ASF Machine Built Stage 4 Track Series EJ257 Subaru Shortblock
The ASF Machine Track Series Subaru EJ257 Shortblock is capable of supporting 1050bhp and 9000rpm.*
We utilize brand new cases from Subaru and fit them with Darton Flanged Ductile Iron Sleeves for high boost and elevated cylinder pressures after laser probing and machining counterbores for leak free operation on a Haas 3 axis CNC mill. Counterbores are heat cycled, stabilized and deburred prior to Darton sleeve installation. The flanges are then step decked on our in house Rottler and fitted with a receiver groove and a copper or stainless o-ring (dependent on customer useage application) to keep the head gasket in place.
Cylinder bores are prepped on ASF's in-house industry standard Sunnen CV616 power hone while clamped with the appropriate torque plate to induce deflection similar to that of a cylinder head being fastened down to it with ARP fasteners. Piston to cylinder wall clearance dependent on customer application.
The factory Subaru/FHI Nitrided 79mm stroke steel EJ257 crankshaft is dynamically spin balanced on our Sunnen Balancer to less than .5 grams, force over plane.
The nitride hardened Subaru crankshaft glides on King XPG Race Bearings for Bull's-Eye tolerances, blueprinted via Sunnen dial bore gauge, ASF uses actual physical clearances down to the nearest .0001"; NOT how much it crushes wax.
Brian Crower Pro I Beam Connecting Rods connect the Diamond 2618 Forged Pistons to the FHI STi Nitrided crankshaft. Optional compression ratios are 8:1, 9:1 and 10:1 based on either a 51 EJ255 or 56cc EJ257 chamber. Total Seal Advanced Profile rings are utilized with a stainless top ring, napier 2nd ring and STD tension oil rings to keep your boxer's combustion chambers clean.
Head fastener sizes are optional, on most street cars we prefer to choose a better alloy such as ARP's Inconel 625 or L19 opposed to weakening the case with larger fasteners. For those that need every bit of horsepower we do offer a 1/2 head stud option but we do so utilizing custom ASF Machine steel threaded inserts that will not yield the way aluminum does when head fastener torque measurements start creeping above 120 ft lbs.
The case is sealed and clamped by Subaru fasteners, Subaru o-rings and Subaru seals.
Handbuilt by ASF to order, please allow 3-4 weeks for delivery.
*When assembled, installed & calibrated by a qualified technician; using the appropriate supporting parts including but not limited to: fuel system, turbocharger, engine management and oiling system.