Balancing the Crank

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Welcome VR.
I see no one jumpng in on this so I will offer notes from the Tech Digest.

It's 52 for the 850, 54 for the 750 and 85 for the 750 Atlas.
It also goes on to explain 'Engine component matching is more useful than balance factor tweaking, especially with iso-framed machines...'

The Dunstall Tuning Notes backs up the 52.

I don't have any personal experience with crank building so I can only offer what I have in print. Hope it helps.

2 VR's ????? One is rare enough, let alone 2.
Do you have any issues with your fiberglass clad tank?

Thanks for the post. I was just kind of confirming my suspicions, but as the old saying goes if you hear it twice it might be true and if you hear it three times it must be true.

Anyway, as for the fibreglass tank, the trick is to mount it differently than how Kenny mounted it and to vent it, at least that is my hope at this point. The 'All Alloy' is really rarer of the two, as there were only 6 made. It's 'All Alloy' VR is my favorite, although the two T160s are pretty close. It's actually an addiction, bikes, cars, you can never have too many.
Scooter sez

"You can have too many ex-wives and kids" :?

Workshop manual also confirms 52% balance factor for the 850.
Yes, a WET balance factor of 52%, is correct. Use 63% for a DRY balance factor, per the 850 Commando Workshop manual.

I saw this over at the BritBike forum and assume its the same query from the same person - though we all know what happens when we assume. :oops:

Anyway - was the "dry" bf in my 1971 Workshop Manual? Did I miss it? As I asked at BritBike - is this common knowledge?

I'd really hate to have made a mistake like that with a rather pricey hopped-up motor.
Hey Doc,

It must not make much difference if you use a 52% or a 63% balance factor with a dry crank. After all, L.A.B. has a 1973 manual that shows 52% dry and I have a 1975 manual that shows 63% dry. (I assume the ‘73 manual refers to the same 850 engine as shown in the ’75 manual.) What’s more, as MichaelB pointed out, component matching is actually more important than the balance factor in reducing engine vibration.

That was the 1973 06 5146 manual I got the 52% dry info from, dealing with all models from 1970 to 850 Mk 1 and it doesn't make any distinction between 750 and 850 as far as the balance factors are concerned, so whether the 52% figure applies to both models I don't know or whether it was something that was overlooked when the manual was printed?

My 1975 manual certainly gives the 63% dry figure.
Well, isn't this interesting. Nothing is simple with these bikes is it?

My manual, p/n 063419, says "52 percent dry" in the Technical Data section. There's no copyright date but it covers 750s only, through the 1972 model year.

Perhaps balancing is just a waste of time and money? Maybe it's better just to have the pistons/rods matched and get the rods magnafluxed and polished?


Personally, I would concentrate on matching the components, especially the rods. If you purchased quality forged pistons, their weights will not vary much. A triple beam balance scale works well for weighing components. And a dermal tool can be used to remove material.

" a dermal tool can be used to remove material."

Be careful, a misguided "dermal" tool can cause excessive bleeding. I prefer a "dremel". (and I think Jason does too!) :lol:
Ron - I think I have the only remaining known Dermal tool. And you're right, a Dremel works much better, plus their much easier to source.

Ahh, but you can Zyglo aluminum for cracks ! It's a fluorescent dye penetrant process we used when I used to help build drag race engines. It's made by the Magnaflux people.
Hmm. If I have the pistons matched (sounds like a good idea) I think I'll let a professional do the dremel work. Sounds like another good example of "don't try this at home", at least for me.

And perhaps the dermal tool is best left to the dermatologists! :lol: :wink:

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