How To Measure Subaru FA/FB Engine Cylinders

How To Measure Subaru FA/FB Engine Cylinders

This procedure is taken from the Subaru service manual and will be used as a reference only. We always recommend that a Subaru specialist carries out work.

Tools Needed
Precision Straight-Edge and Feeler Blade (Gauge) Set
The photos below show the main tools required to perform the block measuring procedures accurately. (Although this photo shows a cylinder head gasket surface being checked for straightness, a similar procedure is used when checking the block deck surface.

Telescoping ”T’ gauge Set & 4″ Micrometer
More economical than the Dial Bore Gauge Set shown below, “T” gauges may be used in conjunction with a 4” micrometer to accurately measure both the out-of-round and taper (referred to in the Service Manual as “Cylindricality”) dimensions of the cylinder bores.

Or a Dial Bore Gauge
A Dial Bore gauge is the easiest and quickest way to measure cylinder bores accurately although significantly more expensive to purchase than a set of “T” gauges.

STEP 1 – Before attempting any measurements, perform a thorough cleaning and careful visual inspection of the block. Look for any cracks, scoring, or other damage that would make it unsuitable for reuse. If your inspection reveals damage which renders the block unsuitable for reuse, repair the vehicle with the applicable remanufactured or replacement short block assembly.

If repairs appear as they will be cost-effective (less than 90% of replacement cost), continue with the following measurement procedures. These measurements should be performed at room temperature (about 20c degrees). If the ambient temperature is higher, it will be necessary to chill the block evenly then perform your measurements before it warms back up significantly. A non-contact thermometer can be used to monitor the block temperature during this process. Any questionable measurements should be repeated after re-chilling if necessary. Similarly, if the block is colder than this temperature, allow it warm evenly to this temperature before performing your measurements. The clearance specifications provided in this bulletin are the same between FA and FB engines, turbo and NA models. Individual component size dimensions will vary based on application and the information stamped into the block at the areas shown in the illustration below.

STEP 2 – Measure the head gasket (deck) surface for warpage. After CAREFULLY cleaning the head gasket surfaces, inspect the top sealing surfaces of the cylinder liners closely for evidence of compression leakage and/or coolant loss. Using a precision straight-edge and feeler blades, check the deck surfaces for warpage in all 8 directions as shown in the illustration on the below. If the warpage measures less than .25mm (.001”), the deck surface is within specifications. If the measurement exceeds the specification, the block will need to be split to in order to determine the block (deck) height measurements. Block height is measured from the mating surface of the block halves to the head gasket /deck surface. Although the standard height is 205.0mm (8.071”), the minimum height specification is 204.9mm (8.067”), a difference of .1016mm (.004”). Regardless of the height dimension, if the warpage is more than .1016mm (.004”), the block is no longer serviceable. If the deck surface is determined to be within specifications, proceed to STEP 3.

STEP 3 – With the block split and at a temperature of 68 F, measure the cylinder bores for excess taper (cylindricality) and out-of-roundness. Measure the cylinder bores in both the piston pin (A) and thrust (B) directions and at the 4 specified heights as per the illustrations below

Calculate the amount of taper or “cylindricality” (C) at each measurement height shown in the illustration above using the formula below:
IMPORTANT NOTE: The “limit” specifications provided are radius dimensions (1/2 of the diameter).
C= [(L) – (S)] ÷ 2
(L) = the largest value measured for cylinder inner diameter
(S) = the smallest value measured for cylinder inner diameter
Limit: .03 mm (.0012”)

Calculate the amount of out-of-roundness (R) at each measurement height shown in the illustration above using the formula below:
R= [(A) – (B)] ÷ 2
(A)= the larger value (difference) between the measurement results taken in the wrist pin direction and the thrust direction of the cylinder bore as shown in the illustration above.
(B) = the smaller value (difference) between the measurement results taken in the wrist pin direction and the thrust direction of the cylinder bore
Limit: .03 mm (.0012”)
If any results are beyond specification, replace the shortblock assembly.

STEP 4 – Measure the piston to cylinder wall clearance.
Using a micrometer, measure the piston diameter across the skirt areas (perpendicular to the wrist pin) as shown in the illustration below

Subtract each piston’s diameter measurement from the corresponding cylinder’s largest bore size measurement to determine the clearance value. The specification for maximum allowable piston to cylinder wall clearance is .01 – .03mm (.00039 – .00118”). At this point, if any of the pistons are found to be out of specification (or have excessive clearance) all 4 must be replaced and the cylinder block bored and finish-honed as required to achieve the specified clearance

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