Engine balancing

Has any of you gone the extra mile of engine balancing when rebuilding an older engine?

Besides weight matching of rods and pistons (which can be easily done at home), would the extra step of balancing the crankshaft be warranted? Supposing that the factory performed a crankshaft balance at producton time, in this case a 1984 bike, would anything throw the crankshaft out of balance?

Would balancing the crankshaft at home (via spinning on high quality bearings) be a viable solution?

Any thoughts on this subject are apreciated.
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http://fj600.blogspot.com

 

I suppose you could go as deep, (into your pockets), as you want to when rebuilding an engine. You could do as much balancing, blueprinting or porting of the heads as you want. But the biggest question you have to ask yourself is, "What am I going to use this bike for?" If it is for drag racing or raced on a track then I say go for it, do all you can do to squeeze every bit of horsepower that you can get out of it. But if your answer is to ride it to and from work, run a few small errands, ride with a few friends, etc. I say spend that extra money on new chrome, new paint, new...

 

Well, I use the bike as main means of transportation, but no racing or anything fancy like that. I just happened to get a spare engine cheap, and want to rebuild it. I have time, since it's spare, so I'm riding the bike now, as I rebuild the other one.

The reason I ask is because since the engine is open, and I'm doing thing to it, why not take a few extra days to do something extra. I do not mean to put more money into it, but try to do all on the cheap at home.

I plan to get some high quality bearings (apec-7 or apec-9) and see if I can do crankshaft balancing that way. For sure I'll do the static balance (match weight of pistons and rods).

It's a learning experience for me, this spare engine rebuilding. Some things I decided to do only because I haven't done it before and am curious of the outcome.

Thank you.

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http://fj600.blogspot.com

 

I worked in a manufacturing facility that produces crankshafts exclusively for the likes of the Viper and new Z06 LS& powered Corvette.

Take it from me how you are talking about balancing the crankshaft is a disaster in the making.

Furthermore if they were balanced once they do NOT need to be rebalanced unless machine work has been done and material has been removed from areas other than the mains or concentric O.D.

Balancing cranks is a fine art. You must have the proper weighted bobweights, you must know your tolerance and exactly where the weight is out of spec at.

One would be surprised at how accurate crankshaft balancing truly is. I have personally seen it done down to .299 grams... And this is on production machinery with $5K plus balance masters and $5K plus bobweights.

Also no two balancers will read the same... so keep this all in mind...

 

I would have to question if it is necessary. Since these engines are designed to run at a redline of 9500 RPM some balancing of the rods and pistons had to have been done. Mike Oberle has shown that porting is an option that does improve the intakes. He is know here as Injuneer. If anyone has pulled and weighed the rods it would be him. You can PM him for more info.

 

If you want a motor that purrs and is smooth at all revs you would definitely weight match the reciprocating parts.

On the recent rebuild of my Turbo I took about 8 grams out of each piston. I don't recall the exact weight. I'd have to look in the notebook. They were within .3g with about an .5g spread. I added some oiling holes to the wristpin journals and piston skirts while I was at it...

The rods, as I recall, were within .5g with 1g total spread. These are static weights by the way.

The cranks have a decent balance (mine did). But if you have the crank ground for oversize bearings (undersize journals) it should be balanced again.

The home static balance is probably not a good idea. You may achieve a good balance but the harmonics introduced during rotation are a wholly seperate problem. A dynamic balance of the crank and static weight balance of the reciprocating parts is a good combo.

 

Ryan, thanks for the info. BTW, I thought that bobweights are only used when balancing V engine, not inlines. In anycase, in the light of what you mention,I don't think I'll attempt the crankshaft balancing myself. I would try to at least test for balance on a "bearing" device that I plan to build. There's a guy who builds such devices for balancing tires.


Injuhneer, I take your advice into consideration. However, would it not help to at least weight match the rods and pistons? Surely a dynamic balance would be superior, but would a static balance actually hurt?

Thank you all kindly for your replies.

 

Well the bobweights are used on all the cranks we balanced. The Sister Shop, Callies Perofrmance Products, also uses bobweight on their cranks.

I believe they even use the bobweights on the 4 Cylinder Lycoming aircraft cranks...

Now if I remember correctly they are weighted according to the piston/rod weight. Also the added weight will help amplify and unbalance.

Like we said, unless your doing some crank work I wouldnt try balancing it.

WHen you talk about balancing down to .3 grams it would be very hard to detect it staticly. Especialy the angle. When balancing cranks if you remove to much material from a c'weight then you start to get into real trouble. Unless you have some Heavy Metal (Basicly carbide/tungsten powdered/pressed material) you can forget about it...

 

You don't need bobweights on a standard inline 4. There's always 2 up and 2 down, so as long as the pistons and rods are balanced to each other, they "bobweight" themselves.

 

So how do you dynamic balance a crank? And one thing came to mind about static weighing the rods. How do you decide from where to remove the extra weight?

 

Well, I'd like to hear how a professional does it. Moreover, I'd like to know the principle used by the balancing machine to determine where and by how much an adjustment is to be done.

If I were to build such a machine I'd do the following. If you think about a wheel, with a weight attached to one spot. that wheel will rotate around an axis, but the force felt on the axis will be higher on the other side of the weight in question, and pulling in the direction of the weight, due to the centrifugal force. This force present on the axis can be measured by a sensor. If you know the diameter of the wheel and the rpm, then you can calculate the speed of travel of the weight, and thus calculate the size of the weights. The adjustment would have to be done by removing some weight there, or adding some on the other side. For a crankshaft I imagine that such sensors are embedded in the two ends which hold the crankshaft in place. I believe that the trickiest part of such machine would be the actual sensors; they would have to be very precise in measuring that force induced by a heavier part of the crankshaft. Once you have the force measured, even the tiniest calculator could be used to calculate the size of the weight that needs to be added/removed. As for location, again, the sensors would tell clearly the direction of the force. The rest of the machine is just the supporting machinery that implements rotating the crankshaft with a controllable speed.

If I were in anyway connected with a racing team/shop, I'd give it a try to build such a balancing machine. But for my "one time" need I'll just close my eyes and do only a static balance.

 

Ohmega, I know a good engine shop in Toronto that specialty is race engines. They could more then likely dynamically balance your crank for you.

 

You're pretty much right on how they work, ohmega. The crankshaft is placed on two V-blocks with a belt around the center main journal, and a magnetic sensor is placed on the front end of the crankshaft. You program in the counterweight configuration and size, then the crank is spun to about 150-500 rpms (depending on the machine), and the sensor measures the imbalance. I'm not an engineer, so I can't tell you how exactly the sensor works. Most modern balancers will then tell you exactly where to drill, and when you program in the drill size, exactly how far down to drill.

For the rods, you need to balance the small end and large end seperately, as the large end is rotating weight, and the small end is reciprocating. You can remove material from the top or bottom of the rod, but on these motors there's not a whole lot of material there to take. But as Injuhneer stated, the balance should not be off by much. For the pistons, most of the time you can remove some material from underneath the crown of the piston (be careful of the valve reliefs though, a piston too thin can be disastrous), or sometimes from the lower side of the pin boss. Depending on skirt thickness, you can sometimes shave a bit off of the inside of the skirt. Not having one in front of me at the moment, I can't tell you 100% where I would take the weight from.

I would be interested to know how you got 8 grams out of the pistons, Injuhneer! Did you happen to take some before/after pictures? That's a pretty significant weight savings on such a light piston to begin with.