1981 Mustang Engine Information – 255 cubic inch V-8 (4.2 L Windsor V8)

The 255 V8

This engine started out its life as a great 302 ci engine, not so after it was de-tuned and modified, it was really sick after the Ford modifications. After modifications, it ended up being a 255 ci engine. Fortunately for mankind, the engine only saw two years of service and was retired. Thank God the 302ci (5.0L) was brought back in 1982.

Following the second oil crisis in 1979, the 302 cu in (4.9 L) “5.0” engine was dropped in favor of a new 255 cu in (4.2 L) V8 due to its better fuel economy. The 255 was the only V8 offered in 1980 and 1981. Basically a sleeved-down 302, it produced 119 hp, the lowest power ever for a Mustang V8. The 255 was mated only with the three-speed automatic transmission.

Engine Basics

Year	1981
bbl	2 barrel
Cylinders	8
Power (hp)	120 hp
Torque (lb/ft)	194 ft-lbs
Compression	8.8:1
Displacement	255 cu / 4.2 liters
Bore x Stroke	3.680 x 3.126

Engine Specs (255 cid V8)

• Cylinders front to rear: 5-6-7-8
• Cylinders front to rear: 1-2-3-4
• Firing order for most v8 engines except for 302 HO & some 351’s
• Valve arrangement: R. Side I E I E I E I E Front to rear
• Valve arrangement: L. Side E I E I E I E I Front to rear

Cylinder compression pressure	101 PSI minimum
Oil pressure	40-65 @ 2000 rpm
Spark plugs	ASF42C set to 0.050
Cylinder head warp limit	0.003 in 6″ or 0.06 overall
Thermostat starts to open / full open up to 1988	188-195 / 221 degrees
Radiator cap pressure	16
Timing	10 BTDC with spout disconnected
Valve stem to rocker arm clearance non-HO	0.096-0.014
Valve stem to rocker arm clearance – HO	0.123-0.146
Valve stem to rocker arm clearance non-HO allowable	0.071-0.171
Valve stem to rocker arm clearance – HO allowable	0.098-0.198
Timing chain deflection	.5 “
Manifold Vacuum	18 inches at idle
Distributor Rotation Counter Clockwise	Counter Clockwise

Engine Torque Specifications

All torque specifications provided in this table are those recommended by FORD. If you use special bolts, follow the torque specifications provided by the manufacturer. I recommend that you use the 3 step torque technique where you torque down the bolts in three equal steps. Once you have reached the maximum recommended torque setting, go around one more time. Do not torque in a circular pattern. Always torque cross to cross.

Cylinder head bolts except 1993 flanged	55-65, then 65-72
Cylinder head bolts 1993 flanged bolts	25-35,45-55, then an additional 1/4 turn
Upper Intake manifold bolts	12-18
Intake manifold to block bolts	23-25
Oil filter to insert engine block	20-30
Exhaust manifold bolts non- 1993 Cobra	18-24
Exhaust manifold bolts 1993 Cobra	26-32
Valve covers	10-13
Flywheel/drive plate bolts	75-85
Rocker arm fulcrum bolts	18-25
Oil pan to engine block bolts 1979 small bolts	7-9, then10-12
Oil pan to engine block bolts 1979 large bolts	11-13, then 15-17
Oil pan to engine block bolts 1980-1987	9-11
Oil pan to engine block bolts 1988 & up	6-9
Oil pick-up tube to main bearing cap nut	22-32
oil pump mounting bolts	22-32
Crankshaft sprocket bolt	40-45
Crankshaft pulley crank bolt	70-90
Camshaft retaining plate bolt	9-12
Main bearing cap b olts	60-70
Connecting rod cap nuts	19-24
Timing chain cover bolts	12-18
Water pump bolts	12-18
Thermostat housing bolts to 1987	9-12
Thermostat housing bolts 1988 & up	12-18
fan pulley to hub bolts up to 1989	12-18
fan clutch to water pump 1990 & up	15-22
Shift lever attaching bolts	17-25
Auto transmission pan bolts C4 & C5	12-16
Auto transmission pan bolts A4LD	8-10
Auto transmission pan bolts AOD	6-10
Engine mount to block bolts	85
Engine mount to frame nuts & bolts
Carb to intake 2150 2V	14-16
Water pump to block	12-18
Oil pan drain plug	12-16
Pedal support brake bracket	13-25
Spark plugs 1979 – 1987	10-15
Fuel pump to block	19-27
Wheel lug nuts	80-105
Front hub nut	17-25, back off nut, re-torque to 10-15
Bellhousing to engine bolts	28-38
Pressure plate to flywheel	12-24
Driveshaft flange	70-95
clutch self adjuster nut	17-26
Fan pulley to hub	12-18
Fan clutch to water pump	15-23

