I'm reconditioning a 1970 Fiat 500 that has had a front disc brake conversion. I am attempting to replace the front knuckles but they are no longer from a Fiat 500 and appear to be a Fiat 850 part. As I search for a replacement, its clear there are some difference between the 850 Special and the Coupe/Spider knuckles.

http://www.ricambio.co.uk/69-fiat/5...set-of-stub-axles-steering-knuckles--fiat-850

http://www.ricambio.co.uk/69-fiat/5...s/1351-stub-axles-steering-knuckles--fiat-850

The response from Ricambio is that the "...difference between the Stub Axles on the 850 is bearing size of the shaft." He suggests a kingpin repair kit instead but am famously lazy and would prefer just to replace the part.

Can anyone expand on how I might determine which part I have?

I'm lazy too so I won't bother answering your query!

Al.

Ok. afaik the 850 Special didn't have front disc brakes but the 850 Coupe did. (I haven't worked on an 850 Spider). Knuckle assy shown reminds me of the 850 Coupe.

How to determine which part you need?

Find out who modified/built your car and ask them which model parts are used.
Find a Fiat dealer which was in operation back when the 850's were current.

Find someone who worked in a Fiat dealer parts dept. back in the day.
Find someone who worked on Fiats back in the day.
Contact a business that modifies Fiats e.g. Middle Barton Garages in the U.K.
Find owners (if any) of the 850, Special and Spider/Coupe local to you, maybe visit to check out these cars. Such owners may have worked on the front suspension of their cars and encountered the part problem you describe.

Measure across the thrust washer fitted between the knuckle and stub axle and compare to the thrust washer included in kingpin overhaul kits to try to narrow down the variant of 850 parts used on your car.

Order up one of each type of knuckle, compare with what you have and return the wrong part for refund/credit if agreeable with the supplier.