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by: Cobalt327, Crosley
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[edit] Overview

As everyone probably already knows, the bellhousing is the mechanical/structural connection between the engine and transmission, and also serves to protect the clutch, flywheel, throw out bearing and other related parts in manual transmission applications, or to protect the torque converter in an automatic transmission application.

[edit] Applications

The bellhousing may or may not be an integral part of the transmission. In some cases like the early Hemi and the Ford flathead V8, the bellhousing was cast as part of the engine block.
1938-’48 Ford 59A
331 cid 1951 Hemi bellhousing detail

Early on, the bellhousing was almost always made of cast iron due to the ease of manufacturing, strength and low cost. Later, aluminum came into use more and more until now where they are almost exclusively made of aluminum alloy for the weight savings. Aftermarket bellhousings may be made of cast aluminum or steel.

In certain racing classes, a sanctioning body may require a “scatter shield” be used on both manual and automatic transmissions. This can be a specially made bellhousing or an added-on ballistic blanket-type affair. The point of it is to contain the shrapnel in the case of a catastrophic failure. It’s not unheard of for a failed flywheel to saw its way into the driver’s compartment, or to have pieces penetrate the floorboards, and this can even endanger spectators. Such a failure happened to Don Garlits and this accident- while almost costing him a foot- was the impetus for him designing the first truly successful rear engine Top Fuel dragster. Racing bellhousings that meet SFI 6.1 certification are able to contain a 13 inch (33 cm) diameter, 30 pound (13.6 kg) flywheel made of steel, cast iron or comparable material, that explodes between 8,000 and 9,000 RPM. Any material can be used to make such a bellhousing as long as it can pass the test.

Ever since there were hotrods being made, there have been transmissions from one make mated to engines of a different make. To that end, custom bellhousings, engine plates and/or adaptor plates are used to join them together. This continues to this day.

[edit] Adaptors

TCI and Trans-Dapt make adaptor plates for mounting a BOP transmission to a Chevy engine or vice versa. They are plenty sturdy for all but the most demanding circumstances, and the cost is relatively low.

TCI adaptor p/n 230001 for Buick, Cadillac, Oldsmobile, Pontiac engines to Chevy transmissions

Wilcap adaptor on early Hemi for using TH350 transmission
Wilcap offers an adaptor to mount the TH350 tranny to the early Chrysler Hemi (above).

[edit] Automatic transmission bellhousings

Many automatic transmission bellhousings are an integral part of the transmission case, and as such, are not removable. However, in some cases the bellhousing is replaced by machining the case to remove the old bellhousing and a specially made bellhousing is attached to the case by the front pump bolts.

There are OEM designs used by Ford (as far back as the early ‘50s) and GM (some 4L60E) that use removable bellhousings. GM also used “uni-case” bellhousing bolt pattern on some of their transmissions, allowing them to be used across a wider range of GM engines.

[edit] Ford bellhousing bolt patterns

Below are three common bellhousing bolt patterns used by Ford.


Warning Note:

  • The small block Ford pattern was also used on the 351 Cleveland as well as some some big block-sized V8, V6 and the 300 cid I6 engines.
  • The 335 "big block" pattern was shared with some 385 series engines (429/460). A very few 400 blocks were cast with the small block bellhousing pattern as well as the normal 335 series pattern- which was left undrilled.

[edit] Aftermarket Powerglide bellhousings

The venerable GM Powerglide (that can be made w/o using a single OEM part) has probably been used behind more high horsepower engines than any other automatic transmission out there. To connect the heavy duty race Powerglide case to various engines, different bellhousings are used.

J.W. Performance bellhousing that will replace the stock bellhousing of a TH700R4

Below are bellhousings used to mount the Reid Powerglide transmission case to various engines.

Bellhousings Reid Powerglide50-60-30-40-20-10-001.jpg
From left to right: 4.6/5.4 L Ford Mod motor; SB Mopar; BOP; BB Mopar; Ford 385 series; SB Ford; Chevy.

[edit] Manual transmission bellhousings

[edit] Bellhousing to transmission bore size

There are four main bore diameters used in the Chevy/GMC bellhousings:

  • 4-1/4": Nova inline 6.
  • 4-3/16": rare
  • 4-11/16": common Muncie/Saginaw car and SM420 truck applications
  • 5-1/8": common SM465 truck applications
  • 5.6": less common NV4500 truck applications

The 5-1/8" truck bellhousing bores can be reduced down to the commonly used 4-11/16" bore size by using a machined ring available from the aftermarket.

In the case of Chevy bellhousings, there are several that are often preferred for use in modern vehicles. One is the casting number 3899621 bellhousing that fits the 11" clutch. Truck bellhousings have to be selected carefully if used in a passenger car application. Some will have a larger bearing housing hole for the transmission input shaft. There are adaptor rings made to allow them to be used. Later bellhousings used in the GM F-body cars with the Borg Warner T5 have the transmission tipped over, and there are bellhousings that have provisions for mounting a hydraulic clutch slave cylinder. A page showing this and other bellhousings and associated info is from

The bellhousing has to be aligned correctly to the crankshaft centerline. This will allow less wear and a smoother drivetrain. Offset dowels are available to correct this alignment if it’s found to be off. More on this page by Hurst.

[edit] Casting numbers of some common Chevy bellhousings

3704922 1955-'56 Cast Iron 10.5" clutch 168 tooth flywheel (1st. SBC cast iron bell housing)

3733365 Late 1956-'59 Cast Iron 10.5" clutch 168 tooth flywheel (2nd. and last cast iron SBC bell housing, identical to 3704922)

3764591 1960 Aluminum 10.5" clutch 168 tooth flywheel (1st. SBC aluminum bell housing. Same size clutch fork boot as 37333365)

3779553 1961-'62 Aluminum 10.5" clutch 168 tooth flywheel (Identical to 3764591 except with larger clutch fork opening)

3779553 1961-'62 Aluminum 10.5" clutch 168 tooth flywheel (Identical to 3764591 except with larger clutch fork opening)

3788421 1963 Aluminum 10.5" clutch 153 tooth flywheel (Identical to 3858403 except with smaller trans bearing hole)

3872444 Early 1965-'66 BBC Aluminum 11" clutch 168 tooth flywheel (Identical to 3788421 except with a larger trans bearing hole)

3858403 1964-'72 SBC Aluminum 10.5" clutch 153 tooth flywheel (Basically the same as c/n 3840383. Identical to 3899621. Identical to 3788421 except with a larger trans bearing hole)

3899621 1966-'74 BBC 1973-82 SBC Aluminum 11" clutch 168 tooth flywheel (Identical to 3872444)

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