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Discussion Starter · #1 ·
I thought id post some info i got on the Brit Bike site. might be of some intrest to some of you.

John Healy - Moderator

From information supplied to dealers I got the following information about non-unit 1953 6T modles: The information indicated that they used a 68% factor not 85%. 85% is for 650 and 750 unit construction twins.

The weight of an original 1953 6T connecting rod (with integral big end bearing) is:
upper half 95 grams
lower half 275 grams.
Total weight of rod 370 grams.

The math would go like this:
TAKE THE WEIGHT OF THE UPPER HALF OF THE RODS FROM THE TOTAL WEIGHT OF THE RODS TO GET THE ROTATING WEIGHT thus:
Total weight 2 connecting rods 740 grams
Total weight upper half 2 connecting rods 190 grams
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Total ROTATING WEIGHT 550 grams

ADD THE UPPER WEIGHT OF THE TWO RODS TO THE WEIGHT OF THE 2 COMPLETE PISTONS thus:
Total weight of 2 pistons complete 560 grams
Total weight upper half of 2 rods 190 grams
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Total reciprocating weight 750 grams
compromise percentage x68%
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68% of reciprocating weight 510 grams
ABOVE WE MULTIPLIED THE RECIPROCATING WEIGHT BY THE COMPROMISE PERCENTAGE OF 68%

BELOW WE ADD THE TOTAL ROTATING WEIGHT TO THE 68% RECIPROCATING WEIGHT
Total rotating weight 550 grams
68% reciprocating weight + 510 grams
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Total weight to add to journals 1060 grams

1060 divided by 2 = 530 grams to add to each journal to balance.

Now your weights may vary! I weighed a period 650 piston and got 289 grams each. So instead of using 560 grams in the formula I would have to use 578 grams. If you were using a later rod that used a insert bearing shell, the weights of the rod would be different and you would have to adjust the figures used for the rod in the formula.

All good stuff, but I would tell my crankshaft balancer to use 68 % of the reciprocating. I would also be sure that he is used to doing single and verticle twin engines as the theory for doing the work differs from balancing most automotive engines. You are not actually balancing (except for what is called the couple, or side to side, where one side of the crank is a different weight from the other), but moving the imbalance in a manner that is less offensive to the rider.
john
 

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dude, i'm gonna have to have my physics major room-mate decipher that for me. but thanks for posting it.. !!!!


kick ass as it's kinda hard to find info on britt bike repair. i wished i lived in california or arizona, out here in florida, only a handful of shops will work on british bikes.
 

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Discussion Starter · #3 ·
no problem

in other words, balance the crank with the correct percent of reciprocating weight. 68% -preunit 85% -unit Vincent 46% i think is Harley 50-51%
 
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