As might be expected, the cylinder heads in a Fiero are common to many GM engines. There were many variations so care must be exercised in getting the correct heads.
'84-86 2.5L XJS:
The designation "XJS" is stamped on the front face or right rear of the block. The intake valve has 0.3137" diameter stem. The exhaust valve has 0.3132" diameter stem. The push rod length is 8.3".Pontiac 6000 '84-86 2.5L Identification XJS on block Chevy Celebrity '84-86 2.5L Identification XJS on block Buick Century '84-86 2.5L Identification XJS on block Olds Ciera '84-86 2.5L Identification XJS on block Chevy Citation '84-85 2.5L Identification XJS on block Olds Omega '84 2.5L Identification XJS on block Pontiac Phoenix '84 2.5L Identification XJS on block Buick Skylark '84-85 2.5L Identification XJS on block
'84-86 2.5L XJS:
The designation "XJS" is stamped on the front face or right rear of the block. The following non-XJS heads will interchange if the coolant outlet and positions are switched and the push rods are included. The heads below must have identification number 10020699 or 1552.Pontiac 6000 '82-83 2.5L Chevy Celebrity '82-83 2.5L Buick Century '82-83 2.5L Olds Ciera '82-83 2.5L Chevy Citation '82-83 2.5L Olds Omega '82-83 2.5L Pontiac Phoenix '82-83 2.5L Buick Skylark '82-83 2.5L
'84-86 2.5L XJS:
The designation "XJS" is stamped on the front face or right rear of the block. The following heads will interchange if the coolant outlet and positions are switched and the push rods are included. The intake valves have 0.3422" diameter stems. The exhaust valves have 0.3419" diameter stems. The push rod length is 8.4". The heads below must have identification number 10022373, 10027767, 10030838, 10030839, 12360333, 1373 or 1767.Pontiac 6000 '84 2.5L w/o XJS designation on block Camaro '84 2.5L Chevy Celebrity '84 2.5L w/o XJS designation on block Buick Century '84 2.5L w/o XJS designation on block Olds Ciera '84 2.5L w/o XJS designation on block Chevy Citation '84 2.5L w/o XJS designation on block Firebird '84 2.5L Olds Omega '84 2.5L w/o XJS designation on block Pontiac Phoenix '84 2.5L w/o XJS designation on block Buick Skylark '84 2.5L w/o XJS designation on block
'84-86 2.5L XJS:
The designation "XJS" is stamped on the front face or right rear of the block. The following non-XJS heads will interchange if the coolant outlet and positions are switched and the push rods are included. The intake valves have 0.3422" diameter stems. The exhaust valves have 0.3419" diameter stems. The push rod length is 8.4". The heads below must have identification number 10037282, 10038231 or 12360335.Pontiac 6000 '85-86 2.5L w/o XJS designation on block Chevy Celebrity '85-86 2.5L w/o XJS designation on block Buick Century '85-86 2.5L w/o XJS designation on block Olds Ciera '85-86 2.5L w/o XJS designation on block Chevy Citation '85 2.5L w/o XJS designation on block Buick Skylark '85 2.5L w/o XJS designation on block
'84-86 2.5L w/o XJS:
When present, the designation "XJS" is stamped on the front face or right rear of the block. The following heads will interchange if the coolant outlet and positions are switched and the push rods are included. The intake valves have 0.3422" diameter stems. The exhaust valves have 0.3419" diameter stems. The push rod length is 8.4". The heads below must have identification number 10022373, 10027767, 10030838, 10030839, 12360333, 1373 or 1767.Pontiac 6000 '84 2.5L w/o XJS designation on block Camaro '84 2.5L Chevy Celebrity '84 2.5L w/o XJS designation on block Buick Century '84 2.5L w/o XJS designation on block Olds Ciera '84 2.5L w/o XJS designation on block Chevy Citation '84 2.5L w/o XJS designation on block Firebird '84 2.5L Olds Omega '84 2.5L w/o XJS designation on block Pontiac Phoenix '84 2.5L w/o XJS designation on block Buick Skylark '84 2.5L w/o XJS designation on block
