is the vega bolt pattern 4 x 100mm??
No. Just over. 4 x 4" or 102 mm.
Actually it is 101.6 mm - very odd, but I know it well since I've been looking for a larger diameter wheel in that pattern. Since the 13" tires are becoming very scarce that's going to be the alternative if keeping the original 4 x 4" - and I've got a Cosworth with the special factory alloy wheels. I've got enough performance tires in stash to fit all my Cosworths, but by the time those are worn out who knows if anyone will be selling 13" anymore.
take your hubs and axles to a machine shop and have them re-drilled to the popular 100mm pattern. Your wheel choices are unlimited then..
I'm not positive, but besides the bolt pattern, wheels center on the center portion of the axle. Care should be done (IMHO) when getting wheels to make sure how they center (locate) on the axle. Some use the lugs to do it, but others - I believe - use the hub center. Best to be safe when doing this. There have been some extensive discussions on the Cosworth Yahoo ( https://groups.yahoo.com/neo/groups/cosworthvegas/info) Group. Search there for that discussion. Although it focuses somewhat on the 101.6 bolt pattern, other discussion are on going to the 5 lug, etc. Just more information for you to consider. For the Vega owner, there is a way to move to the 5-lug front/rear and that opens up a tremendous number of options for wheels.
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