The "421" Chevy
bellhousing
is a 153 tooth
aluminum bell with a small 4-1/8" transmission
hole and
no z-bar mount provision.
This bell is
essentially the same as a 3840383 bell with a
small transmission retainer hole.
The "421" is a
one year only (1963)
bellhousing that was made
to fit the 4 speed transmission on Corvettes and
Full sized Chevrolets of that model year.
In order to fit any other 4 speeds to this bell the
center retaining
hole would need to be enlarged to
he more common 4-5/8" diameter or the transmission
front bearing retainer would need machined to fit.
Since the
3840383 and sister
3858403are plentiful
and cheap, I would avoid the
3788421 unless it is
appropriate to your application.
Front View
In the picture above this bell looks like any other
Chevy 153 tooth
bell. But if you look closely you
can see that the pivot ball stud location appears to
be farther away from the center hole. This is just
an illusion. The stud location is the same as on other
General Motors Housings.
The trans is mated using four 1/2"-12 threaded holes.
Part #, Engineering code, Date code
The passenger side clutch fork knock out is were
you will find the part
number and any engineering
change codes.
I've read that the Four dotes
in the
circle on right pertain to a date code,but will take
more
research to substantiate.
(How does December fit into the half circle?)
Top View
My example has "GM" cast into the upper portion.
Fork Side
This example doesn't have a provision for a Z Bar
mount stud.
Bottom view
Once again this view looks the same as the "383"
and "403" bell housings
Side View
Passenger side view shows almost straight bell to
engine mounting flange typical of 153 tooth fly-
wheel clutch housings.
The 1938-1965 Chevrolet parts Catalog lists
this flywheel housing as part number 3788419
with the note (Ident. No. 3788421)
Inside View
This bell utilizes a one piece inspection cover that
is attached with four 1/4"-20 bolts.
Inside Close-up view
In the above view you can see that in order to open
up the center hole
1/2' (to make it compatible with
common GM 4 speeds), the reinforcement
around
the hole would be virtually eliminated.
3788419
0.683 HOUSING, Flywheel and Clutch
Applications per the 1938-1965 Chevrolet parts catalog effective date October 1, 1964
63 PASS., CORVETTE (exc.409, police).....(Ident. No. 3788421)
Disclaimer:
Factory correct applications are your responsibility. GM made part and part # changes with no rhyme but a reason (no production interruptions at any cost).
Application charts and part numbers are for replacement parts that were available at the time of parts book printing.
My observations come from the parts I have in my collection or other reliable sources your parts may differ.
If Factory correctness is your goal, do more research, join car clubs,
forums and buy an assembly manual if one is available for your Project Car (you won’t be sorry).
This is a fun and interesting part of the hobby!
