|The design also reduces the amount of torque that’s required on the head bolts to seal the gasket, which in turn reduces cylinder bore distortion for better combustion sealing and reduced blowby. The recommended surface finish for the OEM gasket on the 4.6L V8 is 8 to 15
|An Almost Polished Surface|
Although an unusually smooth finish may be required for the Ford 4.6L V8 and certain late model Japanese engines that have MLS head gaskets, smoother is generally better for all engines because it improves cold sealability. One thing you don’t want on the surface of the head or block is scratches. Every scratch is a potential leak path along which fluids and pressure can migrate. If a scratch is deep enough, coolant may find its way into the crankcase or cylinders before the engine is fired up.Or, combustion gases may force their way past the gasket into the cooling jacket or an adjacent cylinder eventually causing the gasket to burn out and fail.
Either way, it’s bad news. So the best way to avoid cold sealing and durability problems is to take the proper steps when refinishing the head and block to ensure the surface finish is within the recommended limits of the gasket manufacturer and/or original equipment manufacturer.
|Can a head or block surface be too smooth? After all, the smoother the surface the better the initial cold seal of the gasket and the less likely you are to have problems with coolant and combustion leaks.Though most gasket manufacturers do not specify a minimum smoothness spec for aluminium heads that have MLS head gaskets, they do recommend a minimum of 30 Ra for engines with aluminium or cast iron heads and a non asbestos or graphite head gasket. The reason for doing so is because soft-faced head gaskets require a certain amount of lateral support from the head and block. A pane of window glass, by comparison, measures about 3 to 4 Ra.When the head is bolted to the block, the metal on both sides bites into the gasket to help hold it in place. You don’t want too much bite when the head is aluminium and the block is cast iron because of the sideways shearing forces that result from the expansion and contraction of the aluminium head. Yet a certain amount of support is necessary to keep the combustion gases in the cylinders from distorting the gasket and blowing past it. This is especially critical in the areas with narrow lands and between the head bolts where there is nothing to keep the gasket in place but the gasket itself. In high output or heavy-duty applications where combustion pressures exerts even greater force against the head gasket, a surface finish that’s below the minimum smoothness spec might lead to premature gasket failure.|
The gasket manufacturers don’t care what type of resurfacing techniques or equipment engine reconditioners use to resurface heads and blocks as long as the Ra numbers end up in the recommended range. It is possible to achieve an acceptable surface finish for a soft-faced
head gasket on most cast iron and even aluminium heads by milling, grinding or belt sanding.
For MLS type gasket applications, though, and even some of the aluminium heads that require a smoother than usual finish (like the 2.3L Quad Four) milling or grinding may be the only way to achieve the lower Ra numbers. But even this may require an investment in new equipment.
Several manufacturers admitted that the spindle and bearings in older milling and grinding equipment were not designed to meet the resurfacing requirements of today’s engines. Consequently, some older machines may not be capable of producing the smoother finishes.
Here at Auto Services Doncaster we use a state of the art Rottler SF7M Block and Head Surfacing Machine.
We can achieve the surface finish required by all the worlds major gasket manufacturers from 2 Ra to 125 Ra.