Identify 1966 Harley Davidson Engine Numbers

A Harley-Davidson’s engine number gives a great deal of information about the bike.

Before 1969, Harley-Davidson motorcycles used the engine number as the vehicle identification number (VIN); there was no separate VIN engraved on the frame. Because engines were sometimes swapped out in these older bikes, it’s not unusual to find Harley-Davidson motorcycles with engine numbers that don’t match the VIN on the title, and this can lead to titling problems when the motorbike is sold. For this reason, it’s important to know where to find a bike’s engine number and read it.


1. Find the engine number of your 1966 Harley-Davidson motorbike by looking on the left side of the engine crankcase. Write the number down.

2. Check the first two numbers, which represent the year the bike was manufactured. They should read “66.” If they do not, your bike does not have the original engine or it has been titled mistakenly.

3. Check the letters after the first two numbers. These identify the model of the bike. BTH is a Bobcat 175 cubic centimeter (cc) two-stroke single three-speed transmission; C is a Sprint, 250 cc four-stroke single; FLB is a 74 cubic inch (ci) overhead valve (OHV) twin sport solo hand shift; FLFB is a 74 ci OHV twin sport solo foot shift; FLHB is a 74 ci OHV twin super sport hand shift; FLHFB is a 74 ci OHV twin super sport foot shift; H is a Sprint 250 cc four-stroke single trail model; M50 is a 50 cc two-stroke single three-speed transmission or a Sport 50 cc two-stroke single three-speed transmission magneto ignition; SS is a Sprint 350 cc four-stroke OHV single; XLCH is a Super CH Sportster 55 ci OHV twin; and XLH is a Super H Sportster 55 ci OHV twin.

4. Check the last numbers after the model identification code. These are the production run number.