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Conversion >> SM420 >> Swapping a SM420 into a YJ
Swapping a SM420 into a YJ
by David Moulton
When my 4 cyl YJ's transmission started to grind going into 3rd
gear I started to look for alternatives. The AX-5 that came in my Jeep
and most 4 cyl YJ's is not a bad transmission for the stock Jeep, but
when you swap on larger tires and start wheeling frequently, it is
only a matter of time.
Since my Jeep is spending more time off road I wanted a
transmission that could take abuse and enhance my Jeeps performance.
After some research I decided on the GM SM420 transmission. This
transmission was used on trucks up to 2 1/2 tons so it will stand up
to anything my 4 cyl can throw at it.
SM420 Side view. Photo by David Moulton.
Besides the fact this transmission can take the off road abuse there
are some other benefits. The first gear ratio is 7.05:1 vs the stock
AX-5 first gear ratio of 3.93:1. This greatly increases off road
performance when rock crawling, or tackling steep climbs and descents.
This gives a much greater level of control, and eases the level of
abuse to other components. The stock crawl ratio is 43:1 and the SM420
increases this to 78:1. A very significant increase!
With the clutch components used, you can kiss the troublesome
internal slave cylinder good bye forever.
The overall length of the trans is shorter than the stock combo,
the rear driveline can be lengthened about 3/4" of an inch. Every bit
helps when dealing with a short wheelbase vehicle like a Jeep. Most
other transmissions that offer the same benefits are longer than the
SM420.
With this transmission, if you decide to swap in Chevy power at a
later date you will be ready, only needing a bell housing and clutch
component change depending on what engine you choose.
With the Advance Adapter kit , it is possible to switch to a Dana
300 transfercase just by switching the spud shaft that comes with the
kit. Of course that would only be if switching the front axle to a
passenger side pumpkin, as in a conversion to Scout axles.
The street performance with the four cylinder will suffer a little,
you seldom use 1st gear on the street, and there is a big gap between
3rd and 4th. You lose fifth gear altogether, but if you are running
big tires then it's not used much anyway. First and reverse are non
synchronized, you will have to be stopped for these to engage
smoothly. Of course reverse isn't synchronized on the AX-5 either.
If you know anyone who has a truck with a truck transmission like
this you might want to drive it first to see how it feels to you. The
SM420 is an old truck transmission and it doesn't shift as lightly or
as quickly as the stock transmission. Definitely not for drag racers.
It is louder than the stock tranny, especially first thru third.
Fourth is about the same as the original transmission.
There is no provision for reverse lights on most SM420's so you
will have to wire in a switch for this purpose.
Hard parts, like gears and shafts are no longer available from GM,
but synchros, bearings and gaskets can be had from 4x4 shops such as
Hicks 4x4, or Border Parts. However, the transmission themselves are
cheap and if a gear or shaft is needed, a whole trans can be obtained
for less than $100 US in most salvage yards.
SM420 passenger side.
Photo by David Moulton.
The SM420 was used in Chev and GM 1/2 to 2 1/2 ton trucks from 1947
thru 1967. There are some differences you might want to look for. I
don't know the cutoff years but later SM420's have a front bearing
retainer with an oil seal on the input shaft, earlier units have an
oil deflector that can't always do the job when being turned at the
revolutions four cylinders are capable of. This results in a small
amount of oil leaking out and while usually not causing any problem,
it smells, and could find it's way onto the clutch, causing slipping
problems.
Some SM420's have a tubular shifter and some have a forged shifter,
the forged shifter is easier to bend, it can be heated with a torch
and pulled into the position you want, whereas the tubular lever tends
to collapse when bending. The shifter will have to be bent to give
your hand clearance at the dash when shifting to first and third.
Very late SM420s have a reverse light switch.
Modification to Chevy bell housing for installation on 2.5L AMC I4
engine. Photo by David Moulton.
The bell housing used for this swap is the bell housing from an 1986
Chevy S-10 with a 2.8 V6 and a five speed. The hydraulic slave
cylinder, pressure plate, throwout bearing, clutch release fork, and
fork pivot ball from the S-10 are also used. The clutch disk to use is
from a 83 Jeep CJ-7 with a 4 cyl engine. The original pilot bushing
from the YJ can be used.
Modification to Chevy bell housing for installation on 2.5L AMC I4
engine.
