E: Engine Crankshaft Balance Method

[Bill Thomas]

Dave , You are better at Maths than me, What is the balance factor as it is now ?.
I think most Vincent's have a bit of vibration somewhere, Maybe just get it nearer than what it is now ?.
That is why Big Sid told me about lead weights in the handle bars.
I am told I am wrong, But I think the higher the compression pistons the more they vibrate.
I have used 12 to 1, On the track and because it was a road bike, Used it on the road also,
I remember a younger racer asking me if his very fast Twin race bike should vibrate like that !, When running it in the paddock, I said Yes it is fine.
For you I would maybe add compression plates, To get it down to near 7 to 1,
I run 9 to 1 because I still like the punch when giving it one !,
I have a newer type piston for my new bike I am building, But in my head I am thinking I should use one of my old type Specialloid's, Lots to think about.
You have helped me with mine. Cheers Bill.

 

[Bill Thomas]

Just a thought Dave, I hope you are running a bit more clearance on your new pistons ?,
We don't want another Nip Up !.
Also it has been said that it is important to have the conrods central in the cases/barrels, Because any Tip on the piston = tighter clearance, Top one side bottom the other. Cheers Bill.

 

[LoneStar]

Bill,

The new pistons are Carrillo 8:1, same CR as previous - I never experienced excess vibration with that CR in the past. I think the factory sent 8:1 CR bikes to markets where decent gasoline was available, like the US. But these are forged rather than cast - given the fate of the previous ones, the extra strength is welcome. The clearance specification is .003, but I'll be running .005 based on advice from others and from an abundance of caution.

I don't know the balance factor of the engine pre-disaster. I didn't check it when installing the pistons years ago, and didn't bother to try determining it when taking everything apart recently. I expect it was near to the factory 46%, since the rods were stock and the Omega pistons weigh c. 475g complete - which I think is close to the weight of original Specialloid or Hepolite pistons.

Thanks for the tip on centralizing the rods!

 

[Vincent Brake]

Dear mr lonestar.drilling is more effective when on outside of cranck

You can multiply by X/45
(45 being half the stroke)
And X the radius whre you drill.
It can go as up to 1.9 or so

I don't know if you took this into account

 

[greg brillus]

Five thou piston clearance is pretty much a max figure, unless you live in a hotter climate than me (Queensland, Australia) for a twin that is........ I run 0.0035" on all the rebuilds I do, never had a piston nip up ever. Use an old main bearing inner race, linish the ID of the bearing so it slides quite easily over the crank main shaft, then use this as a tool to work out how much (if any) shim spacers you need to shift the crank assembly to the right, and thus centralize the rods in the bores. Sometimes they do not line up exactly on both rods, so just average it out as best you can...........Good luck ......... Greg.

 

[Bill Thomas]

Dave, I thought it was interesting to show you a photo of the flywheels for sale , VOC Spares.
Although they look like they have a Big Pin ?.

 

[mercurycrest]

Thanks, everyone, for the informed discussion!
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.
Dave

I'd just like to remind everyone that Phil Vincent personally told Marty Dickerson that "Richardson' is wrong, the Balance Factor is 35%" when Marty asked him why all the stock cranks he checked weren't 47%.
Cheers, John

 

[Bill Thomas]

Here, I have just had some fun.
Taking Dave's 1208 g which needs 604g for 50% balance.
Dave says it's 113 too heavy, So adding 604 to 113 =717g.
Which means at the moment, As is !, It has a balance factor of 59.5%, Which I make 718 !! Close ??.
So after what we have been told by Bob Dunn, Was that in Oz Review ?, It should be smooth when rode fast ?.
Just going for a lay down !, My head hurts .

 

[timetraveller]

When going for the low clearance pistons it is really important to ensure that the base of the cylinder barrel is exactly at right angles to the bore. If the tilt is fore and aft it wont matter. If it is sideways then you will quickly have a piston melt down. Every one that I have checked, or seen checked, was out by several thou.

 

[Peter Holmes]

Slightly off subject, I wonder how far out of true the crank can be to be useable,
With the 560 I want to build, Just in case I balls it up !.

I know you will not be the first to do this 60cc enlargement, and I know that 50cc can propel a moped/motorcycle along reasonably Quickly (no pun intended) but in Comet terms what are you hoping to gain, will it turn a Tortoise into a Hare.