Machining 101: The engine block

Machining 101: The engine block

June 26, 2018 0 By Ray Bohacz

An engine can be broken down into three categories: block, rotating assembly and cylinder head. Though they are all important, the block houses the other two and is paramount to a successful engine rebuild. Thus, the first installment in the three-part engine machine shop series will explore proper block service procedures.


These are the steps that need to be executed on the block for any engine rebuild:


  • Chase (clean-up) the threads in all tapped holes.
  • Remove all burrs and casting slag from block interior.
  • Tap the main galleries at the front of the block so that pipe plugs can be installed.
  • Align hone or align bore the main caps and saddle.
  • Machine the deck for straightness and proper surface finish.
  • Hone the lifter bores.
  • Bore cylinders.
  • Hone the cylinder walls to create the proper surface for the ring set employed.
  • Clean in a jet-wash machine.
  • Paint exterior for rust control.

I will expound on theses areas with an explanation of why they are necessary.


Fastener holes


All threaded holes should be chased with the correct bottoming tap to eliminate any burrs and dirt that might skew torque wrench reading and thus, impact the final quality of the rebuild. Many an engine has failed due to improper torque from unclean threads.


The area around the bolt holes should be checked for “thread pull”. If present, it can be corrected with a file, chamfering, or counter boring. This step is often overlooked and can mean the difference between a gasket lasting the lifetime of the engine or a premature failure.




Burrs and casting slag on the inside of the block should be removed with a high-speed grinder. This aids in removing bits of sand or casting that might jar loose while back in service and damage the engine. Though the slag would have been there from the day the block was made, the years of thermal cycling and the handling during rebuilding often has it break loose from its mooring.


Oil gallery plugs


Some engines use soft metal plugs in oil passages and often better engine builders like to replace these with threaded pipe plugs. This assures a leak-free fit since the soft plug may not seal tightly in an engine that has seen many years of use and thermal cycles. If a gallery leaks the engine’s oil pressure will be low and may result in failure or at the least a repair.


Align bore/align hone


If the main bearing bores (where the crankshaft resides) are not aligned it can be corrected by boring the saddles into alignment. Distortion of the main caps and saddle occur slowly over many years of use and heating/cooling cycles. This causes the block to warp and distort. This results in a misalignment of the main bearing tunnel. Warpage takes place over time. The original main bearings and crankshaft will compensate for this by wearing unevenly. If a reground crankshaft and new bearings are installed in the block without correcting for this, rapid wear occurs and engine failure soon afterwards.


Another concern is main bearing cap stretch from high rates of load on the engine and lugging. This too occurs over time and the original parts formed to the misalignment and new components will cause binding.


If a main cap(s) are ever replaced the block must be align bored.


Align honing is the same theory as align boring but removes less material if the warpage is not too great. Whereas an align bore uses a cutting bit, an align hone uses stones on a mandrel to remove material.


Every engine rebuild may not require an align bore but an align hone is an essential step of a proper rebuild.




It is possible for the cylinder head mating surface on the block to develop irregularities that can cause compression and coolant leakage. The flatness of the block can easily be checked using a straightedge and feeler gauge. A good rule for most engines is the maximum deformation cannot be more than 0.004 inch.


Decking also produces the proper surface for the head gasket to marry to. It is read in RMS (root mean square). Each supplier identifies the surface finish for the head gasket to perform properly. The deck should not be perfectly smooth even though it needs to be straight.


The industry standard for surface finish is the microinch. One microinch is equal to one-millionth of an inch.


No surface is actually perfectly smooth if measured under the standard of microinch. A machined surface has thousands of minute grooves of various depths.


A tool called a profilometer is used to measure these grooves and assigns a value read as RMS. The higher the RMS the coarser (rougher) the surface. For example, a smooth glazed finish on a cylinder wall may have an RMS of between 5 and 7. Most piston ring manufacturers recommend 25 to 30 RMS for cast iron rings and 20 to 25 RMS for chrome plated rings. Moly rings need a very smooth surface of 10 to 15 RMS. While a proper deck surface for the head gasket to seal may require a surface of 30 RMS or higher.


Every manufacturer provides a deck surface RMS value for the head gasket to seal properly and provide the designed service life. Sadly, most engine builders are not aware of this and do not posses a profilometer to test it.


There is another important reason to deck the block  — many cylinder boring tools known as a boring bar require the deck to be straight since they fixture to it. If the deck is off then the boring job will make the cylinders crooked. Better boring equipment known as a boring center and not a boring bar, references from the main bearing (crankshaft) journals to keep the relationship true.


Cylinder boring/honing


Almost every engine that is being rebuilt requires the cylinders to be bored. This procedure removes metal from the cylinder wall making it larger so any taper or out-of- round condition can be corrected and new, oversize pistons employed. If the engine is fitted with removable cylinder liners, the liner itself is usually replaced instead of boring and honing.


Honing is done with stones and is used to create the final dimension and install the proper crosshatch on the wall surface for piston ring sealing and life. The crosshatch is read in RMS.


Honing alone cannot remove enough material to correct warpage or the loss of concentricity in the bore. With rare exception can a cylinder in a farm engine be only honed and not bored and honed. A poor quality rebuild will only hone the cylinder.


A high-quality shop will use a rigid honing stone on a dedicated automatic-stroke-honing machine. A brush hone for a specific surface finish to meet piston ring manufacturers specifications may follow it up. They should not use a large hand-held drill.


So, take a walk through the shop and have them explain and show you each piece of block equipment before you decide to spend your money there.


Those familiar with race engines will acknowledge in that venue it is common to bore and hone the cylinder with a stress or toque plate in place. This simulates the inherent distortion to the bore when the cylinder head is attached and the bolts tightened. Torque plates are application specific and for the most part and not available for engines that one will find on the farm.


Lifter bores


The bore that holds the valve lifter or tappet is the most overlooked item in block service. The wall needs to be cleaned and measured against the new liters being used. Any rust, glaze, burrs, or high spots will evoke scoring and a possible failure. If the lifter bore is excessively worn it can be honed to the next lifter diameter size that is offered or it can be fitted with a bushing if the wear is excessive.




A high-quality machine shop will most likely wash the block a number of times before it is assembled. At the least it will be washed on disassembly and then before being assembled. The most effective and least time consuming method to accomplish this is with a dedicated machine that operates with heat and pressure. The colloquial term for this is a jet-wash machine. It is possible to do a good job washing the block with special soap and water along with dedicated brushes to clean the cylinders, lifter bore, and oil galleries, but it leaves too much room for error. Cleanliness is essential when assembling an engine.


The reality of it all


You may have a local shop that does not posses all of the latest equipment but is still very capable of doing an excellent job performing engine block work. For example, a trained eye from years of experience will know a proper deck surface finish or cylinder crosshatch though they may not have a profilometer to assign a number to it. The important thing is that the shop recognizes and performs all the steps for proper block service and does it with equipment that is accurate.


The easiest way for the shop to lower its cost to you and increase its profit margin is by eliminating most of the procedures to make the block like new. Many farm engines are only bored and honed with that being called a rebuild. The end result is you have an engine that is not what it is suppose to be and from my experience, is probably a waste of time and money. There is no quick and easy way to do things right.