A-body swap guide by Richard Durost
- Richard posted this on MTS message board. Thank you Richard.
GM A-Body Swap Guide
Posted by Richard Durost on 9/3/2002, 2:36 pm
Here's the one I developed based on my own experience. There are
plenty of holes, so please pitch in with more details.
I'd be especially interested in getting the detailed trans mount
information required to put the Cad trans in an A body.
The section on heads isn't strictly swap-related, but it was in
response to a question on the table at the time.
I put a 500 in my '69 LeMans, which is a GM "A" body. If your
car already has a Turbo 400 trans that'll make life easier. Cadillac
uses the Buick/Olds/Pontiac bellhousing pattern, so you'll need an
adapter to use a Chevy trans. An unmodified Turbo 350 trans won't
last behind 500+ lbs.-ft. of torque if you ever get traction, while
the 400 (which all Cads came with) is fine. While the Caddy trans is
a 400, it's got an extra-long tailshaft, so you'll have to shorten
the driveshaft if you want to use one in an A-body, as well as rework
the trans mount.
You'll need an Eldorado rear-sump oil pan (with matching pickup,
dipstick and tube) if your engine doesn't come with one. If you can't
extract the dipstick intact from the oil pan donor (very likely)
you can order a new one from Maximum Torque Specialties, at 414-740-
1118. Also get MTS's catalog and tech guide. Tons of useful
information. You should also get a copy of Doc Frohmader's book "Big
Inch Cadillac" for $20 or so
http://www.ridetech.com/DOCBOOK/BIGINCH.HTM). Best purchase you'll
EVER make if you want to build up a Cad motor.
Take the Eldorado pan and cut out the front drain plug and replace it
with a flat plate for crossmember clearance. Once you've got the
Eldorado pan, the next problem is that the Cad motor mounts are way
too far forward to line up with the A-body crossmember. I unbolted
the existing motor mount pedestals from my crossmember and replaced
them with flat plates about 7"x 12"x1/2" thick (3/8" probably would
have been fine), extending forward. There are two basic types of '68-
'75 Caddy motor mounts, both of which use a single large peg bolt
that goes through the frame crossmember. The rear-drive style has a
bolt that sticks straight out from the side of the block (45 degree
angle to the ground), while the Eldorado has a bolt that angles down
more. For an A-body you need the Eldorado mount.
The adapter plates require a slot for the single big "peg" bolt on
the mount, and a round hole for the locating bump. Shorten the ears
on the motor mounts that bolt to the block and redrill the holes 1/2"
closer to the body of the mount. This compensates for the thickness
of the plates.
Replace the (external) Cad oil pump with one from a later 368/425
(Melling # M-58G) to tuck the filter closer to the block for
clearance. This will allow you to use a Purolater 30005 full-size
filter (it's an AMC part number) without hitting anything.
Now that you've got the motor in the chassis, the next problem is
figuring out the driver's-side exhaust manifold (the passenger side
is fine). Start by relocating the lower end of the steering column
1/2" outboard for clearance. The column passes through a sheet metal
plate that is in turn bolted to the firewall. Just redrill the
firewall holes 1/2" farther out. You'll need to cut a new foam gasket
to seal the firewall.
The rear-drive driver's-side iron manifold nowhere near fits. The
Eldorado unit dumps directly out to the side, which isn't close
either. I used a HIGHLY modified set of Sanderson shorty headers
designed for a big Chev in a Chevelle but it was a lot of work. In
the future if I wanted to use cast manifolds, I'd cut off and
relocate the outlet from the Eldorado part. That requires finding
somebody who can properly weld or braze cast iron, of course. There's
been some discussion here about another brand of shorty headers,
maybe Blackjack, that sound like they might need less massaging to
work. The Caddy flanges are available from MTS.
Using the motor mount scheme above, the fan lined up perfectly with
the LeMans radiator and fan shroud. If your heater hose exits the
passenger-side head at the rear, relocate it by drilling and
tapping the unmachined flat boss on the outboard side of the housing
below the thermostat in the Caddy engine block.
Replace the intake with the excellent Edelbrock unit. A Holley 3310
750 cfm with a 14" low-rise open-element air cleaner, and a 3"
element, fits under the hood of my LeMans.
One thing I don't really like about my swap is the poor ground
clearance below the drain plug on the bottom of the Eldorado pan.
I've bottomed out a couple of times. The pan really needs to be
shortened and widened. I just replaced the bump stops on my control
arms with traction bar snubbers, which are taller, to limit downward
travel and keep the plug off the ground, but it still makes me
nervous. If you work out something better, please let me know!
I used a Cadillac HEI distributor in my car, but MSD has a Caddy
distributor if you feel like spending some money. The MSD has a
smaller diameter cap.
The MTS Tech Guide and the Big Inch Cad book go into this in great
detail, but essentially there are two types of 472/500 heads--a 76cc
unit used from '68-'73, and a '74-'76 120cc unit. The piston used
under each of the two types was different. Putting 76cc heads on
your '74 with the small-dish pistons would result in something like
13:1 compression, which will destroy the stock cast pistons sooner
rather than later even with race gas. The 76cc heads flow better,
particularly on the exhaust side, and have a combustion chamber with
superior quench properties. The '70 unit is considered marginally
superior because it lacks the cast-in and drilled passages for the
air injection, and thus weighs a few pounds less.
The Caddy rear end in an odd dog. It's not like any other GM part. No
other gears interchange. Some posi units were offered as an option
and in commercial chassis (hearse, ambulance) applications, but I've
never seen one. It's too wide for an A-body, and the mounting points
are different anyway. I put a 12-bolt non-posi from a Monte Carlo in
my LeMans. The 2.73 gears are ideal for street use with a non-
overdrive trans. I've heard that even hard-core Caddy racers don't
usually go over a 3.73. My LeMans came with 3.55 gears, which were
WAY too much. The car actually feels a lot faster with the 2.73
because they take advantage of the Caddy's torque.