Crankshaft Balancing???

Joined
May 12, 2019
Likes
5
Location
Annapolis, Maryland
#1
Hey,

This might seem like a silly question, but do the 701 style crankshafts need to be balanced, like a 4 stroke crank, when upgrading/rebuilding? I'm still learning when it comes to these 2 stroke motors but I have plenty of experience with 4 strokes. I ask because I am looking to do a big bore stroker build this winter and I have yet to see any mention of crank balancing on the forums.
 

yamabro

Team Faith
Joined
Mar 28, 2007
Likes
127
Location
Dickson, Tn.
#2
No.
The 2stroke cranks are pressed,... so balancing would be impossible due to the rods being slung around.
Some builders do true the webs and Static balance them to insure that the webs weigh exactly the same.
 
Joined
May 12, 2019
Likes
5
Location
Annapolis, Maryland
#3
Interesting. I have read and seen a few videos on truing them up. I will deffinately be looking into that more. If you or anyone else, have any good links to info on crank rebuilding and truing them up, please post them. But from the sound of it, I should actually be able to do that all myself in my machine shop at work. Looks like I might actually be able to do my winter big bore stroker PV build without breaking the bank.
 
Joined
Feb 16, 2019
Likes
8
Location
Western NY
#6
I always had a problem with that method of truing cranks, if you put your dial indicators right above your point of contact in the v block, or the rollers in this case, most of what your measuring is roundness of surface your indicating on. To measure runout with this method you have to offset your indicators axially along the crank from the point of contact with the supports. I realized this is the same method described in most service manuals etc, but to me seems incorrect. Last time I trued a crank I just put it in a lathe, made sure the first web ran true, then measured runout on the second web. I was pretty sure also that people did balance two stroke rotating assemblies with press together cranks. Haven't seen people talk about it in a 701 or anything though more of a high revving single cylinder thing I think, like a yamaha kt100 or something. A twin should run a lot more balanced to begin with.
 
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