Valve Specifications

Minimum valve margin width	1/32
Intake & exhaust valve seat angle	45 degrees
Intake & exhaust valve face angle	44 degrees
Intake & exhaust valve seat width	0.060-0.080
Intake & exhaust valve face runout limit	0.002
Intake & exhaust valve seat run-out limit	0.002
Intake stem diameter Standard	0.3416-0.3423
Intake stem diameter – 0.015 oversized	0.3566-0.3573
Intake stem diameter – 0.030 oversized	0.3716-0.3723
valve stem to guide clearance – standard	0.0010-0.0027
valve stem to guide clearance svc. limit	0.005″
Exhaust stem diameter – standard	0.3411-0.3418
Exhaust stem diameter – 0.015 oversized	0.3561-0.3568
Exhaust stem diameter – 0.030 oversized	0.3711-0.3718
Exhaust valve stem to guide clearance std	0.0015-0.0032
Exhaust valve stem to guide clearance-limit	0.0055
Valve spring pressure – intake 1979, 1980 valve open	190 to 212 @ 1.36″
Valve spring pressure – intake 1979, 1980 valve closed	74 to 82 @ 1.78″
Valve spring pressure – intake 1981, 1982 valve open	196 to 214 @1.36
Valve spring pressure – intake 1981, 1982 valve closed	74 to 82 @ 1.78″
Valve spring pressure – exhaust 1979 -1981 valve open	190 to 210 @ 1.20 “
Valve spring pressure – exhaust 1979 -1981 valve closed	76 to 84 @ 1.60″
Valve spring pressure service limit	10 % loss
out of square limit	5/64
Installed height intake 1979 – 1987 non HO	1 43/64 -1 45/64
Installed height exhaust 1979 – 1987	1 37/64 – 1 39/64
Valve lifter diameter standard	0.874-0.8745
Lifter bore clearance	0.0007-0.0027
lifter bore clearance service limit	0.005

Crankshaft Specs

Connecting rod journal diameter	2.1228-2.1236
Connecting rod journal out-of-round / taper limit	0.005″
Connecting rod Bearing oil clearance desired	0.0008-0.0015
Connecting rod Bearing oil clearance allowed	0.0008-0.0024
Connecting rod side clearance std.	0.010-0.020
Connecting rod side clearance allowed	0.023
Main Bearing journal diameter	2.2490-2.2482
Main Bearing journal out_of_round limit	0.0006
Main Bearing journal taper limit	0.0004 per inch
Main Bearing journal runout – Standard	0.002
Main Bearing journal runout – limit	0.005
Main bearing oil clearance 1979-1981 #1 journal	0.0001-0.0015
Main bearing oil clearance 1979-1981 #all other journals	0.0004-0.0015
Main bearing oil clearance 1979-1981 #1 journal, allowable	0.0001-0.0017
Main bearing oil clearance 1979-1981 #all other journals, allowable	0.0004-0.0021
Crankshaft endplay – Standard	0.004-0.008
Crankshaft endplay service limit	0.012
Cylinder bore diameter	3.680-3.6845
Cylinder bore out-of-round limit	0.005
Cylinder bore taper limit	0.01
Piston to bore clearance limit, 1979-1987	0.0018-0.0026
Piston ring side clearance, compression rings -Standard	0.002-0.004
Piston ring side clearance service limit	0.006
Piston ring side clearance oil ring	Snug fit