Hi Drew,Personally i think this realm of expertise is best left to the experts.The balance factor usually for an M20 is 60%,but how this is achieved I cannot answer.
Unless you have made some serious alterations to your motor,I cant see why you need to re-balance the crank.
North Basset Downs Crankshafts will balance it to exactly what you tell them
Cheers Rick
email (option): richardholt@rocketmail.com
Whatever the balance factor the engine/frame assembly will always vibrate as it is impossible to achieve 'perfect' balance with this engine type...
It's just a question of where in the rev range and how badly it does so as the frame reacts to the vibration frequency produced by the engine...
The BSA balance factor was actually 58% but 60% is an easier figure to work with...
As Kieth says, there is no absolute requirement to rebalance the assembly unless the piston weight has been changed by more that 1 1/2oz.
However, BSA did fit these engines, all with the same balance factor, into different frames...So the level of vibration may not be the same for all models..
Though finding out what is the best balance factor for any given frame would become a time consuming exercise I would think...
So, ignoring the above and getting back to the 'standard' balance factor I have found there can be some benefit in rebalancing a standard crank assembly.
Originally they were assembled with a production tolerance and no doubt there were times that was not strictly adhered to...
I have had standard assemblies rebalanced to the 'standard' balance factor and it has still been necessary to remove material from the flywheels to achieve this accurately....Ian
email (option): ian@wright52.plus.com
Hi Keith,
If it is one of Ian's pistons,
it weighs about 460Gr (Asssy) which is close to original M20
+20 downsize piston.
I've just finished rebuilding a flywheel with NOS bigend pin, rollers and conrod,
And took it to be parallel fixed and balanced in a shop.
The guy was serious and took the piston and all for the balance.
I should pick it up tomorrow.
Hope it will be OK and not vibrate worse than original...
Noam.
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email (option): noam10@gmail.com
You can not have too many pliers Noam.
email (option): spacemonkeym@gmail.com
Yes..
It's not mine..
I just used the work space of an old BSA shop.
We agreed on something like 4 Pounds for using their bench for the work.
They have allot of tools,
Most of them arn't even worth scrapping...
But it's easier to work there than in my house..
The third photo is at the flywheel workshop.
Noam.
email (option): noam10@gmail.com
I had my 720 rebalanced even though it didn't require it theoretically, based only on measurement of piston weight against an original....
To achieve the correct balance figure accurately removal of some material from the flywheels was still required...
I put this down to production tolerances, or non adherence to the laid down production tolerances, at the manufacturing stage of the flywheel assembly...
Inspection regimes in factories will vary according to the importance of the part(s) being manufactured..
Where very tight tolerances are applied to a critical component (such as some of the guided missile giro compass parts I used to manufacture at British Aerospace) every component would be checked for compliance to the manufacturing tolerances...A 100% inspection regime which ensured no defective parts 'slipped through the net'
Less important parts may be tested at levels of 1 in 10 or 1 in 50 for example and with some parts only the first and last parts made in quite large batches would be tested...
In these latter cases it meant it was possible for some parts to fall outside of the specified tolerances but this was not deemed to be critical..
These systems would have been applied at BSA and I would think it is most unlikely a 100% inspection regime would have been in place in the crankshaft balancing dept....These regimes are very time consuming and therefore expensive to operate...
It was stressed to me when I worked in Production Planning not to 'over specify' accuracy (tolerances) on components or the inspection levels for exactly that reason...
Having rebalanced the flywheels irrespective of measured piston weight I have found this engine to run quite smoothly compared to others...
I have found that worthwhile on the longer trips I have done on the bike...The extra time and expense may not be worth it for everyone though..Ian
email (option): ian@wright52.plus.com
Hello guys.
I had allot of surprises with my flywheel "Doctor"
(The guy at the workshop)
He said that "He used to manufacture light aircraft plain motors",
That "He used to build flywheels for competition 2T bikes" and more...
He said that "He balanced "Harley" engines to a point you can stable a coin on the handlebar to stay"..
The last surprise is the flywheel balancing:
I asked for a 60% balancing factor, but when I went to pick it up today and asked if he reached this factor,
He said that he did not, but "He can do it if I want to", but he "Did better"...
And he showed me that when the flywheel is on a flywheel Balancing Stand,
And there is a weight which weighs like the piston tied to the smallend bush,
And you rotate the flywheel, it stops wherever you left it...
He said that he "shaved" (actually, drilled) about 50 grams off the wheels.
Mostly from the top, as far as I could see.
