Original Mk1 water pump seal fitted to almost all US models and early European models has a spring that is a bit weak and a design that causes it to vent water if slightly overpressured (e.g. by fitting a 7 lb cap to the header tank). Seals with inferior springs (identifiable by the marking "GACO CF28783") found their way into the parts supply chain in the late 80's leading to significant reliability issues. For more information see [].
There is no short term problem if a correct new seal (marked "GACO WPF087") is fitted, but it may be best to modify the installation slightly for a long term solution.
Mk2 seal incorporated a modified timing cover (seal seat is smaller diameter), jackshaft (impeller pilot diameter is smaller than MKI) and new design impeller (Aluminium with ceramic insert). Fitted to later & some factory reconditioned engines and most Meraks. The seal itself (but not the ceramic seat) can be sourced easily as it is nothing more than a PacSeal 600 (5/8 inch type 68) industrial seal.
MK1 water pumps are easily distinguishable from MK2 water pumps in that all MK1 water pumps have a bronze impeller whereas all MK2 water pumps were fitted from the factory with an aluminium impeller.
Don James (JBM, Kent Ohio) supplied a seal with a stiffened spring in the mid 1990's. The spring was made from a vanadium steel alloy rather than stainless steel. It was later discovered, however, that although the vanadium steel alloy spring was stronger than the original spring, it suffered premature corrosion failures. If you have a JBM seal you can retrofit the spring from an OE seal to create essentially a reproduction seal (slightly weaker spring but one that will not fail from corrosion).
A water pump modification that uses a PacSeal 601 (3/4 inch type 68) seal in place of the expensive and unreliable MKI water pump seals. The PacSeal 601 type seal easily seals well over 7 psi. so you can run the cooling system at higher pressures to suppress localized boiling. You can even use a coolant recovery cap (which are available only in 7 psi and up). Unfortunately, this modification can only be done when the engine is out for overhaul or some other work because you need to trim one surface on the jackshaft slightly (i.e. in a lathe). Once the mod is done, you can still go back to the MKI seal if you ever want to.
A MK1 seal is approximately $275 (March 2010 price).
The PacSeal 601 is approximately $12 from McMaster.com. A Viton version PS601V is available from Superior Seal for about $30 in quantities.
An equivalent seal for this modification is available in Australia from Pump Seal Supply,(02) 9773 3084, Unit 7-9 Ladbroke Street, Milperra NSW. The seal is a .750 Shaft Size, Head Type 68, and .406 seal thickness. The price is $10.00.
Modified Timing Cover
Note that this means that the modification can only be performed with the engine out of the car.
The Final Result
I have posted drawings so you can do this modification yourself. If you don't have the confidence to do it yourself, I can provide a kit. See Ordering machined parts for ordering machined covers.