Recommended installation procedure for maximum seal life and an oil-tight engine.

To gain access to the seal and housing, the sump and flywheel first have to be removed.

Install a new crankshaft rear main lip-seal in alloy housing with the aid of the correct press tool.

For optimum service life, install a double lip seal that has a Viton or Silicon material construction.
Viton or Silicon oil seals offer an extended service life as they have a temperature rating of 200 degrees Centigrade and are abrasive resistant.
BEFORE installing the seal, carefully remove the internal spring and ensure the over-lapping ends mesh sufficiently.
From experience it has been noted that oil leaks have occurred when the over-lapping ends of the spring have disconnected (or the spring has been displaced during seal installation)
The new seal should be gently pushed into the housing until the seal bottoms out on the housing shoulder.
Secure the seal housing gasket to block with Loctite Forma-gasket No.3 or Wellseal (non-hardening sealant).
Apply Loctite Forma-gasket No.3 to housing mating face.
Clean/polish rear main crankshaft journal and then lubricate with molybdenum grease or engine oil.
Carefully slide alloy housing and seal onto crankshaft rear journal Install 5/16" UNC bolts and spring washers so they are finger tight only.
With the internal jaws of a digital vernier, measure between the crank journal and alloy seal housing (every 90 degrees)
Four measurements should be made at convenient points 12, 3, 6 and 9 O'clock should then be compared.
Ideally the four measurements should be identical (seal and housing perfectly centralised) 

From experience the gap is usually between 0.150" and 0.160" for the Ford alloy seal housing, part number 730E-6335A
Note - The original rear oil seal housing has a perfectly circular internal opening and a sump pan gasket groove 7.00mm wide.

The housing should be lightly tapped with a soft face mallet to equalise the gap measurements.
Once the four readings are the same, the 5/16" UNC bolts should be tightened to the correct torque.
This simple but effective procedure ensures maximum life of the seal by ensuring it runs concentric with the rear crankshaft journal.