Just a theory. It'll have to do until someone with direct knowledge weighs in.
Space Chain.
This write up from Azusa is excellent: http://kartdrome.com/index.php/technical-documents/azusa-engineering-history-of-35-space-chain PDF also attached.
With an inboard clutch with Space Chain on a Mc91X (and probably McX), the chain will eat into the block at the PTO seal boss, and into the seal itself.
As explained in the Azusa write-up, Space Chain came along after these motors were produced.
When the Mc92 came along (1974?), it's PTO was different in 3 ways (that I know of). No keyway, LH thread and 0.100 longer. The last one fixed the "eating disorder."
The "eating disorder" can be lessened by tapping the seal a little deeper into the block. Keep an eye on it, and keep a couple of extra seals on hand... not sure what else you can do.
A while ago, I looked around for regular (non-Space) #35 chain to solve the problem on my Mc91B. All I could find was "play chain," which is nowhere near adequate for use with a Mc9X because it kinks up almost immediately. At first I assumed that it was a lubrication issue. Nope. The actual pins & bushings bend, turning it into a crooked "bar" of scrap metal.
If anyone knows of a regular (non-Space) #35 chain with hardened pins & bushings, at a comparable price, I'd love to know. A couple of years ago I looked through McMaster-Carr's offerings in #35 (tons), but found nothing viable.
Attachment: Chain_35_space_Azusa.pdf (Downloaded 7 times)
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