aomcc square 4 rods and engine balance

[david.anderson]

I have just received my order for a set of AOMCC conrods for my 4. They are certainly a lot lighter than the originals and clearly a lot less metal. Even the big end bolts are only 5/16. I certainly hope they are a lot stronger.
In the fitting instructions that came with the rods, under Engine Balance it recommends dynamic balancing and states.
“additional weight will need to be added to each of the bob-weights. E.g. slugs of high density ‘Mallory’ metal. As a rough guide AOMCC rods will need 24 grams of additional weight. Original Mk1 rods need 44 gms and late Mk11- 120 gms”
When I first built my engine it vibrated badly as speed increased so I stripped the engine and statically balanced it. Dynamic balancing meant the roller mains would have needed removal from the shafts and that meant destroying expensive lipped rollers that I had recently fitted, so I opted for the cheap alternative and statically balanced the shafts myself. The result was a near vibration free bike.
Has anyone fitted these rods without dynamic balancing as I don’t think that I will bother (especially if the balance is improved by the lighter rods)? Or has anyone taken the advice in the guide and added 24 gms to each bob weight and refitted without a dynamic balance (possibly just a static balance check) and what was the result? Thanks
David

[nevhunter]

You would think they would know what to do but I can't understand why lighter rods need more weight to be added to the bobweights. The SQ$ is a unique design where opposite forces cancel each other out diagonally. Individual dynamic balance would be a good principle but to what factor? Nev

[brenton.roy]

Nev, I know it is a typo, but SQ$ isn't far from the truth..

[david.anderson]

Nev
The balance factor for the square 4 cranks is quoted 0. The instructions also advise that square 4 cranks were only ever static balanced by Ariel.
When I first built my 4 the one thing I did not do was balance the cranks. I had heard the theory that counter rotating one up one down cranks was perfect engine balance and the square 4 was super smooth no matter what, so I did not worry with balancing. As I ran the bike in and increased speed so the vibration increased. At 60mph it was uncomfortable so with 1200 miles on the bike I ran it to 90mph and the vibration increased with speed so I stripped it and checked everything. I had the cranks magnaflux crack tested and asked for a dynamic balance. I was advised that I would have to remove the rollers from the drive side crank and as that is not easy without destroying them I decided to static balance the cranks myself. (I had paid an extraordinary amount for the lipped rollers locally and subsequently found that I could have bought them from Drags, including postage for a fraction of what I had paid). My rear crank was perfect static balance but the front crank required a 10mm hole about 10mm deep in the flywheel to correct it. Not much for the vibration it caused. So I reassembled and have a virtually vibration free bike.
As such I do not feel like going to the expense of dynamic balancing when I fit the new rods as the lighter rods should reduce vibration further. But I have wondered if I should add the suggested weight to each of the bob weights while I am on it. If properly done it will not affect the static balance, which I would check again in any case. Maybe someone has added weight to the bob weights and/or dynamic balanced and can comment on engine smoothness before and after.
Square$. How true. Yes the rods cost me $820Aus including postage and paypal charges, but in the long run it is good insurance for an engine that has new valves, guides, springs, pistons, liners, reground cranks, new mains, cam bushes, steel oil pump body, distributor bushes, 300w alternator plus new chains and sprockets, then for the rest of the bike add gearbox bushes and bearings, recorked clutch, wheel bearings and brake linings, rebushed carb and new jets, paint and chrome, exhaust system, aluminium repair and purchase of missing parts. I have never added up the total cost and would be afraid to.
David