Some questions on balancing

mattrgray

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Joined
Aug 30, 2017
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103
So I've had this same engine for a year, my first engine and its going good. High comp head, porting, expansion chamber, porting (haven't matched the transfers on the case yet) and with a 44 tooth sprocket it does over 40mph at 10000+ RPM. It seems that I got a very nice crankshaft from the factory with very little wobble, but my bike vibrates like hell, and I mean the kind of hell where your feet go numb after 30 seconds of riding at high RPM. I have a cutout in the piston skirt so the intake port is fully open at TDC and I drilled a small hole with a channel going down to the wrist pin. I've seen people say things about balancing crankshaft and pistons and drilled into the crank, so what is this all about? Is there a forum going into detail on this? Please let me know anything you guys know, thanks, -Matt.
 


machanic

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Jan 6, 2014
Messages
354
Looking for the same thing myself, I have my engine apart at the moment and see little attempt at balancing the crank by the factory, there is a small void in the bolt on flywheel weights but it looks more like a casting flaw. Found a spreadsheet calculator for balancing by Jaguar in the resources but I could not get it to download.
 
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FurryOnTheInside

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Sep 23, 2013
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3,938
I'm surprised that there are engines still being sold with the bolt on weights. I thought that was all done with now.

The general advice for balancing on Jaguar's dragonfly site, I took to be referring to the generic no-name (eBay etc) engines that everyone outside north America seems to be stuck with.

I took a leap of faith as I knew I wouldn't want to keep stripping down the engine in the first test rides. So I drilled radially into my one piece crank weights 3/8" holes about 15mm deep (measured halfway in the radius to account for the pointed drill bit tip) and lightened the piston by just 1.5 grams. I did lighten the upper section of connecting rod slightly but not much and it's impossible to weigh without splitting the crankcase so idk how much exactly.
I thought these to be conservative modifications to give me a starting point for future fine tuning.

Anyway I don't know how much worse it would be without these mods but I didn't care to start from zero, and my bike's vibration is not unpleasant at any RPM, just a soothing massage really. :)

Could be partly because of my retarded timing key, rigid fork and unsprung saddle, of course. No undamped springs on my bike; and the engine is very rigidly mounted in the frame.
 
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6blueoval9

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Aug 28, 2017
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1,120
I'm taking a rather daunting approach to lighten everything..a bunch of little details that by themselves seem a worthless attempt but combined together give just that little bit more.

Simple things such as smoothing every and all sharp edges on the piston (excluding outside upper edge)..just taking that edge off everywhere adds up not to mention this is added insurance to stress crack failures ..The main lightening of the piston will come from small dimples drilled at the underside of the piston dome area, spaced apart as close as possible with as many of them as I can fit in there...Not sure just yet what size bit I'll use but I'll remove about a millimeter of material or more (depth) with each dimple.

My rod will get both sides ground down then semi knife edged. ( Not the journal sides)..The journal side edges will get smoothed to also help prevent possible stress crack failures.

The crank I'm not sure yet...it's supposed to be a very well static balanced crank but I'm not so sure removing from the rod and piston won't throw that balance off to a certain degree.

I guess if I need to spend the money at a local speed shop to re balance the lightened assembly then that's what I'll have to do.
 
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