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+--- Thread: Balancing Conrods (/showthread.php?tid=5317)
Balancing Conrods - Jamie - 27-11-2020
I wish to balance the conrods on my three-bearing engine before assembling it, primarily to reduce avoidable vibration. The flywheel and crankshaft will balanced as a unit.
Three of the rods were of a very similar weight and by smoothing casting lumps and seams, they are all now 185 grammes. However, the fourth rod, a replacement, weighs 207 grammes or so. I have ground off any casting lumps and seams, but this has only reduced the weight by about a gramme.
I am worried about taking off too much metal in the wrong areas and weakening the rod. Can anyone advise me how much metal can be safely removed and from where so that I do not compromise its strength? Failing that, I shall leave this rod as it is and place it away from the centre bearing.
Regards,
Jamie.
RE: Balancing Conrods - Zetomagneto - 27-11-2020
Are you balancing end to end, or just overall weight, if the latter increase the weight of the other 3 by
the use of washers under the BE nuts
RE: Balancing Conrods - Howard Wright - 27-11-2020
Hi Jamie
As Zeto says adding weight to the others is the way to go. However making up that difference needs more than just washers. I had one rod lighter than the others and made up this special washer to add sufficient weight.
Cheers
Howard
RE: Balancing Conrods - John Cornforth - 27-11-2020
As a guide, the density of rod metal is about 7, so 21 grams will have a volume of 3 cc or 3000 cubic millimetres.
That's about the size of a big lump of sugar and sounds like rather a lot of metal to be removing.
When I rebuilt my 2 bearing engine, I didn't know that there were early and late pattern 2 bearing rods with different weights, about 320 and 370 grams respectively but with some variation. I only discovered this after weighing my newly re-metalled rods and finding one light one
. After contemplating filing off metal or attaching lead weights, the solution I went for was to buy another late rod.I then ended up with rods of 361, 363, 370 and 372 grams, so I placed the 370 and 372 at cylinders #1 and #2, and the 361 and 363 at cylinders #3 and #4. This has been successful.
My notes from the time tell me that a big end bolt washer weights 0.25 grams, a gudgeon pin weighs 37 grams and a piston weighs 135 grams.
RE: Balancing Conrods - Chris KC - 27-11-2020
As John suggests, I'd look for another rod - 21g is a lot to remove.
I don't think placing it 'away from the centre main' is going to help you in any significant way.
Not saying don't use washers - it's a way to achieve some measure of balance - but if I needed another 20g of material I'd rather it was also adding something to the strength of the rod rather than simply adding to the applied stresses.
RE: Balancing Conrods - Colin Reed - 27-11-2020
HI Jamie,
You need to weigh both ends of the rods separate ie support say the small end and have the big end on the scales and vice versa
If as you say you are having the crankshaft and flywheel balanced they will need the the weight of conrods including gudgeon clamp bolt because they will need to know this to apply the correct balance weights during balancing
this is why they will need the rods to check the weights
Colin
NZ
RE: Balancing Conrods - Jamie - 27-11-2020
Thank you for all of the replies. I am only trying to balance the overall weights, not the end and weight of the rods. I confess that the thought of removing that volume of metal fills me with apprehension. It had occurred to me that I might be better off to look for another conrod. I can see the adding weight to the other rods is one way of balancing the weights, but as has been observed, this will add more strain to already flimsy components.
I suppose the next question is: does anybody have a 3 bearing conrod for sale?
Regards,
Jamie.
RE: Balancing Conrods - Chris KC - 27-11-2020
In a happy world you might find someone with 3 heavy rods and 1 light one!
RE: Balancing Conrods - dickie65 - 27-11-2020
Is that Nippy or Ulster rod in the piccy with the custom washer ??
Hi Jamie,
I should have some three bearing rods in hiding about 25 miles from home though,
I think that there are a couple of three bearing crankshafts in hiding as well.
I have no need of three bearing bits anymore as my special with a three bearing engine was sold 12 years ago.
If its dry at the weekend I will take the 65 up to have a look.
I will have a look in the garden shed as well, just remembered I bought so oddments which are hiding in the shed.
RE: Balancing Conrods - Howard Wright - 27-11-2020
Hi Dickie
Yes well spotted. It’s a Nippy rod.
Cheers
Howard