OK, I'll jump in where angels fear to tread on this one.
This is a perennial question on the List Dieter.
Your impression is very reasonable and probably typical of people with a
casual acquaintance with Alpines and Tigers. To be as brief as possible,
Tigers are not modified Alpines, but were specifically built by the Pressed
Steel Limited division of Rootes to be Tigers. Also, the chasses were sent
to Jensen for final assembly rather than on Rootes' Alpine assembly line.
There are several distinguishing features on a genuine Tiger chassis that
are essentially impossible to duplicate starting with a series IV or V
Alpine. These are the basic facts of the matter. From here on, Tiger owners
tend to bifurcate along two extreme points of view, the first being , if it
walks like a duck and squawks like a duck, then it's a duck; or, if it isn't
the original chassis with the original VIN and rivets, then it's a fake and
ought to be hauled away to car purgatory.
Nobody has tried to argue that an "Alger" can't be everything a Tiger is,
depending on how much care and effort is put into the "conversion". Also,
not very many argue that a prospective buyer doesn't have the right to know
the car's provenance. How much more is a "real" Tiger worth? I don't have
any good numbers to gauge that, but the bidding price for an "Alger"
invariably drops once the "cat" is out of the bag. The reasons are partly
esthetic and partly practical. One likely problem with an Alger is
registration, since the VIN was probably taken off a wrecked Tiger and
installed on the Alpine chassis. This is an arguably illegal procedure and
has the potential for grief down the road - like worst case scenario, the
state haul's the car off to the crusher.
I've tried to be both brief and fair to both sides of this controversy.
Tac'd in S.D.,