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Machining 101: Engine blocks

Updated: 09/08/2014 @ 10:28am

An engine assembly can be broken down into its block, rotating assembly and cylinder head. Though the three are all important, the block houses the other two and is paramount to a successful engine rebuild. Thus this, the first installment in Successful Farming’s three part series on hiring an engine machine shop services will explore proper block service procedures. In that regard, the steps that any machine shop needs to perform on the block for any engine being rebuilt must include:

  • 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.

Cleaning all Fastener holes

All threaded holes in a block must be chased or cleaned 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, thread pull it can be corrected with a file, chamfering or counter boring. This step is often overlooked by machine shops and can mean the difference between a gasket lasting the lifetime of the engine or a premature failure.


Burrs and castings 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 the engine is back in service thus causing damage. 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. Often high quality 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 major repair.

Aligning main caps

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 and cooling cycles. This causes the block to warp and distort. The result is a misalignment of the main bearing tunnel.

Engine block 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 an engine will fail soon afterwards.

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