Can anyone shed some light on what this is used for? It goes between the shock absorber and chassis.

This plank here forgot to put it in when I put the New Delphi Strut in!

Click image to enlarge

This gasket is probably not vital in terms of operation, but is there to prevent metal to metal contact (especially if different metals are involved - electrolysis etc.) I reused the old ones when I changed my struts.

You should be able to get the gasket on without taking the whole thing apart again, support the hub, just undo the top nuts and lower the hub to create the clearance you need. .
Experience is the only genuine knowledge, but as time passes, I have forgotten more than I can remember
Volvo V70 P2 2006 2.4 Petrol 170bhp Estate SE
MG Midget Mk1 1962

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I thought that might be the case in all honesty, I looked at it and realised a new one didn't come with the Delphi strut and told myself I will have to re use the old one, and still forgot and realised afterwards!

I think i'll just order two when I make the next Island 4 x 4 order, as it will be free shipping, at about £1.10 inc VAT each I might as well just use new!

Just a pain in the butt!!