Modern AMC sixes/years:
199 - 3.75" bore, 3.00" stroke
"short deck", 6.125" rod, 1966-1970
232 - 3.75" bore, 3.50" stroke
"short deck", 5.875" rod, 1964-1970
"tall deck", 6.125" rod, 1971-1979
258 - 3.750" bore, 3.895" stroke
"tall deck", 5.875" rod, 1971-89
242 (4.0L) - 3.875" bore, 3.440" stroke
(note that most pubs round 3.875" to 3.88")
"tall deck", 6.125" rod, 1987-2006
Only two deck heights. Tall deck is 9.528-9.534". There's a 1/8" difference between short and tall (half the rod length difference). Short should be 9.278-9.284". Tall is from a 74-76 AMC factory service manuals, prior to 74 deck height wasn't printed. Deck height changes slightly over the years - 77-82 states 9.487-9.493", 93 Jeep factory manual states 9.429-9.435". Don't know if the blocks actually vary over the years to alter compression or there are errors in the manuals.
Note that two different bell housing sizes were used. 64-71 used the "small" diameter bell as used on prior AMC/Nash sixes. Starter is on the left hand side. 72-06 used a larger diameter bell, same bolt pattern as the AMC V-8. Six cylinder bells are roughly 6" deep, V-8 bells are roughly 8" deep. Use corresponding transmission with bell to swap. 71 and earlier also use a Borg Warner auto which uses the same crank flange as stick shift models. 72+ use a custom made Chrysler Torque Flite auto trans (AMC bell housing pattern is the only difference from Chrylser models). The stick shift flywheels interchange, but the Chrsyler flexplate will only bolt to 72+ cranks. The crank flange has an indentation that aligns the flexplate.
Heads varied, but all mid 1980 and prior engines used a cast iron intake and exhaust that bolted together and 1/2" head bolts. Mid 1980 and later used an aluminum intake separate from the cast iron exhaust manifold and 7/16" head bolts through 1986. 1987 moved to a cast aluminum valve cover and back to 1/2" head bolts.
Cranks are 12 counterweight through mid 1980 (corresponds with head bolt and manifold changes), four counterweight after. There doesn't appear to be any strength differences in the cranks, though the older crank is heavier. The only noted difference is the lighter crank can spin up slightly faster (we're talking milliseconds here!) and the heavier crank has more stall resistance due to greater rotating mass. Rock crawlers make good use of the heavy crank, but breakage reports are about the same for both.
There's no way to look at any engine and tell what size it is except for the 4.0L -- it has no provision for a mechanical fuel pump. There is a code stamped on a flat on the block near the block/head parting line between cylinders #2 and #3 on the right side (spark plug side). The fourth character is the engine code.
A - 199/1V, 70
A - 258/1V, 71-79
B - 258/1V low compression (Jeep only), 71-74
C - 258/2V, 76-89
E - 232/1V, 70-79
F - 232/1V low compression (Jeep only), 71-74
G - 232/2V, 70-74
J - 199/1V, 66-69
L - 232 (1V or 2V), 64-69
MX - 242 EFI (4.0L -- Jeep only), 87-06
note: may have just "M"
The first character of the engine code is the year -- that's how you tell the 70 199 and 71-79 258 apart! From 1980 on the first character is the last digit of the year, and zero is used. Before then it's more complicated! The code started in 1959 with "1" used for 1959. Just count forward through 9, then skip zero and start over. So "1" could be 1959, 1968, or 1977. Knowing what engine sizes were built during a year and some engine details is necessary to decipher the code correctly. So code 108A15 could be a 68 199 or a 77 258. Check starter position. 68 would be on the left, 77 on the right. Clear as a bell, right??
Incidentally, the 2nd and 3rd characters are the month the engine was built, and the last two are the day of the month.
Last edited by farna; 08/15/08 12:59 PM. Reason: added block deck height
