It is 3 special aerosol cans.
Step 1- clean it all over with the #1 cleaner supplied, and wipe dry.
Step 2- spray #2 red dye all over the surface where you suspect there may be a crack- on a Mini crank this means all the fillet radii on the bearing journals, the oil holes, and also the c-clip slot in the crank tail.
Leave it on for 10-30 mins. Dye will penetrate any surface cracks present.
Then wipe off with a clean rag lightly moistened with #1 cleaner.
Step 3- spray #3 fixer (white powder) all over it lightly.
Any cracks will be evident as a red line in the white surface coating.
You can buy the kit, not sure of the cost.
I salvaged mine 10 years ago from a previous employer when they sold the gas cylinder manufacturing plant, lock stock & barrel.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.
