The engineering number is assigned by Ford engineering when a part is approved for production. This number is used by engineering and the assembly plants.

The service number is assigned when the part is ready for the parts distribution system. A different number is used because the finish or packaging of a part for service is different than it is for production.

The service number is the one the Ford parts people use. They don't care about the engineering number. But you may need to--the engineering number is the one appearing on many parts.

Casting Numbers--Casting numbers are special engineering numbers on a casting to assist in identification at the plant. As explained in the text, the numbers cast into the part apply only to the basic casting. One casting can be machined several different ways, creating different engineering and service numbers.

Using casting numbers to identify a part is like playing horseshoes. It doesn't count much when you're close, but it does count and a casting number may be the only number you have.

Dissecting the Numbers--All Ford part numbers-regardless of whether they are engineering or service numbers-consist of three distinct groups: prefix, basic part number and suffix.

Prefix--The four-digit alphanumeric prefix tells the year the part was released by engineering for production, the car line the part was originally released for and by what Ford engineering group--chassis, body, engine, etc. In the case of a service part, the prefix identifies the division the part is for--Lincoln-Mercury or Ford.

For example, the following part numbers are for a 427 Medium-Riser cylinder head originally released in 1965. The year is indicated by the first two digits. The decade is indicated by the first letter--D for the '70s, C for the '60s, B for the '50s and so on. The following number is the year in that decade--5. The third digit--usually a letter--indicates the car line for which the part was originally designed. For example, most 427 engine parts have the A designation, because they were originally designed for Ford cars. The accompanying list [at end] gives most car-line designations.

The last digit, or letter in the prefix in this part number indicates the part was released for production by the engine group: A is for chassis, B is for body and E is for engine--makes sense because it's an engine part, This applies to engineering parts.

The last figure in a service number prefix refers to the car division--Z for Ford, Y for Lincoln-Mercury. Other letters refer to special parts, such as X for the extinct Muscle Parts program or M for the current Motorsports program.

Basic Part Number--Regardless of whether it's an engineering or a service part number, the basic part number is the same. For example, 6049 is for all finished cylinder heads, 6303 is for finished crankshafts and 6010 is for finished blocks.

The basic part number for the unfinished casting for these parts is different. Referring to the cylinder head again, the basic part number for the finished part is 6049, but the basic part number for the casting is 6090.

It's relatively easy to put a casting number on a part while it is being cast, so it's the casting number that appears on the part--great for Ford and terrible for the guy who's trying to identify it. Also, the number that appears on a casting may not include the basic casting number--you don't need a number to tell a block from an intake manifold. The casting number generally consists only of the prefix and the suffix, or C5AE-F for the cylinder head.

Suffix--A part-number suffix generally tells you the change level of a part, regardless of whether it is applied to the casting, the finished part or the service part. A signifies a part produced as originally designed; B indicates It was changed once, C twice and right through the alphabet, excluding the letter l. When the alphabet has been gone through once, the suffix grows to two letters--AA, AB, AC and so on.

How does a change at one stage affect the other two numbers? A casting change affects both the finished and the service parts, so the suffixes on all three numbers will change. An engineering change will change the service part--and its suffix--but won't necessarily change the casting.

A service part and its number can change independently of the casting part and its number and the engineering part and its number. Got that? This is simply because the service part is generated after the others in the scheme of things.