General Motors Casting Number 3899621 Clutch Housing
The "621" Chevy Bellhousing is a 168 tooth
Flywheel aluminum bell with 4-5/8" center hole.
It was used on Chevrolet big block, small block
and six cylinder engines requiring an 11" factory
Considering the wide range of original usage, and
the fact that it was in the General Motors parts
catalogs until at least till July of 1987, you would
think these units would be plentiful. Yet, this clutch
housing is so highly sought after, that there are now
at least three reproduction versions available.
Transmission Mount View
This is a Chevrolet car transmission to Chevrolet
engine adapter. All GM "car" transmissions (Saginaw,
Muncie, T 10, ST-10), can be used with this clutch
housing. The only exception is the 1963 small
bearing retainer transmissions.
The bottom of this housing is rounded and hard
to use to determine the difference between this
168 tooth housing and a 153 tooth housing.
Passenger Side View
In this picture you can see the casting number,
engineering codes and starter "bump".
The starter bump identifies this housing as fitting
the Chevy 168 tooth flywheels.
The casting number and engineering codes are
very light (on most but not all) original 621
housings. All aftermarket housings )that we have
seen) have thick casting numbers, and no codes.
Here you can see the standard Chevrolet V8
block bolt pattern, and GM cast into this part.
Transmission Mounting Holes
The transmission to bellhousing attachment holes
are 1/2"-13 threads.
Drivers Side View
The drivers side is straight and can't be used to
differentiate this housing from a 153 tooth flywheel
Engine Side Ball Stud Hole
The zbar ball stud hole, that is used on 1965-70
full sized cars has been present and threaded, on
all used 621 bellhousings we have seen.
Note: The location of this hole is different and
shouldn't be used for 1973-84 GM truck z-bar
Here you can see the Chevy V8 and 1962-88
straight 6 cylinder bolt pattern (top). The four
clutch cover or inspection plate 1/4"-20 threaded
holes. And the starter "bump" for 168 tooth
The GM part #3899621 clutch cover has the
4 5/8" (4 11/16") center hole, and the typical fork
boot hole. These castings, unlike Chevy 153 tooth
housings, are left hand driver only. With no provisions
for the clutch fork to exit on the right side. As you can
see opening up the center hole for truck transmissions
would severely weaken this bellhousing.
0.683 HOUSING, Clutch
There are too many applications to list.
We believe the #3899621 clutch housings first appeared at the end of the 1966 model year to replace part #3872444
We have not seen any part numbers listed in assembly manuals. This is probably because the engine plants installed the clutch housings, then shipped them to the assembly plants.
This part number does not necessarily indicate "Heavy Duty Clutch". The Corvette parts books list part #3858403 (Casting #3840383) for special high performance 427 cubic inch engines (L88, ZL1).
Most parts that I describe and picture, have identification on or with them, others I can identify from other sources and deduction (educated guess).
Factory correct applications are your responsibility. GM made part and part # changes with no rhyme but a reason (no production interruptions at any cost).
Applications are for replacement parts that were available at the time of parts book printing, and when possible model specific assembly manuals.
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 wont be sorry).
This is a fun and interesting part of the hobby!