E: Engine Crankshaft Balance Method

Monkeypants

Well Known and Active Forum User
Non-VOC Member
When I realized my 1360 crank needed a rebalance, preferably dynamic, the possible effect on the interference fit worried me a lot.
I did some research and found a Harley tuner shop that had worked with Hines to develop a dynamic crank balancer that runs the crank with rods in place. This is a $200,000 machine.
I almost had them talked into doing my 1360 crank, but in the end they declined making the changes to their machine. They wanted to do it but it made no sense for 1 crank which isn't even a stock Vincent crank.
So I opted to push one side apart to get the balancing done and the other side to put things back together again, final assembly.
The interference fit dropped from 14 tons pressure on the take apart to 12 tons on assembly.
Still plenty I think as new Vincent cranks at factory aimed for 7tons pressure needed for assembly.
I recall John Mcdougall saying that some of the old ones come apart with hardly a flicker of the needle, maybe half a ton. I guess if it's been rebuilt a few times in 60-70 years it's to be expected.
All of which makes those new cranks more interesting....

Glen
 
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vibrac

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Its another point where you need to say" is this the best use of my time?" there are many companies nowadays that offer dynamic balancing at a reasonable rate.
I am the worst culprit, I must stop saying "that looks interesting!" and start asking the first question more often and I should remember to engage in effective productive time and that at my age time is also a dwindling resource:confused:.
 

Bill Thomas

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VOC Member
Thanks Vincent, Got it, Your 445 got me going for a bit :) , Thought it was LL something.
My Wife's 12 year old Grand Daughter helped me with the % is Times, Never knew that !.
Many Thanks, Bill.
 

LoneStar

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VOC Member
Thanks, everyone, for the informed discussion!

Re. my basic approach of trying to muddle through myself, rather than delegate to Qualified Professionals -

- I enjoy messing with old bikes, so the time spent isn't a loss
- Not sure why drilling and weighing should require professional skills or equipment

Re. buying a new crankshaft, which would be of better quality -

- I'm cheap, and my current crank (being free) is 2500 USD less expensive than a new one
- The original cranks, despite their low-grade steel construction, have provided millions of riding miles since 1947 without a lot of failure. A new crank would no doubt be safer in a modified engine on a race track - but what benefit would a rider notice on the road? My current flywheels are now assembled with new rods, and true to .001 on the drive side, .0015 timing side.

Balance factor - I settled on 50% after reading a variety of opinions (including OVR - thanks Martyn). While experienced people can be found endorsing values from 35% to 66%, the original value of 46% can't be too bad, and most people think a bit higher is smoother. As my "frame" is original Vincent, no need to deal with variations due to Egli frame structure or similar. And since by most accounts, anything from 45 - 60% is acceptable, there's no need for great precision when balancing.

I'm still looking over all the posts on the balancing process. I see the benefit of removing weight from the crankpin side rather than adding to the opposite, but it will require removing quite a lot of material. Assuming equivalence of removing weight to adding it 180 degrees away, I'd need to remove 17.3cc of steel near the flywheel rim at the crankpin.

This would be a new axial hole through both wheels of c. 2cm diameter (no room), or equivalent enlargement of the existing ones. But I'm a bit scared of drilling or expanding axial holes there, due to the risk of damaging the rod big ends or getting swarf into the bearings.

Alternatively, it would require radial holes of 1/2" diameter with total depth of 14cm, or 3/4" holes with total depth of 6cm.

Given these factors, I'm still looking at adding tungsten plugs. True, the flywheel assembly doesn't need to be any heavier - but given it's current weight, I doubt an extra 135g will be noticeable.

Dave
 

LoneStar

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VOC Member
I would open out the top 3 holes.

I think you were lucky !!, The rods snap there and the bit that is left bangs a hole in the cases.

What make piston was that ?.

Bill,

Sourced from Ron Kemp years ago; cast 8:1. Embossed 'RAK' on one side, 'VOC' on the other. I think they were made by Omega.

Dave
 

Monkeypants

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Non-VOC Member
It sounds like you have it under control.
For the record, Maughan's current price list shows 320 GBP for each flywheel including mainshaft. Big pin is 100 GBP.
So 740 GBP for the crank plus whatever they charge for assembly.
Their assembly charge was pretty reasonable as I recall. They do send them out for dynamic balance, but even that was quite a low cost.
I guess they get a volume discount!


Of course rods are needed too, however in this instance rods are also required for the existing crank.
 

LoneStar

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VOC Member
That's a pretty good deal; my figure was from Coventry (probably reselling Maughan), and included assembly, truing and balancing. But buying from Maughan direct involves shipping across the Atlantic - expensive for a heavy assembly, and often slow.

On further reflection, I think I can remove sufficient metal by drilling 4 axial holes near the crankpin, 7/8" diameter by c. 1cm deep.
 
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