The products listed under stripper rubbers as 3558/9, 3658/9 and 3758/9 may be considered
in effect energising to some degree (and are so-called by TOT) and are replaceable separately
from stripper rubbers with which they may be used. But it is truer to say they fill the
cavity in a larger box, so economising on large sizes of stripper rubbers. It is possible
to mix hardnesses eg a 3659 energiser with a 3554 insert to make a 1.75" system. We believe
the all-in-one systems of the larger rubbers, or 3652 to 3657, or 3752 to 3757, or 3852 to 3857
to be better, offering less scope for leak paths. Similar remarks apply to the TOT radial packers.