Sourcing parts for my Spider project, I've been amazed at the price difference between various specialists and other suppliers such as Rock Autos. The main problem is finding the correct part as specified by AR.
This site gives a useful cross reference especially as many of the specialists are offering Payen parts at 2 or 3 times the price of other suppliers.
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I found it here:
Payen | ENGINE PARTS (UK) LTD
A quick example, looking for a TS head gasket AR number 60608895
Cross references as Payen part BW710 (also listed as Goetze 30-028420-00)
Rock Auto, Payen BW710: £13.82 inc vat
Specialist, Payen BW710: £33.00 inc vat WTF?