The 250 is in the same family as the 200, making the swap relatively straightforward. Nevertheless, there are some items which need to be addressed in order to make the switch. These comments apply to the swap performed on a 1966 Mustang. Later model Mustangs, Falcons, Fairlanes, and Mavericks may differ in details, but the same items should be considered.

Engine similarities and differences

The flywheel to crankshaft bolt pattern is identical to the small block V8 and the 300ci six cylinder. Although Small Block V8 flywheels will fit the 250ci, the balance is incorrect. The 200ci flywheels will not fit because the bolt pattern is smaller. The 250ci and 200ci oil pumps and pickups are different and the oil pans are different due to the 250's added width. The 250ci rods are longer than 200ci rods. Valve pushrods are different due to length. The 250ci water pumps are larger as are the crank and water pump pulleys.

The 200ci and 250ci do share the same pistons, fuel pumps, valve covers and camshafts (although the lift and duration vary over the years). All 144-250ci cylinder heads and rocker arms will physically interchange, but there are variations in combustion chamber size, carburetor mounting pads and pushrod ends. The motor mount mounting bosses are identical and in the same location relative to the bell housing face. Exhaust manifolds will interchange across all years, but there are variations for emissions and exhaust pipe size.

Things to consider when swapping the 250ci.

Height, width, and length need to be considered. The 250ci is 1.66” taller than the 200ci. On early Mustangs this poses a hood clearance problem. The original early air cleaner is too tall to clear the hood. However a Maverick or Late (79-82) Mustang air cleaner is lower and will provide needed clearance. Because the carburetor is now higher, the throttle linkage also needs to be raised either by bending the linkage or extending it. The original lines from the 200ci fuel pump and the vacuum lines from the carburetor will also be too short and new ones need be made.

The 250ci is approximately 1.5” wider at the motor mount bosses. The original Mustang mounts can be used, but they will place the engine higher in the compartment because of the way the brackets are angled. Elongating the bolt holes in the engine mounts to allow the engine to drop slightly will help, but the proper solution is to use Maverick 250ci mounts and brackets. The Maverick mounts are designed differently and will require new bolt holes in the frame.

The harmonic balancer and pulleys make the engine about 1” longer overall. This makes it difficult to install an engine driven fan. Even if there were adequate clearance lengthwise, the water pump on a 250ci is higher which reduces clearance between a fan and the top radiator hose. The easiest solution is to add an electric fan.

Transmission options

Note: The bell housings for the 250ci are the same as the 289/302ci and the 5.0 V8's.

Manual transmissions

An original 65-66 2.77 non-synchro 3-speed cannot be adapted because there are no suitable bell housings and the torque of the 250ci would quickly shred that tranny. The same applies to the 4-speed Dagenham transmission.

The 67-up 3.03 3-speed and 4-speed top loaders can be used by employing a 250/289/302ci bell housing. The Tremec 5-speed will also fit using this bellhousing. The late model 5.0 bellhousing and T-5 transmission will bolt on and standard T-5 swap procedures apply. For the really ambitious, it's even possible to adapt a T-56 six speed using the same procedure as used to install one on a V8. The bottom line: if it will fit a small block Ford V8, you can adapt it to the 250ci.

The 300ci flywheels and bellhousing's will also fit the 250ci, but since most of these came in trucks, you will have to check for clearance and release mechanism compatibility.

The 250ci flywheel is has "0" imbalance since the engine is internally balanced. The stock flywheel is 13.25" in diameter and has 157 teeth, just like the 289/302ci and the 5.0 V8's.

Automatic Options

Although the original C4 tranny will not bolt on without a bellhousing and torque converter change, it opens the door to a number of other transmissions. If you are determined to keep an original 200ci C4, you will need to change to a V8 C4 bellhousing and torque converter.

Late model AOD/AODE's can be used behind the 250ci as can any other automatic (C4, C6, FMX) which has a small block pattern. To use an AOD, a 164 tooth flexplate from an AOD equipped 300ci six must be used.

Other items to consider

The starter position is different than a 200ci. On most 200's the starter is above the oil pan alongside the block. The 250ci starter location is identical to the V8, it's below the block alongside the oil pan. Although the 200ci starter will bolt on, the orientation of the engagement cover is wrong. A proper 250ci or small block V8 starter should be used. A 3.8 V6 starter will also fit and is smaller and shorter if header clearance is an issue. Tilton or Ford Racing gear drive starters for a small block V8 will also work.

If using a stock exhaust manifold, the 200ci exhaust pipe is bent wrong and will not clear the 250ci starter position. The stock exhaust pipe from either a 69-70 Mustang 250ci or a Maverick 250ci will clear the starter. Headers designed for a 200ci may not fit for the same reason, but because different designs may work on multiple applications, you will have to trial fit them yourself to make that determination.

Accessory brackets from the 200ci will not fit. The mounting bosses are located differently. Cars equipped with AC and PS will need the brackets from the 250ci.

The 250ci water pump is larger and the outlet is larger than the 200ci. A Maverick lower radiator hose fits most applications.