Full Circle Crank for a WB820; Worthwhile?

[Doug Tenney]

Looking for an opinion from "gray beards!" Recently, my son and I picked up another US820, to have as a spare. We weren't desperate, but the price was right, and it's basically brand new. Anyway, it is a reverse-rotation engine (CCW at the PTO), with a 3/4" diameter non-stepped PTO shaft. Our "main" engine has the typical kart shaft 5/8" stepped to 9/16 shaft. We're about to buy a better clutch, but we'd like to buy only one! (Presently, we have a Noram clutch with stamped drum).

According to what I've been able to learn over the years, on US820's, the differences between CCW and CW motors are:
1. Flywheel: magnet North & South poles are flipped.
2. Crank: keyway for the flywheel is in a different location. Also, CW motors use LH threads.
3. Pull starter & cup: obviously, they are opposite.
We bought a used flywheel yesterday from a local guy. He also has a CW crank that he is willing to sell (reasonable), but it has been converted to full-circle counterweights. He bought it used, has never run it himself, and he thinks it might be one of Jim Akerman's, based on the pictures he showed me in a November 1969 article in Modern Karting. It has two thin aluminum rings, about 3" OD, 1/2" wide, forming "molds" around the stock counterweights. The "molds" were then filled with hard yellow epoxy, thus making it a full-circle crank.

Of course, I understand why folks stuff the crankcases on 2-strokes. But, is there a real performance benefit? Have they been known to come apart at high RPM?

We run our US820 on a '63 Blitz kart, on gasoline. All stock, except V12 intake with 2 HL carbs. Box muff. We're thinking about stepping up to a pipe, alcohol and as I said, a better clutch. What do you think?

[Terry Schmitz]

Not sure how they perform yet but, here are pics of one of my 820 cranks

fullcircle820crankc.jpg

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[Jeff Campbell]

Doug,
The full circle crank is one of those incremental performance improvements, the crank by itself is not going to be one of those "wow" what a difference deals. The highest benefit is with motors running a muffler, since the crankcase pumping is the main action scavenging the engine (expansion chambers greatly aid scavenging, and the benefit of a full circle crank will be less). I have seen many converted full circle Mac cranks come apart, mainly do to the fact that to make things fit, grinding is required on the original crank near the rod journal, this weakens the crank there, I've seen many snap there and the resulting devastation is not pretty.

I attached the November 1969 Modern Karting article written by Jim Akkerman for those who have not seen it. For engine builders, these articles are a joy to read, just like the WB820 article reprinted in this month's VKA magazine.

I remember a few years ago someone have all of Jim Akkerman's full circle crank tooling (Bender and Mac) for sale on Ebay along with a huge lot of engines/parts. I would have loved to have bought that.

Jeff

[Doug Tenney]

Jeff,

Thank you for your detailed response. I would not have guessed that the type of exhaust would've been a factor, but once I read your response, it all makes sense. I knew at least one of the "gray beards" would step forward!

Thanks again,
Doug.

[Gary Wlodarsky]

Doug....

Just an FYI...Jeff is far from a ""Grey Beard"" but, he is one of the more ""techie"" guys on here...maybe you better send him an apology( he is kind of a big guy too ....)..lol

gw

[ted johnson]

Don't forget that while Jim Akkerman's Bends won at least one National Championship, that most of Jim's exploits were in enduro racing, where top end performance is paramount. The average Sprint Bender is a torque monster, and the loss of bottom end with the full-circle crank may be a detriment. You also need to consider the EXTREME port modifications Akkerman did with plastic tubing and Devcon poured around the tubes, which were then pulled out to leave the dead smooth runners. I doubt that a sprint kart with an 820 would benefit with a full-circle crank.

[Dick Teal]

Ted is right on. If you watch the really fast dual and single sprint karts race today, you'll see the massive torque he's talking about. The dual Bonbright 820 that I play with has so much torque I have to coast through the corners and then roll into the throttle. If I touch the throttle in the corner the front end gets light and I lose some steering control. I could change the gear ratio but this teaches me to drive better. I suppose that I'm a little too old to be doing this but it sure is fun.

The real trick is to get the right balance of torque and top end together in an engine that has to use a single speed gear set like we do. You also need great tuning and throttle response.

Dick Teal

[steveohara]

" the loss of bottom end with the full-circle crank may be a detriment."

Care to offer an explanation for how a full circle crank would cause a loss of bottom end????

The folks at Rotax, Yamaha, IAME etc will all want to know
Steve O'Hara

[Tom Smith]

Some of the Big-J 820s used full circle cranks. For what's it worth here, a guy I know that raced those engines in C-open said they had more bottom end and insisted that I needed to use full circle cranks in my engines for just that reason.

Mc billet crank .jpg

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[david a luciani]

hi tom,
i have that exact crank .
i bought it at TBO a couple of years ago for 5$.
do you know who made them?
i put in in an open mc91 and that puppy really cooks.

thanx dave