How much difference if the tonearm is mounted 5mm

If you are mounting the new arm in the stock VPI hole then I doubt it will work. The pivot to spindle distance for a JMW-9 is 223 mm. Which is 9 mm longer than your new arm. That's very unlikely to work.

I am probably restating what the others said above. It comes down to the protractor geometry in question, which as the others said, starts with spindle to pivot distance. It's just that simple.

If your VPI came with a protractor, you will know immediately if you can mount the cartridge so that the stylus touches the precise point on the protractor that is the optimal overhang setting. If you can't dial in the overhang point of contact, tracking distortion will be increased.

If you don't have a VPI protractor, you can buy one from VPI, or buy an adjustable protractor off eBay.

I can't give you precise information on the added distortion effects, but suffice to say, each protractor geometry is set up to "optimize" minimal distortion results based on the approach and theory of the particular geometry in question.

Good luck.

A 5mm deviation in pickup arm setup is a huge error. You want to avoid it.

Let me make it clear. When i mounted the arm as the manufacture suggested, 212mm STP, the cartridge could not reach the cross point on my Dr. Feickert NG Protractor (I extended the cartridge all the way out from the Pivot) it was 4mm short. Unless i have a longer head shell or i will have to mount the arm 4mm closer than spec. (5mm to be sure).
If I mount it closer, and use the Protractor to align the cartridge, will the arm be performed as normal?
Thanks to all who have responsed so far.
Calvin

If you mount the arm the 4mm closer I would suppose that it would be fine...(Harry said he or anyone else can not detect a bit off)...however, as for ME....I would want it absolutely correct. ....just a question....why would you want to exchange arms...VPI makes a very good arm...the 3D is wonderful.