multi cyl ic engine balancing

[ozzie46]

Can some one point me to info on balancing a 4 cyl inline engine? I have been searching and the only thing I came across was something ETW wrote in ME back in the 50s. I found a lot of info on full size engines but don't know if it is useful for model engines.

 Specifically, how to balance piston, rod and crank throw.

 Thanks Ron

[Longboy]

What you find for full size engines can....or may not apply to models. Todays inline 4's incorporate balance shafts to tend to secondary imbalances. I have read that 2 litre  4's was about the limit in the old days as larger displacement engines would have heavier reciprocating/ rotating components bringing on those secondary undesirables. I don't know of any modeler's inline 4's using balance shafts.  On a model it probably isn't necessary because of physical size. You can beat some of this secondary vibration with some beefy crank web throws and flywheel on your model as well as a light weight piston/ rod assembly. As for primary balance, you have a two pairs of crank throws at different lengths ( three main bearing engine) 180degs apart as well as their pistons and rods. You already won as this relationship is a primary balance. https://en.wikipedia.org/wiki/Inline-four_engine

[ozzie46]

  Thanks for the reply Longboy.

  So I just need to make sure the piston/rod assemblies are the same weight then?

  Ron

[gbritnell]

Ron,
I have my small 4 cylinder OHV engine and the much larger Holt engine, 4 cylinders also. With a flat plane crank basically the weight of the components, pistons, rods and wrist pins, going up in one cylinder offset the weight of these parts going down in another. That's not to say that this condition makes for perfect balance as other factors are involved but with the small reciprocating mass in a miniature engine they aren't going to vibrate like a full sized engine. My 4 cylinder OHV engine is capable of about 7-8000 rpm and it's quite smooth. (.750 bore x .875 stroke)
gbritnell

[ozzie46]

Thanks George. The engine has a flat plain crank with 3 main bearings, .875 bore, .750 stroke overhead cam. So this is all good to hear that I don't have much to worry about as far as balancing goes.

I have Steve Hucks Lil Demon plans that I want build in the future so I guess things change for V-8s then? Especially since I would like to scale it up to .875 or 1.000 bore. Mainly to get away from the 0-80 and 1-72 screws. Don't know if that will actually happen as I have to learn how to scale it.

 Ron