Archive: July 2008

During our extensive Subaru engine development program, the one thing we noticed is that the factory Subaru engine block's cylinder bore dimensions are very inconsistent right out of the box. This is fine with a factory cast piston because there is very minimal expansion in a cast piston. However, this can be a huge problem with forged pistons that are designed to expand when hot. If a forged piston is designed to have .003" of piston to cylinder wall clearance, what happens when the bore is oval by .002"? If you were to drop in a forged piston without honing the EJ257 cylinder block, the result is an eventual catastrophe.

First off, understand that the EJ257 cylinder block is more of a "case" than it is a block. Because it is virually hollow and its walls are quite thin, it is subject to severe distortion when head and block fasteners are torqued. Here are some tips for honing a Subaru engine block:

1. 1. Torque the case halves together with the fasteners you will assemble the engine with.



2. 2. Always bore and hone the cylinders with a torque plate and dowels in place. You should always use a head gasket (used ok) of the same make and model as your new gasket and also use the studs or bolts that you will be using in your final assembly. Steps one and two will allow the cylinders to be honed while "distorted" in its final assembled state.



3. 3. Always follow the piston manufacturer's recommendation for piston to cylinder wall clearance. If in doubt or if the engine will be used for severe duty, add .0005-.001" to the recommendation. It is generally safer to have more clearance than less with forged pistons.



4. 4. Always follow the piston ring suppliers recommendations for bore finish and crosshatch angle. Same with the piston ring end gaps.



5. 5. While a cylinder bore's perfection comes from the machinist's experience with your particular engine block, it is good practice to let the cylinder block cool before finish honing if there was a lot of material removed from the bore.

Follow these 5 simple tips and you should have a good running EJ257 if the engine is assembled correctly. If you haven't picked it up already, you should know that a "drop in" forged piston just will not last very long in a Subaru engine block without custom boring or honing from an experienced machinist.


Here is a factory EJ257 block straight out of Subaru's packaging with a custom designed Cosworth deck plate (available winter 2008) with the new Cosworth H-11 tool steel head studs.


Here is a measurement of the cylinder on axis with the crankshaft. You can see that it is .0011" from zero (the dial bore gauge was "zeroed" using the guage point on the piston).


Here is the thrust side of the cylinder bore measuring in at .0012" from zero. The cylinder bore is oval by .0023"! A "drop in" forged piston that called for a .003" piston to cylinder wall clearance definitely would not last very long in this cylinder block.