E: Engine Crankshaft Balance Method

Bill Thomas

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My Old one is no Tortoise, In road trim without a fairing And me 6 foot plus, Was Timed at 99.8 mph
At the end of a Flying 1/4 mile, But it was Blue then :) .
It's just a bit of Fun, Most people think the poor old Comet could do with a bit more punch,
Although I think it's more the gap between gears.
It's something I have always wanted to do, Not sure I am good enough ?,
And it may go on to be a pipe dream, And never get done.
Not as keen as I was !, I don't Need another Bike.
Plus I just like looking at the Bike as it is, Lots of bits from other Bikes I have Raced,
I don't want to take it apart, I think I am going Crackers ?.
 

Peter Holmes

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Bloody hell Bill, I think that time would embarrass my Rapide, but I never feel the need to those speeds anymore, I used to cringe watching the road going twins at the Ramsey Sprint, all I could see was £s going up in smoke.
 

Bill Thomas

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Bloody hell Bill, I think that time would embarrass my Rapide, but I never feel the need to those speeds anymore, I used to cringe watching the road going twins at the Ramsey Sprint, all I could see was £s going up in smoke.
Me too now, On my Pension if one went Bang, Can't see it ever getting back together.
You must remember Tony's Comet with a Rod out the front ?.
And Peter C from Oxford with a Viscount did his cases at 30 mph !!.
So as you say mine is a pointless job, I will never beat what I did with my old Comet,
I threw my Leathers away a few weeks ago, Not that I could get into them now :) .
And I can't see me running against clocks on an airfield again.
So there is no hurry !!.
 

LoneStar

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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.

Greg,

Thanks, good advice. On the clearance, I'm in Texas and it gets above 100F quite often in the summer. I'm influenced by a fellow Texan who seized up a Comet with these pistons, running the specified clearance. Of course there may have been other factors there, but there's no big down side to having a bit more clearance.
 

LoneStar

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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%.

Yes, all very confusing. PEI in 'Motorcycle Engineering' also quotes 35%. On the other hand, sworn testimony of many builders since then tells us that 46% - 60% is the range where vibration is best controlled, with stock frame. As a decision is necessary, I've chosen 50%.
 

LoneStar

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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 :) .

Bill,
I don't think you can reverse-compute the old factor like that. The engine had original Vincent rods, but now Carrillo are installed. These have higher overall weight (660g vs. 620g) but lower reciprocating weight (187g vs. 226g), and both factors influence the balance.
 

Bill Thomas

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I told you I was rubbish, I bet if you just took a bit off, Like opening out the holes like David Bowen told us ,
No 4 Of this thread, It would be enough, But I have already said too much. Cheers Bill.
 

Dinny

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I run my twin in Reno, NV, It hits 100F in the summer at 5000 feet. My low expansion pistons have a 4thou clearance with no issues.

Maughans did my crank and I want to say it was about $450 dollars to rebuild/ balance. They shipped it back to me and it cost $120 and was well packaged.

It might be worth asking Maughans what they use as a balance factor?

He also did my A crank because I could not see why I would use anyone else. Taking the twin crank home in checked luggage was interesting though!!

Cheers
Mark
 

vibrac

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Ran the comet racer on four to six thou for years never had any seizeure however there is always my lubrication advantage to consider :cool:
 
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