High RPM is a killer for these units. As Rono says they were intended for 5500 to 6000 RPM. They are very sensitive to heat and will burn the drive balls up if they do get too hot. My McCullouch setup uses the original 3/4" wide V belt and the McCullouch provided pulleys. I've never played around with pulley size for boost much. I did run a smaller diameter top pulley from a later Paxton for a while. It was about 4" diameter. Ran quite a while but eventually the head unit failed. On my roots supercharger I'm using a double V belt setup. On these units its a matter of pulley diameter top and bottom and a formula to determine amount of under or overdrive you get. The roots unit set up for street or competition use will allow higher RPM. Picture below is the SN-60 style unit with the smaller top pulley.


Y's guys rule!
Looking for McCullouch VS57 brackets and parts. Also looking for 28 Chrysler series 72 parts. And early Hemi parts.

MoonShadow, 292 w/McCulloch, 28 Chrysler Roadster, 354 Hemi)
Manchester, New Hampshire

I should add that Chuck and Mike actually run these superchargers and they are correct about pulley size and tooth count determining the internal RPM of the SC head unit. There are different combinations you can try on the top vs bottom pulley, if you are using toothed pulley's I know Chuck runs a V belt set-up, but I'm not quite sure how that works. Regardless, 35,000 RPM on the Ball Drive is about the max you want to spin. There is a formula you can use to determine the number. I got this info from Gord McMillan, REX High Performancxe, who makes a blower kit for these superchargers.

Rono

  Ron Lane,  Meridian, ID

John Erb rebuilt my VS 57 and changed over the internals to the more modern SN-60. If you can find a shop manual for one you will learn a lot. I did. At 5,500 RPM at the crank, the internals on the SC are spinning around 35,000 RPM which is about the max you can go. Is your Head Unit a Direct Oil (DO) plumbed from the block, or is it self contained? If it is self contained, It should have had fresh Type F trans fluid put in. Also the spring pack that puts pressure on the ball drive can create flat spots on the ball bearings causing them to burn up. Turn over the supercharger every couple of weeks when not in use. Hope this helps.

Rono

  Ron Lane,  Meridian, ID

It should spin fairly easy with a hard spot each revolution. These superchargers are turning an extreme RPM and will get hot. Occasionally an external oiler dipstick pump shows up on ebay so the oil has extra cooling. If yours is tight its surely due for a rebuild. I've used John Erb and Craig Conley for parts, advice and repairs. 
John Erb 
4019 ponderosa dr. 
Carson City, NV 89701
johnerb@sbcglobal.net.


Y's guys rule!
Looking for McCullouch VS57 brackets and parts. Also looking for 28 Chrysler series 72 parts. And early Hemi parts.

MoonShadow, 292 w/McCulloch, 28 Chrysler Roadster, 354 Hemi)
Manchester, New Hampshire

Depends on the pulley sizes, but given the typical set up 5500 was about the limit. Not that guys that knew what they were doing didn’t manage more. I seized an SN-60, and replaced it with an SN2000 on Craig Conley’s advice. You’re almost certainly due for a rebuild, or it will seize. There’s a number of guys that do that, and when you pick one talk to him about pulley sizes, rpm range etc. As an aside, it takes a pretty good cam and head work to make a 312 go 7000rpm and make power, even with a blower.

miker
55 bird, 32 cabrio F code
Kent, WA
Tucson, AZ

I have a SN-60 paxton on my 57 312 that had not been run since the 70s. It worked well on my first few shake down runs up to 5500 rpms  then 2 7000 runs and back to the pits it felt warm not hot. Now I try to spin it after a few months and spins very hard is 7000 to much and if so is there a fix I plan to rebuild it so if there's a fix now is the time1