I've tried this, don't do it.  It will most likely never warm up fully,
run rich all the time, dilute your oil with unburned gas and ruin your
engine.  Remember there's over four gallons of coolant in there, nearly
twice the typical four cylinder system capacity. This large a system
really needs a t-stat.

Replace the t-stat and if you still have problems after bleeding the
system then it is time for a new radiator.  Note: You may be able to
change it without bleeding the system if you can clamp hoses off and
not lose more than a pint or so of coolant (this works on the 1.9, but I
don't know about the 2.1 cooling system).  The system will bleed itself
of small bubbles like this, just check and  refill the overflow tank
each day until the level stabilizes.
--
Stuart MacMillan
Seattle

'84 Vanagon Westfalia w/2.1
'65 MGB (Daily driver since 1969)
'74 MGB GT (Restoring sloooowly)

Personal mechanic for:
'70 MGB GT (Daughter's)

Assisting on Restoration (and spending OPM):
'72 MGB GT (Was daughter's, now son's)
'64 MGB (Son's)

Stripped and gone but their parts live on:
'68 MGB, '73 MGB, '67 MGB GT