Engine Balance for a Two Cylinder Verticle Engine 2

[LittleRedAx]

I have a two cylinder engine. The cylinders are side by side. It is a four stroke motor and the the crank thows are 180 degrees apart. So, the engine goes fire, fire, intake, intake, fire, fire...like a john dear tractor. I am replacing the original cast iron pistons with new, much lighter weight pistons. Also, the flywheel is very heavy. My question is, what do I do about balance? and should the weight only come off the counter weights, or should the the flywheel get lighter as well?

 

[GregLocock]

Sizing the counterweights is a black art, as it is not entirely obvious what you are counterbalancing. If you are willing to experiment then re-assemble it with the original crank and see if it is OK. If it isn't take off a little of each web, REBALANCE the crank, and try again. An out of balance crank will have far worse effects than an incorrectly counterbalanced one. Plenty of engines have no counterbalance. The reason you need counterbalancing is to reduce the loads in the main bearings - one approach is to treat the engine as a pair of single cylinder engines, and work out the bearing force for each, and size the counterweights appropriately. This is crude, but it does more or less work, it just isn't the lightest solution.

What are you trying to do with this engine - you are obviously spending $$, but trying to turn this old sows ear into a modern Japanese motorcycle engine is going to be tricky?

The flywheel size may be doing the rest of your drivetrain a favour as its high inertia will absorb a lot of the destructive vibration from the crank. Making it smaller is a separate decsion from the counterbalancing. Again, make sure it is balanced before you run it again.



Cheers

Greg Locock

 

[evelrod]

I tried to ring up my balance shop earlier today but, no luck. I have never had a problem changing from one piston mfgr to another as far as balance is concerned . The only cast iron pistons I ever saw was in a flathead Plymouth/Dodge 6 and my dad replaced them with aluminum. I don't recall any BIG balance problems.
I did the balance on my mini a few weeks ago and paid no attention to the crank aside normal balance. The rods were balanced gross and end for end, the pistons were balanced seperately. Net result, the bugger spins up rather nicely.

Plus---I have driven a 30's John Deere and rode a Honda 350 Scrambler---you have a problem if you want smooth, IMO.

Rod