SOHC.co.uk Forums > CB350/400
Heavy duty hyvo primary chain
K2-K6:
They're more or less heavy duty in design spec, particularly for the 350/400 installation.
Some of their attributes and limitations aren't readily apparent in casual observation though.
The bearing pins, along with the holes they run in, are quite smal in surface area. Oil replenishment and its film strength being critical to longevity.
Originally developed to run in 5 viscosity transmission oils, are pushed into compromise a little in these engine oil applications. Because of restricted access into and out of the bearing sites, any impediment effectively starves those of competent lubrication. That resulting in the oil breaking down its film strength (shearing capacity) to both wear and overheat the pins.
Oil flow and low fuel dilution (shear performance reduction) are the prerequisite needs to give long life.
They are in effect self tensioning as the tooth form change along with centrifuge action causes them to moderate their own excess movement and generally not flap around. This effect part of their original designation as "silent" chain type, before development by Borg Warner into the more common usage for transmissions generally, named by them I believe as "HyVo" installation.
Ideally they'd use lowest viscosity engine oil in bringing it closer to the Auto transmission fluid they developed it with for high speed applications. But other restrictions apply here in our shere of regime operation. Cleam undiluted oil though, eminently sensible for all,engine function.
AshimotoK0:
Interesting that the 1968 CB750 prototype that is up for auction at Mecum this month had a Morse / Hyvo primary chain drive....description not entirely correct as the CB750 SOHC Automatic had a Morse / Hyvo primary drive.
https://www.mecum.com/lots/1157654/1968-honda-cb750-prototype/
https://www.cb750prototype.com/
https://www.cb750prototype.com/parts
SteveD CB500K0:
Gosh.
Expected sale price?
Sent from my iPhone using Tapatalk
AshimotoK0:
--- Quote from: SteveD CB500K0 on January 05, 2026, 10:51:29 AM ---Gosh.
Expected sale price?
Sent from my iPhone using Tapatalk
--- End quote ---
My estimate is £250-£500k
Orcade-Ian:
Hi Ash,
Thanks for that link, very interesting!
Just for fun I've copied the link to the 3x CBX man here in Orkney - he likes rare stuff with pipes on both sides🤓
Ian
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