HELP !!!
IS THAT OK?
Ho yes.. by the way..
And he want about 400 Quid for drilling the bore from 84 to 90mm,
Grinding the valve seats,
Re-installing the Valve guides and the balancing.
(He re-installed the guides I put in a day before..
He said that I put it in off the alignment. ("/")
Is it even possible to put a metal guide into a metal tunnel off-center???)
And the barrel was in the oven 150C, and the guides were at -18C FOR A NIGHT
Before I hit it in..
Thanks,
Noam.
email (option): noam10@gmail.com
Hi Noam,did he do it like this?
https://www.youtube.com/watch?v=3tQptjetY4M
cheers Rick
email (option): richardholt@rocketmail.com
Hi Noam,here is another with a bit more explanation,
https://www.youtube.com/watch?v=Jn50F_yKW0s
cheers Rick
email (option): richardholt@rocketmail.com
Rick, that is dynamic balancing, in those videos.
Static balancing more apliccable (and easily done whithout an expensive machine!) on singles, I weigh the small end same as in the 1st video, add the weight of the piston, and then take 40% off that figure, if you want a 60% balance factor.
Take some nuts/washers, and attach to the small end (with a piece of iron wire) till you have exactly that weight, and then use the knife edge setup (round bar works good too!! but turn an adaptor if crank's mainshafts are a different diameters left/right) and see if the crank stops at any position, drill out, Always on side of flywheels where heavy spot is.
Thanks to Ed Abbot, for explaining it to me, a couple years back! (there is a post about it here on the forum, but cannot find it now)
Cheers,
Lex
ps, you can read theories and books etc. etc. till you get more and more confused about the subject!!! the above worked for me!
email (option): welbike@***bike.net
My understanding of it is as follows...The top half of the conrod, small end bush and piston are considered 'reciprocating weight'..the big end cage, rollers, crankpin and the lower end of the conrod with the big end eye are considered 'rotational weight'....Reciprocating weight in this context being weight that is accelerated up to a speed and then reduced to stationary through a repeating cycle when the engine is running...
When balancing the crank (without the conrod fitted) a percentage of the total reciprocating weight and all of the rotational weight is used in the calculation to determine how much weight should be balanced against the bob weights...
If the conrod and complete big end are fitted to the crank for balancing then the weight added (as in Lex's picture) should be a percentage of the combined weight of the complete piston assembly and the weight of the top half of the conrod and small end...in this case 60% of that figure...
If the crank is balanced without the rod (but with the crankpin, nuts locking plates etc fitted), the full weight of the big end cage and rollers and the lower end of the rod with big end eye is added to a percentage of the weight of the upper half of the conrod, small end and piston assembly, and a weight equivalent to that is attached to the crankpin....Ian
email (option): ian@wright52.plus.com
['If the conrod and complete big end are fitted to the crank for balancing then the weight added (as in Lex's picture) should be a percentage of the combined weight of the complete piston assembly and the weight of the top half of the conrod and small end...in this case 60% of that figure...']
The above is from my last post but is incorrect and should read...
'then the weight added (as in Lex's picture') should be a percentage of the weight of the complete piston assembly minus a percentage of the weight of the top half of the conrod and small end ...
In this case 60% the piston assembly but with 40% of the weight of the top half of the conrod and small end deducted..Ian
email (option): ian@wright52.plus.com
Balancing the primary (reciprocating forces) is done to achieve a reduction in vibration but to maintain it in the vertical plane (in the case of an upright single cylinder engine) along the axis of the conrod..
The figure used (60% in this case) is a compromise to achieve the best result for a particular engine and frame combination..
Balancing to 100% is not actually desirable as it transfers the forces from the vertical to the horizontal plane...itself not a desirable thing...
The fact is this subject is extremely complicated and variations caused by the number of cylinders, cylinder layout, conrod length etc. etc. etc. can all affect the final calculation...
I have read up on the subject in the past and have to admit I didn't take in and understand all the theoretical aspects of it...
It certainly can't be explained in any depth within the space available on this forum!...
The methods described in the older books etc. are sufficient to produce a reasonable, practical result for the amateur working in a home workshop with basic equipment....
If a more accurate result is required then I think dynamic balancing by a company that specialises in it is needed...
Fortunately in the UK there is ready access to such companies...Ian
email (option): ian@wright52.plus.com
Thanks allot Ian and Lex.
