It is always nice to get a hold of a complete engine, clutch and transmission package. However, chances are that the Chevrolet engine obtained for a conversion will not have all of the matched parts with it. If so, the following information will be useful in piecing together miscellaneous Chevrolet standard shift or automatic parts.
Note that Chevy engines enjoy the widest parts interchangeability of any group of engines and this is one of the reasons for their outstanding popularity for conversion purposes.
All Chevy V6 and V8 engines continue to use the same block-to-bellhousing pattern as introduced in 1955. This includes Generation I, II and III engines. Some Gen. III engines omit drilling and tapping one hole in the block. The installer can drill and tap this blank boss appropriately and it may be recommended to do so if maximum strength is required.
The Chevy I6 "Blue Flame" (235 cid, etc.) from 1950-1961 use a different engine block pattern than the popular 90-Degree. However, the Chevy 265 cid (and its following 250 & 292 variants) from 1962-1990 do feature the same 90-Degree pattern as the Chevy V6 / V8 family.
The GM (technically a Pontiac) 151 "Iron Duke" also shares this same block pattern. However, the Jeep bellhousings from these engines (1980-1982) rarely fit the full-size clutches of the V6/V8 engines. Flywheels do not interchange.
GM Atlas engines - the new generation of I6, I5 & I4 - use the same standardized engine block pattern as the 1955-on engines.
Chevrolet / GM Big Block V8 engines have the same block pattern. Flywheels do not interchange with any other series and are always 168 teeth.
Buick, Olds, Pontiac and Cadillac block patterns (BOPC) are different. However, note that some of these cars increasingly used Chevrolet engines, supplanting their own V8 designs as GM gravitated towards a more unified powertrain.
The standardized Chevy bellhousing can be of either cast iron or cast aluminum. Chevy V8's from 1955 to 1961 all used a cast iron bellhousing. These bellhousings all had a 4-11/16" diameter transmission locating bore.
All cast iron bellhousings used a removable bottom and front cover. As of this writing these parts are still available from Chevy as #3704923 for bottom cover and #3734908 for front cover.
1962 saw the introduction of aluminum bellhousings. They were produced concurrently with the cast iron versions, though the latter were increasingly only used in heavier duty applications and then phased out by the early 1970's. Many of these bellhousings had provisions to use them as structural, rear engine mounts.
Rear bolt patterns on the Chevy and many other GM applications use the familiar and standard Muncie/Saginaw/Stewart 4-3/4" x 8-1/2" (roughly) transmission bolt pattern.
Lower holes on earlier truck bellhousings housings were still sized for bolt clearance and therefore untapped. Mid-1970's and later versions have bolts that go through lower holes in the transmission and holes in bellhousing are threaded as per convention.
Flywheel clearance is an important issue. Essentially all cast iron bellhousings will accept small and large flywheels (discussed below). However, many aluminum versions will accept only the smaller flywheels. The larger clearance bellhousings will fit both large and small flywheels. One of the most common large bellhousings is the GM #3899621. Though no longer made by GM, Novak carries them new. Other numbers to look for are: 6263756, 460486.
Starting in the late 1970s there were some bellhousings that had the transmission bolt pattern rotated about 13 degrees. These cannot be used as they cause problems in mounting the engine. There are also some late 1970s bellhousing that have the release fork pivot on the passenger side of center. These require clutch linkage that would operate in reverse of normal Jeep linkage.
Four primary locating bore diameters were used in the standard Chevy/GM bellhousing:
The 5-1/8" truck bellhousing bores can be reduced down to the 4-11/16" standard with a precision sizing ring made by Novak. This stock item is available under Novak #BR4.
All flywheels from 1955-1961 are of the larger 168 tooth style. Starting in 1962, a new flywheel size was introduced: 153 tooth. 153 tooth flywheels may be drilled and accept a 8", 9", 9-1/2" or 10-1/2" clutch disc and pressure plate assembly - depending on what engine they were originally used with. The 168 tooth flywheel used 10-1/2" or 11"+ clutches. The 168-tooth truck and high performance car flywheels are drilled for 11" clutch.
Diameters of these two flywheels are 12-3/4" and 14" for the 153 tooth and 168 tooth versions, respectively.
