Torsion bars

"DipodomysDeserti" (dipodomysdeserti) 02/14/2020 at 16:35 • Filed to: None

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I’m getting pretty deep into the top end tear down with my class on the 450. Other than being Honda’s first DOHC engine, there is another peculiarity about this powerplant. Instead of using valve springs, it uses torsion bars. They’re essentially steel rods that hold the valve in place, which are twisted in order to open the valve. They pretty much eliminated the risk of valve lift. Why did Honda put them on a UJM that only spun to 10k? I have no idea, but they’re pretty cool.

Pretty sure we found the source of my valvetrain noise in a flattened cam follower. I plan on regrinding the cams along with some other mods in order to take advantage of those torsion bars. Gonna teach the kids to wheelie once we’re done. Picture of the bike in healthier times:

HammerheadFistpunch > DipodomysDeserti 02/14/2020 at 16:40

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Thats actually pretty cool.  I wonder what the life cycle on those is and how much the rate is adjustable.

If only EssExTee could be so grossly incandescent > HammerheadFistpunch 02/14/2020 at 16:53

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There’s a reason why torsio n bar valve springs aren’t more widespread, and I’m gonna guess it’s life-cycl e related.

DipodomysDeserti > HammerheadFistpunch 02/14/2020 at 16:53

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From what I’ve heard from a couple of old time engine builders, they’re pretty indestructible, especially w hen compared to the durability of the rest of an engine made in the 60s and 70s . To test them you’re supposed to slip some sort of socket on the end and measure the twist with a torq ue wrench. I think they should twist to 60 lb-ft.

Easiest way I could see to change the rate would be to mill the middle of the rod down. I could see the possibility of creating a variable valve system by designing a ratcheting bar. There were a couple shops who built these bikes up pretty extensively, but it doesn’t look like they touched the torsion bars at all.

  http://www.teamhansenhonda.com/parts.htm

DipodomysDeserti > If only EssExTee could be so grossly incandescent 02/14/2020 at 17:33

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They limit your cam profile, and are a lot more expensive to produce. That’s my guess. A lot of guys swap them out so they can run lopier and faster cams.