Reading the chassis number
AC's chassis prefixes tell you the original engine: AE for the AC Light Six, BE for the Bristol, and RS for the Ford Ruddspeed unit. An X after the prefix (AEX, BEX, RSX) denotes an export car built in left-hand drive. Confirm that the chassis number, the car's configuration and its paperwork all agree — and treat any mismatch as a question to be answered before you buy, not after.
Matching numbers, and the honest alternative
A car retaining its original, matching-numbers engine commands a premium. But because these engines have been rebuilt and swapped over decades, many cars now carry a correct-type engine that is not the original unit. This is common and, if properly documented, perfectly acceptable — what matters is honesty and paperwork. A correct-type Bristol engine of the right specification, with its history recorded, is far preferable to a mystery unit or an undocumented swap.
The AC Owners' Club and the Registrar
Your single most valuable authenticity resource is the AC Owners' Club, which maintains detailed model registers and a marque Registrar. Engine changes, restorations and ownership history are frequently recorded there, and the Registrar can often confirm whether a specific chassis is what it claims to be. Before committing to any car, it is well worth checking its number against the register — and this Archive's own Chassis Registry is intended to complement that record.
“A documented, honestly-described car will always be worth more than a "perfect" one with an unexplained past.”
Common modifications — and the red flags
Over the years Acecas have been altered in predictable ways, and a buyer should know them. Steering conversions from the original box to rack-and-pinion are widespread and generally accepted. Left-to-right- hand-drive conversions — and, in some cases, cars converted and then converted back — exist and affect both originality and value. More dramatically, some cars have been fitted with entirely non-standard engines; there are documented Acecas running American V8s, such as the lightweight Buick 215, which transforms the car's character and its value and must be disclosed. Be alert, too, to the wider AC world of replicas and rebodied cars: make sure a car described as an original Aceca genuinely is one, and not a later recreation on another chassis. When in doubt, the Registrar and a marque specialist are the people to ask.