Balancing individual steel flywheels

[nevhunter]

Sorry to say so but that taper is a real mess I wouldn't re use the main shaft either and unless you are careful the shaft will go further into the taper and the nut will bottom on the thread.. BIG angst. Nev

[paul.jameson]

I am with Nev on that one. The mainshaft should be easy enough to replace but the flywheel will be a problem, whether finding a replacement or sorting out the current one.

[SEDoan]

You did see where I was able to get the runout down to 0.004" at the flywheel edge? Is that too much? That is with the nut at about 120ft/lbs. 0.004" is equivalent to the timing side wheel and approximately the roughness of the flywheel edge and the precision of my current fixture. The shaft is not quite bottomed in the taper. Thanks!

[nevhunter]

Mauled up tapers do not wear evenly. and will often settle to a worse position in use which will cause strain on the cases and vibration as well as timing gear whine. Unless you're racing it the iron wheels will be fine cheaper and easier to get. The crankpin in them is tapered and the rod eye bush a different size.. Nev.

[SEDoan]

Well... I've ignored all the free advice and gone ahead and attempted a repair of the mauled tapers. Sorry

First I used bluing and lapping to get the taper as true as possible - no detectable runout. It's better than the undamaged taper. This required that the shoulder below the threads be cut back about 0.025" so the shaft will be that much shorter into the primary. I did a test fit where I tightened the mainshaft into the flywheel and pressed it out again. The shaft pressed out with a small bang - convincing evidence that the taper was taking some load. I then heated the flywheel in a bbq grill to 300F (150C), dropped it on the cold mainshaft and tightened the nut. Checking the runout in a lathe collet shows virtually no runout even after all this abuse so I think it will be OK.

[SEDoan]

With the mainshaft true I could then balance the individual flywheels on the jig. With care it is sensitive to + or - 5 grams on the outside of the flywheel. The goal was to confirm that each flywheel originally had 3 or 4 balance holes drilled near the big end. Prior to filling the holes there had been evidence of 3 factory holes and their placement suggested a 4th.

Since the crankpin, rollers, and rod were not attached to the flywheel their weights had to be measured. This could then be added to 65% of the reciprocating mass of the piston and small end of the rod. Half of this mass would be balanced by each flywheel.
Rotating mass (crankpin, rollers, nuts and big end of the rod) = 802g
Reciprocating mass (piston, pin, rings, clips and small end of the rod) = 649g 65% of this is 422g
Each flywheel must balance half this so (802 + 422)/2 = 612g

I found that the flywheels were balancing less so I drilled the additional holes each flywheel could now balance about 610g. Small magnets make it easy to add or subtract mass on the pin.

I then took Alan's suggestion for a sanity check and stoned an old pin so that it is a near sliding fit into the flywheels. The picture below shows the flywheels and rod together in the alignment jig.

The picture below shows the balance parallels used. It is sensitive to + or - 5g in the pan.

I found that the old pin assembly is 28g lighter than the pin to be used so need to account for that in the pan. (the test pin has 12 rollers in an aluminum cage)
small end of the rod = 201g
weight in pan = 229g
total = 430g
subtract -28g for lighter crank pin
balanced mass = 402g

Balance factor achieved = 402/649 = 0.6194 about 62%

Given the reciprocating mass is 649g every 6.5 grams = 1% of the mass to be balanced, so adding or subtracting 6.5 grams will change the balance factor by 1%. 65% is supposed to be the ideal balance factor suggesting that the equivalent of 19.5 grams more should be removed from the flywheels. However, the piston is 0.040" oversize and I have an identical 0.010" oversize piston that is 17 grams lighter. Presumably a std. piston is even lighter (maybe 23 grams lighter?) reducing the mass to balance to 626g.
If so, 402/626 = 0.642 or about 64%
Since it will be within 1% of the original balance factor I am considering leaving it as is. Anyone see any problems with this logic or my math?
Thanks!