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I notice on their website (and ebay store) that Ultima has gone to all 3 piece cranks. At least on their 100ci motors and larger. Kind of interesting. Harley's been using 3 piece cranks for 20 years now so I guess they work. I've got one in my 120 and have had no trouble. Used to be that the 4" bore motors had a 5 piece and that was a plus for a lot of guys because you could rebuild them with standard components. I think the Ultima's (on all their 3pc cranks) use a proprietary crank pin of 1.620 or something like that. It never bothered me with my 120 because crank assemblies are not out of line cost wise with rebuilding one.
 

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The most important thing with a three piece crank that is manufactured properly is to not introduce sudden and large stresses to them.The largest of which is locking up the rear brake with the clutch engaged.Lots of reciprocating mass concentrated on one pin between the flywheels.
I believe that tapers,generally speaking and comparative sizes being considered, are stronger.
 
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