David...to the extent of my knowledge the "swing" or trailing arms on the 4A
IRS, 250 and 6 are interchangeable. I know I have two slightly different ones
on my right and left...evident by the different bump stop perches. One is
round and welded on, the other cross-shape and cast in. I have measured both
arms carefully and they are identical. The number on one of mine (also
STANPART) is 307053 Vsomething...the other side is a little too dirty to
distinguish.
If you are concerned about and indeed have noticed what appears to be a crack
you might want to have both of them magnafluxed to be sure. I recently read
an article in Car Craft magazine about repairing cast metal such as swing
arms, intake manifolds, etc. It involves drilling a "trough" along the
indentified crack, then welding in material to fill the "trough" then
grinding down to finish. It is a little more sophisticated than my
description and must be done by an expert but since the arms are getting
scarce (I don't believe any of the Big 3 suppliers have them) it is a way to
save one. I could dig out the article and be more specific if you are
curious. If you need a new arm they appear peridically on e-Bay. You also
might want to check some of the LBC salvage yards in you area.
Hope this helps...
Larry Morrison
TR250 in Colorado
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