'85-86 2.5L w/o XJS:
When present, the designation "XJS" is stamped on the front face or right rear of the block. The following non-XJS heads will interchange if the coolant outlet and positions are switched and the push rods are included. The intake valves have 0.3422" diameter stems. The exhaust valves have 0.3419" diameter stems. The push rod length is 8.4". The heads below must have identification number 10037282, 10038231 or 12360335.Pontiac 6000 '85-86 2.5L w/o XJS designation on block Chevy Celebrity '85-86 2.5L w/o XJS designation on block Buick Century '85-86 2.5L w/o XJS designation on block Olds Ciera '85-86 2.5L w/o XJS designation on block Chevy Citation '85 2.5L w/o XJS designation on block Buick Skylark '85 2.5L w/o XJS designation on block
'87-88 2.5L:
The heads below will interchange only if they have identification number 10049100, 10117769, or 12360340.Pontiac 6000 '87-88 2.5L Chevy Celebrity '87-88 2.5L Buick Century '87-88 2.5L Olds Ciera '87-88 2.5L
The following information was posted to Pennock's Fiero Forum by "Ray", Houston, TX:
Better flowing heads for an '87-88 Fiero 2.5L are found on the '90-93 S-10, S-15 and Blazer. Using these heads requires transferring the intake manifold with the heads, but all other parts from the Fiero will bolt on directly. Changing the heads/intake provides a bit more horsepower and better mileage. The exhaust ports on the newer heads are smaller than those on the Fiero but are more efficient. The combustion chambers are different and provide better fuel/air mixing and better cooling.
'87-88 2.5L:
The heads below will interchange if the coolant outlet and positions are switched. Heads must have identification number 10054501, 1006630, 10117770 or 12360340.Pontiac Astro '87-88 2.5L Blazer S10/GM Jimmy S15 '87-88 2.5L S10/S15/GM Sonoma '87-88 2.5L Pontiac Safari (GMC) '87-88 2.5L
'85-88 2.8L:
The following are cast iron heads and must have identification number 14054879 or 1405884. The compression ratio is 8.9 to 1.Pontiac 6000 '83-84 2.8L H.O. VIN "Z" (8th digit) Pontiac 6000 '85-86 2.8L EFI Blazer S10/GM Jimmy S15 '86-89 2.8L Camaro '85-89 2.8L Camaro '90-92 3.1L Camaro '93-95 3.4L Cavalier '85-86 2.8L Chevy Celebrity '84 2.8L VIN "Z" (8th digit) Chevy Celebrity '85-86 2.8L EFI Buick Century '84 2.8L VIN "Z" (8th digit) Buick Century '85 2.8L EFI Olds Ciera '85 2.8L EFI Cadillac Cimarron '85-86 2.8L Chevy Citation '83-84 2.8L H.O. VIN "Z" (8th digit) Chevy Citation '85 2.8L EFI Firebird '83-84 2.8L H.O. VIN "L" (8th digit) Firebird '85-89 2.8L Firebird '90-92 3.1L Firebird '93-95 3.4L Olds Firenza '85-86 2.8L Chevy Lumina Van '90-95 3.1L Olds Omega '83-84 2.8L H.O. VIN "Z" (8th digit) Pontiac Phoenix '83-84 2.8L H.O. VIN "Z" (8th digit) Pickup, Isuzu '91-94 3.1L TBI Isuzu Rodeo '91-92 3.1L S10/S15/GM Sonoma '86-93 2.8L Olds Silhouette '90-95 3.1L Buick Skylark '83-84 2.8L H.O. VIN "Z" (8th digit) Buick Skylark '85 2.8L EFI Pontiac Trans Sport '90-95 3.1L Isuzu Trooper '89-91 2.8L
Fiero owner Steve Steel researched the 2.8L cylinder heads trying to determine the difference represented by the two identification numbers. Here is what he found out from New Jersey Cylinder Head Co.:
"The two casting numbers (879, 884) are identical heads. The manufactures change the casting numbers every once in a while even though they are producing the same castings. Those casting numbers are 100% compatible with each other along with casting numbers 227/327. They will fit a 2.8L and a 3.1L GM engine from '85-94 that uses that style cast iron head."