Question of balance

[aobp11]

Hello David,

Yes that's the way to go.
With your data at hand and using my own con rot data I tried to verify results. Annoyingly I find deviations that are too large to ignore. I might be wrong of course. Hence I show my derivations in detail.

You should have con rod 010312, same as for the other 1954 models (and G3LS/G80 in at least 1955 and 1956). For that con rod I measured 198 g small end weight. (Almost same value as when taking 1/3 of total con rod weight.) I suppose your 70 g measurement was with the complete Hepolite piston hanging down. I assume the piston weight is 510 g (earlier you mentioned 530 g ?).
The magnet centres are at most 10 mm outside the flywheels I think. I take 10 mm, that makes 108.4 mm from flywheel centre, as compared to 46.5 mm for the bigend. Ratio: 2.33 to convert magnet weight to the bigend position. Gives 2.33 * 70 g = 163 g pulling upwards at the bigend.
In your equilibrium the total reciprocating weight hanging there is 198 g + 510 g = 708 g. So 708 g - 163 g = 545 g is available for compensation of the designed total reciprocating weight. With 66% balance factor this translates to compensate 545/0.66 g = 826 g total reciprocating weight. For the piston that leaves 826 g - 198 g = 628 g.

On the other hand you measured that your piston is 59 g too light for 66% balance factor. So you would need a piston of 510 g + 59 g = 569 g to arrive at 66%.

The difference between 869 g and 569 g is quite large.

The other way around, suppose 569 g is correct for 66%. When you mount such a piston the not-compensated for weight is 0.34 * 767 g = 261 g. For balance with magnets that would require 261 g /2.33 = 112 g. But your actual piston was 59 g lighter which makes the magnets 59 g /2.33 = 25 g lighter. That leaves 87 g for the magnets. Again a quite large difference, now between 70 g and 87 g.

Taking 530 g for your Hepolite piston makes things even worse. So does taking taking a smaller distance to centre of the magnets (giving ratio smaller than 2.33).
Taking less weight for the small end helps somewhat but even reduction to 150 g is not enough (gives 608 g in stead of 628 g).
I wonder which weight you used for the small end and which intermediate measurement result(s) you found following the "official" procedure.

Albert

[Dave T_LAPSED]

Albert, please forget the magnets! I was wrong to use them to measure the balance( rest assured I never had them inside the engine!). The method I have used is Phil Irving's and that uses the total weight of the piston etc. and the little end weighed horizontally whilst attatched to the flywheels.
I feel quite confident that I have it right now, thanks.
Dave

[aobp11]

Hmm, the magnets were not the nicest way to follow, but nevertheless their weight can be used.
But if you feel sure about your Irving results then just forget my remarks. I don't know how CS pistons compare to "ordinary" pistons. Your Irving result looks quite reasonable when I think of my 18S pistons, while that 628 g would be very high.
Success,
Albert

[Expat]

Aarrrrrrrrrrrrrrrgh!!!

[Dave T_LAPSED]

YUP

[Dave T_LAPSED]

In the end I found a nos wirewound +20 piston in the good ol' U.S of A .

GEDC1316.JPG

It needed this to balance.

GEDC1313.JPG

and also it fits the bore lovely (2 thou gap)

So I am a happier little puppy, on the first ride up to 50mph it just tingles my feet! Thus fortified I look forward to a more pleasing association with this product of AMC's finest.
The weather looks fair, just off for some running in.

GEDC1324.JPG

More or less finished article above.
Cheers everyone, Dave.