So, If the guy had added the full weight of the piston
To the small end (Much like in Lex's photo)
And then took about 40 gr off the top of the flywheel
In order to make it 100% balanced, (If I understood correctly)
Much like so after adding the heavy pin:
https://www.youtube.com/watch?v=HbsGGSjmpko
(This one is not nine)
It is no good?
If we talk in grams, and as I cannot tell
What it the weight of the top half of the conrod,
And I want a decent balance, (58/60% ?) BSA WM20, One cylinder, sad owner?
...
How much weight (In grams) should I hang on the small end
For a 440 gram piston assy?...
I regret the minute I walked into this workshop !!!!! 
Thanks again,
Noam.
email (option): noam10@gmail.com
Richard[/quote]
When all else fails, read the book.
What it the weight of the top half of the conrod,
And I want a decent balance, (58/60% ?) BSA WM20, One cylinder, sad owner?
...How much weight (In grams) should I hang on the small end
For a 440 gram piston assy?...
[/quote]
Noam, you'll have to weigh the small end as I described, then we can do the math, also weigh the piston with all accesories.
No use guessing at things!!
Cheers,
Lex
email (option): welbike@***bike.net
Thank you Lex !
The guy at the workshop has has the piston weight.
It comes to 440gr, I think. I can get the exact figure.
Ian also must know (Its one of his bug bore pistons)
I called the guy from the shop and talked with him.
I asked if the weight on the smallend was the weight of the piston.
He replied: "No! not at all!"
I could give an arm betting that that what he described yesterday !
Darn it's hard to communicate with him! 
I asked him what is the balancing factor,
And he replied that he does not know, but he can calculate it for me.
He said that the balancing he did should give me a "Comfortable riding"
At least up to 4000 RPM,
which the BSA does not get to work.
I guess that for the sake of my sanity, I will just let him do his thing,
And hope it comes out good and that I will be happy with it..
Last question:
As I mentioned, I inserted a NOS valve set guides into the barrel
After putting the barrel in an oven heated to 150c,
And the guides spent the night at -18c, and I carried it to the work bench
In mug filled with ice cubes,
And then inserted it as the maintenance manual describe,
With a tool I had lathed.
After leaving the barrel at the shop, the guy said that he found
That the guides were off by almost 1mm to 10cmm (0.039 inches to 4 inches)
So he had to take it out and give it to be inserted after cooled with
Liquid Nitrogen, so it's slip into place nice and straight.
Can a valve guide be driven in a crooked way?...
Kind regards,
Noam Zehavi.
email (option): noam10@gmail.com
Noam,
I incline to think he did it OK, but when you have picked it up, you can easily check on what he did, 2 knife edges of some sort are easy to rig up, and add nuts to the small end, till the assy. stops in every spot, weigh the nuts, and small end, and you know what he did.
I would do this, to be sure, it's a pain to tear it all apart again!
I deliberatly did not go into the why and what theories, as that will drive you crazy, keep it simple I Always think, maybe I am a bit too
I learnt a lot from the guy in the picture here:
Bob, from Bob's Indians in Etters PA, USA, he knew exactly what he was doing, and could explain it well too!!! (he's drilling my crank there!)
Cannot comment on the valve guides, do they have a ridge? or did he mean they were pressed in too far?? (or not far enough?)
Cheers,
Lex
email (option): welbike@xxxbike.net
Hi Lex,
I'v been trying to upload photos of how I did the work,
The special drift I used, and so on,
But I keep getting
"Hmmm. Something didn't click. See below for more details."
This guy you used looks like a real professional,
And you are lucky that he really let you know what he is doing
And explained everything..
"My" guy may be good, but is reluctant to say anything about what he does,
He says that he does not "Want to teach" the secrets of balancing
And so on..
Really hard to understand.
Both of us speaking the same language, but it is going nowhere..
Well, I will just have to wait for a good or a bad surprise..
I will be updating.
Thanks everyone!
Noam.
email (option): noam10@gmail.com
Hi Noam,take your mind off it for a while and watch the boys(and girls)building these,and get yourself a flat cap,it helps.
http://www.youtube.com/watch?v=uzYxBbitP_s
http://www.youtube.com/watch?v=-fo7SmNuUU4
http://www.youtube.com/watch?v=A4nGO3SwBqs
Cheers Rick
Thanks Rick.
Great stuff.
I love the documentaries abut factories and industry from 40's-50's,
Like the ones you sent, and from BSA and Triumph factories.
I am surprised every time from the high skills, workmanship and machinery they used.
Cheers,
Noam.
email (option): noam10@gmail.com