G-Body swap guide posted by Richard Durost on MTS board
- Posted by Richard Durost on 9/3/2002, 2:46 pm
I haven't done this swap yet, but the information has been validated
by somebody who has.
GM G-body Cadillac Engine Swap Guide
('78-'88 Monte Carlo, Malibu, El Camino, Grand Prix, Le Mans,
Cutlass, Regal, etc.)
You can use a Cadillac radiator. Use the radiator saddles and top
brackets from the Cadillac. The Cadillac radiator has a heater hose
fitting at the top of the passenger-side tank. Strap the hose to the
battery tray to keep it away from the fan.
If your Cad engine has a heater hose fitting on the rear of the
passenger-side head, remove and/or plug it and put a new fitting in
the boss on the outside, below the thermostat housing. Pre-'71
engines came with this configuration from the factory. This will give
you more firewall clearance.
In order to fit the Cad radiator in the G-body chassis, you'll need
to trim the core supports to clear the tanks. Reinforce the bottom of
the support with 3/16" x 1 1/2" flat stock after trimming it. Make
sure the radiator only makes contact with rubber all the way around.
A pair of Chevy Citation electric fans will work. If you use an
engine-mounted fan, use a shroud modified for clearance.
A stock Cadillac upper hose will work, but you may have to use a
universal ribbed hose on the bottom. Make sure the idler arm bolts
don't rub against the lower hose. You can cut the bolts flush with
the nuts and grind them smooth to make additional clearance.
If you use factory iron exhaust manifolds, on the passenger side
you'll have to eliminate the heat riser valve, if any, and weld the
header pipe to the exhaust flange at an angle in order to clear the
frame. Maintain a minimum 3/8" clearance. On the driver's side,
you'll need to grind the bottom outside edge of the flange to within
1/8" of the bottom stud in order to clear the crossmember.
The most common rear end ratios are 2.41 or 2.73:1. The stock rear
will survive behind a mild motor if you stick with street tires and
it's in decent shape to begin with.
You can use the Cadillac transmission yoke on the G-body drive shaft.
The U-joint bearing sizes are the same, but you'll have to cut out
the ball that's there to center the Cadillac's constant-velocity
joint assembly. Shorten the driveshaft to 42 15/16" (measure your
installation to be absolutely sure), measured between the U-joint
centerlines, to accommodate the Cadillac long-shaft transmission. You
can use a short-shaft transmission, but it provides no functional
advantage, and the driveshaft will have to be shortened anyway. If
the stock driveshaft is too small for your application, you can start
with the 2 3/4" unit from a '78-'82 Camaro or Firebird.
Use the Cadillac power steering pump. If you're eliminating the air
conditioning, you can run a belt straight from the pump to the crank.
A Gates 11A1195 should be close. The stock Cadillac power steering
hoses will usually work. If your steering box has metric fittings,
you can use 3" sections of the hard lines from the steering box as
adapters. The low-pressure return hose can be clamped to the stub of
tubing from the box. Trim the fitting off the high-pressure hose and
join it to the cut line from the box with a 3/8" compression fitting.
To delete the AIR pump, if you can't locate a pulley set from a '70
engine, use a complete pulley set from a '77-'80 425 engine. These
used no AIR pump either. You can seal the air tube fittings on the
front of the heads with cup plugs, or tap them for pipe plugs that
you KNOW won't blow out.
Air conditioning can be retained with minor modifications to the
heater/AC box, with reduced access to the #7 spark plug. You'll have
to cut a clearance well in the outside of the housing to clear the
valve cover. Make sure you don't cut into the A/C gear inside. This
is better done with the top half of the housing removed. Heater-only
cars will require a smaller clearance hole in the housing. Thin sheet
aluminum or fiberglass can be used to create a clearance pocket in
the housing, making sure that you don't run into the squirrel cage
Maintain a half inch clearance between the driver's side exhaust
manifold and the steering column. You can shift over the lower end of
the column by redrilling the holes that attach the lower column
bracket to the firewall inside the car. If your steering coupling has
a grease fitting, you'll need to cut or remove it, or replace the
coupling with one without the fitting.
If you want to run headers, your best bet is to start with big block
Chevy shorty headers designed for a Chevelle (A body) and add Caddy
flanges, available from MTS, Sanderson, or Headers by Ed. The
Sanderson headers require tucking up the #1 cylinder tube and
flattening the bottom of others on the passenger side, and extensive
mods to the driver's side to clear the steering shaft. Other brands
might provide better or worse clearance. An alternative here would be
to use street-rod-technology U-joints and a swivel shaft support to
move the shaft away from the engine. You will need a U-joint at the
steering box, another close to where the steering shaft comes through
the firewall, and a third in the middle next to the swiveling shaft
support (looks like a heim joint) bracketed to the frame.
Create additional exhaust clearance at the firewall by cutting flush
the five sheetmetal screws protruding from the firewall next to the
steering shaft. Flatten the passenger side of the lip around the
steering shaft hole in the firewall and dimple the rest of the
firewall in this area approximately 1/8".
The column shift linkage won't fit. Remove it, being sure to fix the
lever on the column in a "steering unlocked" position. Use a floor
The Cadillac valve cover will reportedly clear a factory power
You may need to cross the fuel line over from the passenger to the
driver's side of the chassis. If so, use a 4' section of hard line
attached to the existing line with a compression fitting. Route the
line in the depression on the back of the crossmember, securing it
with insulated clamps screwed to the frame. Maintain the hard line to
within 8" of the pump, and add a section of rubber hose to allow for
Use a '68-'76 Eldorado oil pan, pickup tube, dipstick and dipstick
tube. The rear-sump pan requires a different dipstick, and the
dipstick tube needs to be inserted in the rear hole in the block.
Knock out the ball bearing plugging this hole and transfer it to the
front hole. After tapping the dipstick tube into its hole, gently
bend the section of the tube inside the pan down toward the sump,
keeping it away from moving parts. If you don't do this, the oil will
read too low on the stick. The front oil pan drain plug should be cut
out and replaced with a flat plate. This plug interferes with the
crossmember and it will be inaccessible anyway. The notch in front of
the sump that's used for front-drive half-shaft clearance prevents
the oil in the small front sump from draining back, so either remove
it or fabricate a drainback channel through it. Heli-arc or TIG
welding is best, but in any case be careful to avoid cracking or
warping from excess heat.
The oil pump from the '77-'84 368 and 425 engines (Melling part #
M58G) tucks the filter much closer to the block for clearance. You
can use a remote filter setup, but the consensus is that this is to
be avoided because it reduces flow and takes longer to build oil
pressure on startup. You can also reportedly space the sway bar down
2" and retain the 472/500 oil pump.
You will need an aftermarket transmission crossmember, such as the
one offered by MTS, or you can fabricate one. When placing the
transmission mount, make sure the engine centerline is parallel with
the vehicle centerline. The engine may end up shifted slightly to the
passenger side of the vehicle depending on how you fabricate the
engine mounts. That's fine as long as the trans mount is shifted a
like amount to keep the engine's centerline parallel with the car's.
If the engine is angled in the frame, you will get serious driveline
vibration due to improper alignment of the U-joint(s).