Although there have been always a lot of criticising about this rather strange water pump, I still think it is a good pump with the possibility to rebuild is instead of a swap, and adjust it to your needs, road, race.
When you rebuild a pump, you have to take it seriously and for a start check the replacement parts, I have seen wrong O rings etc in several rebuild kits.
Beside the Impeller, it is important that the oil, and the water seal are fully home in the housing, (see the pictures)
To rebuild a water pumps I use several home made tools as described in the Sprint manual.
The impeller have to be reworked for a proper seal, look at the pictures and the comment at these pictures.
Of course the opening in the block must be clean and without burs or rust, otherwise it will damage the outer O rings during fitting.
Water pump rebuild tools
Water seal home
Machining WP impeller surface (use old shaft to support the impeller)
Impeller reworked, Little roughness helps to bed in the new seal.
Sins I build the pumps min this way I never had a leaking pump.
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Hans ten Broeke
De Hoge Esch 49
7783 CB Gramsbergen NL