Alright,
after a few honey do's today I'm back home and luckily a post man was in training so his trainer decided to stop by my place and drop off my Tower order containing an OS 905 (Code #71905200) aluminum radial mount. First of all, this mount accomodates the following OS engines: 60FP, 65LA, 61-91FX, 65AX, 75AX, 75AX-BE and the 95AX - that's quite a range of engines using the same identical mount pattern. The first thing I did was to take out my NR Speed/13 and see if it fit as we supposed and it is indeed a perfect mount pattern - this confirms that a similar repertoire of NR engines would most likely fit on this same mount as the Speed/13 does. Being a RE engine, there are some issues with the clearance of the header (as often happens with radial mounts and RE engines) but Dremeling out about half the depth of the mount in a semi-circular pattern centered on the top rear will allow NR headers to pass and mount on to the engine. I have yet to pull out an OS 61VF and a 61RF to check the exhaust status but otherwise I'm certain these will be also be a perfect mount pattern as well as the above listed engines.
I also checked a YS 60FR and the engine also mounts perfectly, side-to-side, on either pair of the mounting holes: front or rear. This confirms the 52 mm center-to-center mounting width between mounting bolts. However, as already mentioned, there is the 20 mm vs 25 mm lug width difference between bolts. As to the thrust washer offset between the NR Speed/13 and the YS 60FR IF the YS is mounted using the position of the rear bolts, the difference in location of the thrust washer (spinner backplate) is exactly 1mm. This is due to the rear bolt center to thrust washer distance differing by 1 mm between the S13 (77 mm) and the FR (78 mm). In other words, if one were to re-use the rear mounting bolts used for a S13 and switch to a YS FR, AR or Classic, the YS engines would end up with a 1 mm larger gap between the nose ring and the spinner backplate. Not great but 1 mm is a little over 1/32" so I believe that might be generally acceptable for a quick swap from NR to YS. If the opposite were done (YS to NR), one might end up a little tight on clearance between nose ring and spinner unless one left 3/32" spacing between the two when mounting the YS initially. Once swapped, the NR would be just shy of 1/16" so snug but workable.
Another way of looking at this current manufacture engine option and replacement if using a radial mount (and using the exact same location for the rear mount bolts) would be to mount the NR using a 1/32" ply spacer between the FW and the mount. Then, if one were to replace the NR for a YS, one would simply remove the spacer and the thurst washer would be in the correct place to within 0.2 mm - perfect!
Next will be to check the thrust washer of the OS engines and compare to both the YS and the NR. Last would be a comparison to vintage engines such as Rossi FIRE's, OPS Supers and Picco's.
David