piston balancing

[harleyracer59]

so my ab07/af01 build has been running great,  except for a high rpm vibration.  I pulled the cylinder to look for the issue and put an extra base gasket to bump up the timing.  every thing looks fine internally so I thought id weigh the piston. going from 40mm to 47mm I figured there might be a little extra weight on it and that could be causing it.  the bbk is a polini honda lead kit.  

  40mm stock piston= 60g  stock wrist pin=10g
 47mm polini piston = 86g  polini wrist pin= 14g

that's a 30g difference!!!!   WTF!  Theres my vibration for sure.,...   
  how in the hell can I remove that much weight from up top??  the stock wrist pin isn't long enough to safely use on the 47mm piston.  id be happy if I could drop 20-25g.  I could accept 15g.   that's still a crazy optimistic goal.  any one have any ideas??? im all ears...

[jackrides]

Pics of inside and outside of pistons, ideally the new one with some miles on it please, maybe with a ruler across wrist pin boss. Bore of both wrist pins?

[oldgeek]

You will have to add it to the rotating mass.

www.youtube.com/watch?v=0hoE_MuFpPI&t=2s

[harleyracer59]

Apr 9, 2019 18:35:28 GMT -5 oldgeek said:

You will have to add it to the rotating mass.

www.youtube.com/watch?v=0hoE_MuFpPI&t=2s

i was trying to stay away from this option. but its the only real way i could think to solve this problem.  that or look for a lighter piston to begin with?

weird thing is my 2 kitted af05e's don't vibrate like this. one has a polini kit with same exact piston in it.  but its stock piston is a little heavier  being 41mm. ab07 is 40mm. and a smaller stroke: 37.4.  ab07: 39.3
  but still a 20g difference. (estimated)

[oldgeek]

The extra stroke is adding to the unbalance?

[190mech]

We dynamically balance aircraft props/engines by adding weights(flat washers) inside the prop spinner back plates..Special test instruments are used to find the right weight and position..Ive often wondered if a metal ring could be installed between the flywheel and cooling fan with holes drilled around the outside,then a small screw and nut randomly attached thru a hole,keep reindexing till it gets better...

[harleyracer59]

I get what youre saying. do you think I could add weight to the flywheel to get rid of this imbalance? or would you have to add weight to the variator side as well?

[190mech]

Lots of the old V-8's had 'external balance' weights welded to the flywheel/flexplate..I bet it will not take much weight to make a change..Dont think it needs both sides of the crank to have equal weight,,

[lupo76]

If, you have to take off wheight from the piston, to make it the same like stock. But you cant.

So you have to add wheight at the crankshaft, cause both of them are machined to fit each other.

There is also a reason why high class engineers develope engines the way they do. At least back in the days with these Hinda scoots. 50k miles guarantee, so to speak.

[190mech]

Just pulled a prop spinner off this morning and got a pic of a balance ring installed on some props..Kinda gives you a visual of how its done..

[benji]

You could try a different piston, and/or do some grinding inside the piston to lighten it.

[ThaiGyro]

Interesting can-o-worms, or good fun and learning experience?
Like the airplane prop above, here is a small block Ford:



and a high performance scooter crank.(Vespa, I think)



I too think you can easily experiment with adding a bit to the flywheel. I would tend to stay away from piston grinding, or crank weighting. It looks easy, but is very time consuming to do correctly.

[ThomasTPFL]

I think I’d try drilling holes in the flywheel.

[SMALL CC TEK]

You could fill the Gudgeon pin or find a heavy one ...

[lupo76]

Apr 10, 2019 23:17:10 GMT -5 ThomasTPFL said:

I think I’d try drilling holes in the flywheel.

Then you`ll have unbalanced mass on the crank itself and you WILL destroy your bearings for sure.