[Amc-list] AMC six specs

[Frank Swygert <farna@xxxxxxx>]

I just posted this on the In-Liners board in reference to how to 
identify AMC six displacement. Wanted to pass it on here since we had a 
specs discussion a day or two ago. I looked up the deck height in the 
TSMs, which may not be correct!
--------------------------------
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.

-- 
Frank Swygert
Publisher, "American Motors Cars" 
Magazine (AMC)
For all AMC enthusiasts
http://farna.home.att.net/AMC.html
(free download available!)

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