While it is possible to have a flywheel redrilled to accept a larger clutch, this must be done with extreme accuracy or the weight of the pressure plate will cause imbalance. To be done properly, a vertical milling machine and rotary table attachment (or CNC milling center) with suitable dial indicator, drills and tap are required.
All Chevy V8 (1962-1985) engines are internally (or neutrally) balanced and flywheels/flexplates interchange between them. The exceptions to this are the 400 CID Small Block and 454 CID Big Block V8's. These each require their own special balanced flywheel. Using a neutral balanced wheel on either of these engines or using either the 400 or the 454 wheel on a neutral balanced engine will cause vibrations that could damage the engine. These external balanced flywheels or flexplates can be identified by the obvious addition of weight to the side of the flywheel facing the engine.
The Chevy V6 most commonly used a 168 tooth flywheel or flexplate. These do not generally interchange with Chevy V8 flywheels and flexplates due to the unique balance of the V6.
In 1986 a crankshaft and flywheel change was made to the improved, once-piece seal design. 1986 and newer Chevy engines will not accept older flywheels due to the different crank flange and bolt pattern. Older bellhousings, however, will work on 1986 and newer engines.
The following chart, courtesy of GM Performance Parts, provides some data and part numbers on flywheels and flexplates, for your interchange information.
Flywheels Reference Chart
|Part #||Outside Diameter||Years||Crank Flange Bolt Pattern||Clutch Diameter||Ring Gear Tooth Count||Crank Seal||Notes|
|14085720||12-3/4"||1955-1985||3.58"||10.4"||153||2 pc.||Lightweight nodular iron flywheel. ~15 lbs.|
|3986394||14"||1970-1980||3.58"||11"||168||2 pc.||Externally balanced 400 cid small-block V8 only and used with balancer #6272225|
|3991469||14"||1955-1985||3.58"||10.4" or 11"||168||2 pc.|
|10105832||14"||1986-1999||3.00"||11" or 11.85"||168||1 pc.|
|14088646||12-3/4"||1986-1999||3.00"||10.4" or 11"||153||1 pc.||Lightweight nodular iron flywheel. ~16 lbs.|
|14088650||12-3/4"||1986-1999||3.00"||10.4"||153||1 pc.||Standard weight|
Flexplates Reference Chart
|Part #||Outside Diameter||Years||Crank Flange Bolt Pattern||Converter Bolt Pattern Diameter||Ring Gear Tooth Count||Crank Seal||Notes|
|471598||14"||1970-1985||3.58"||10.75" or 11.50"||168||2 pc.||Internally balanced engines|
|471578||14"||1970-1980||3.58"||10.75" or 11.50"||168||2 pc.||Externally balanced 400 cid small-block V8 only and used with balancer #6272225|
|471529||12-3/4"||1969-1985||3.58"||9.75" or 10.75"||153||2 pc.||Internally balanced engines|
|14088765||12-3/4"||1986-1999||3.00"||10.75"||153||1 pc.||5.7L HO engine|
|12554824||14"||1986-1999||3.00"||11.5"||168||1 pc.||Increased thickness for HD applications|
|14088761||14"||1986-1999||3.00"||10.75" or 11.50"||168||1 pc.|
From 1955 to 1961, Chevy Small Blocks used a three-bolt flange, mount-on-bellhousing starter. 1962 and later bellhousings will not work with earlier starters or engines.
Starting in 1962 the starters are mounted on the block. These V8's have three starter bolt holes in the block, but only two of these holes are used to mount the starter. The engine blocks are usually drilled to accept two types of starters. Starters with mounting bolts in line with a right angle to the engine centerline are for 153-tooth flywheels. Starters with mounting bolts offset from right angle to the engine centerline are for 168-tooth flywheels. Starters, drives and solenoids are essentially the same. Nose cone castings are interchangeable so a starter can be converted for use with either size flywheel.
On 1955-1961 V8's the aluminum bellhousings can be used by having the block drilled and tapped for the starter. This must be done also on 1955-1961 V8's when using these engines with Turbo 350or 400 or 700R4 automatic transmission, as these transmissions were always used with block mounted (1962 and newer) starters.
The cast iron bellhousings with flange mounted starters can be used on all V8's 1962-on. These can be used with block mounted starters by grinding the lower portion of the housing to clear the block mounted starter casting.