Here is the technical information on some of the Akai Reel to Reel Pinch Rollers. The design of the Pinch Rollers varied more than would be obvious at a purely causual look. Some early sets such as the X-1800SD had the PR residing on the shaft which was 5.5 mm in diameter; while on the 1700 Series and all known later Akai sets the PR sat on a 6.0 mm shaft. Little is known about the M series and some of the other X-Series units; if someone who has these could take some measurements and update me about those sets it would be helpful.
There were also some differences among the PR's in terms of the center brass bearings, as some PR's sat higher on the shaft than others; also the overall width of the PR's rubber tire could vary, wwith the most common being the Hard Types at 11mm. and the Soft Types at 14mm. in width. Unfortunately, this makes for an astounding possible 90 or so variations, which is why almost all of the known Akai models have PR's showing different part numbers from almost all "other" Akai models.
Still among all of this kaos, there are certain consistencies, so lacking any new PR being made during the prior 20+ years, in general the following chart shows that some PR's will work fine on more than one model of Akai. Also, in general, but maybe not in a specific situation, a one size smaller PR can be substituted on a set that uses a larger PR. But in general, they cannot be sub'd, the other way around. Its likely a 37mm PR can be installed in place of the 38mm, or the 38mm could sub for the 40mm.
While the center aluminum hub surrounding the brass bearing can also be seen to vary in size, this probably doesn't matter as much as the actual outer diameter of the rubber tire. In other words, the 38mm Hard Type as seen on the GX-255 has a much larger aluminum hub part than the 37mm. Hard Type ( around 28mm. versus 25mm. ) but it will work on most of the other sets that normally use the smaller 37mm. PR's or the larger 40 mm. whose hub diameter is 31 mm. However, across Soft and Hard types parts cannot readily be interchanged due to several factors being different ( the width of the rubber tire and the height specifically are the most important ).
Various later sets used Pinch Rollers that sat on a 6.0 mm shaft. These
either had a soft rubber with a Circular Depression Ring which was located
a few millameters from the outer edge on both the upper and lower rubber
surfaces; or a more solid rubber which contained no such depressions. The
typical later Hard Type PR is seen in the picture below, which gives a
good idea of what the more solid type looks like...
Examples of some Akai Pinch Rollers having 6 mm. diameter shafts:
Akai Part # // Outer Diameter // Rubber // Machines that it Fits:
MP582164 // 42.0 mm // Soft // GX-620, GX-625, GX-630D, GX-630D-ss. * ( PR's are 14 mm. in width and have Circular Depression Rings
MP275984 // 40.0 mm // Hard // GX-266D, GX-266II, GX-267D, M-11.
MP625048 // 38.0 mm // Hard // GX-255.
MP424023 // 37.0 mm // Hard // GX-280D, GX-280D-ss, GX-285D, GX-286DB, GX-297D, GX-635D, GX-636, GX-646, GX-747, GX-747dbx.
MP533744 // 36.0 mm // Soft // GX-400D, GX-400D-PRO, GX-400D-ss, GX-650D, PRO-1000. * ( PR's are 14 mm. in width and have Circular Depression Rings )
MP531347 // 32.0 mm // Hard // GX-77.