Photo by David Moulton
Some of the original bellhousing mounting bolts are too long and will
have to be replaced. The bellhousing has to have some modifications.
The two lower transmission mounting holes have to be drilled out to
fit the SM420 bolts, cap screws will have to be used because of the
limited room inside the bellhousing. I had to have the cap screws
turned down to fit. It was suggested to have these holes spot faced in
a milling machine for a flat contact surface with the screw, a good
idea.
The starter cone has to have a piece taken out to fit the Jeep
starter. A new inspection cover will have to be made, or the S-10 or
YJ cover must be modified to fit.
The clutch release fork, pivot ball stud has a thick washer under
it, this has to be removed or the clutch fork will hit the pressure
plate. I used Locktight on the ball pivot stud. I also ground off a
small amount on the clutch fork at the top of the pivot ball divot for
a little extra clearance.
A bracket must be made to hold the crank position sensor, I had a
local machine shop do this, and took them the old bellhousing for a
guide. Of course they screwed it up and I had to reposition the hole
when I put it in on a Saturday with the shop closed. This is very
important to get right, in too far and the flywheel will eat your
sensor, not enough and your engine won't run.
Modification to Chevy bell housing for crank position
sensor on 2.5L AMC I4 engine. Photo by David Moulton
Modification to Chevy bell housing for crank position
sensor on 2.5L AMC I4 engine. Photo by David Moulton
The transmission needs one of the mounting ears ground down a little
for clearance when mounting the slave cylinder.
A new line needs to be made to adapt the S-10 slave cylinder to the
YJ clutch line. Since the 91's have the line as an integral part of
the clutch master cylinder I decided to keep the quick disconnect and
had a local hydraulic shop make up a line with fittings to mate the
two together. I used a coat hanger bent to the shape necessary, and
gave that to the shop for a guide.
Before modifications for slave cylinder.
Modification to SM420 transmission for slave cylinder on install
to 2.5L AMC I4 engine. This picture shows the transmission before
modification. Photo by David Moulton
After modifications for slave cylinder
Modification to SM420 transmission for slave cylinder on install
to 2.5L AMC I4 engine. Photo by David Moulton
Modifications for slave cylinder line
Line for slave cylinder on install to 2.5L AMC I4 engine. Photo by
David Moulton
Modification to transmission tunnel to install SM420 in YJ with
2.5L AMC I4 engine. Photo by David Moulton
A notch has to be cut in the floor pan for clearance of the shift
tower, I cut two but believe now only one is necessary, see the
picture for details. The tunnel cover also needs to modified for the
new shifter position. The transmission lever also needs to be bent so
that it will clear the dash.
The skid plate will need a notch cut for the transmission oil drain
plug. This is at the very front of the skid plate and I cut mine with
an angle grinder.
The front driveshaft must be shortened 3/4", this is important,
because the shaft if not shortened could be pushed through the
transfer case, ruining the whole case. My rear shaft is a little too
short but good enough. I am waiting to modify it when I do another
lift in the future.
SM420 transfer case adapter to install SM420 in YJ with 2.5L AMC
I4 engine. Photo by David Moulton
The transfercase is adapted to the SM420 using Advance Adapters kit
#50-9703, the kit uses the stock transmission mount and shift linkage
mounts. The top two bolts of the transfercase shifter linkage are no
longer used and the shifter is a little unstable, this could be fixed
by using a piece of flat stock and running it to one of the
transmission cover bolts. The shift lever comes through the floor in
almost the same location as stock.
That's about it, I love this transmission off road. It is just about
bomb proof! The Swap is not difficult and was the first time I took
out a transmission by myself. It does demand some caution in the
placement of the crank sensor, and attention to details, but overall I
consider it well worth the effort.
A special thanks to Dion Davis and Paul Weitlauf, I bugged these two
guys a lot during this project. Without Dion it would have never
happened. I'd also like to thank Kris Shattmann, Jeff Gent, David
Givens, David A. Woodring, Cliff VanDerhyden, Terry Howe, Larry Soo,
Ed Cox, Curtis Prost, Jeff Reynolds, Jim Williams, Jason Lockwood,
Jarek Skonieczny, John Nutter, Rick LeBlanc, Michael L. Courtenay, and
some non-virtual helpers, Chad Loyd, Andrew Meyers, and Robert Mills.
I would also like to give a special thanks to Courtney Kerr.
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