Not sure why I elected not to get this done when the assemble was balanced. Does side the side clearance take off enough weight to make a difference in the balancing?

Motor is a half bridge 20B that I still haven chose a builder (or I'd defer to them...).

 

I personally would go ahead and get the rotors side cut.
I have had 2 engines fail from rotor tips striking the side housings and sticking the side seals in.

Usually the machining I have seen (and done) is on the rotor tips just outside the area of the holes drilled for balancing.
Just taking off material here should take weight off the rotor quite evenly (if rotor is chucked up in lathe on center) as you are machining material off of surfaces that are already all machined to the same axis. That is Rotor face is machined to the same axis as the rotor flank.
As long as you don't get into the area where the balancing holes is you should be taking off the same amount of material on each rotor tip.

In this way the rotors should stay balanced to themselves.

I have seen some rotor flank machining that cuts the whole rotor side up to the oil seal lips for a side cut. I wouldn't do that. I have always seen just the rotor tips strike the side housing from wobble or tilt- I haven't had the whole rotor flank rub the side housings.

Now, your machinist must make sure he takes off the same amount of material from each rotor to keep the rotors balanced to the other rotors.

There is no way to keep the 3 rotors balanced to the front and rear counterweights/e shaft (if you had balanced rotating assembly versus just balanced rotors), but the amount of material removed from the rotors for rotor tip side clearance isn't much. I would imagine its within the fudge factor of the balance just like trying to account for how much liquid oil versus foamed oil is in the rotor when accounting for oil weight within the rotor.

 

Rotor weights were within a few grams, but not matched perfectly so I'm not going to get worked up over that. Mainly wondering if it's going to throw off balancing of the counterweights and if it's worth $500 to get that redone...

https://www.rx7club.com/attachment.p...0&d=1400896299

 

You're not removing all that much metal.

Hell, I clearanced my rotors by chucking them onto a brake lathe and hitting the sides with an angle grinder. After 50,000 miles of abuse on an UN-rebalanced assembly (normal redline 9500 in competition, 10000rpm at dragstrip, have missed shifts that saw "higher" than that, and then there was the event where I broke 3rd gear so I had to keep it in 2nd, accelerating past the 10k RPM tach's peg 3-4 seconds before the end of a straight...), the rotor bearings looked just like they did when I'd installed them. And no evidence of hitting the side housings.

Good enough really is good enough.

 

Thx! Will defer to the builder but this is reassuring...

 

For reference, the amount of material you remove when clearancing is going to be only slightly more than the amount of metal that will wear off of the side and corner seals between rebuilds.

The amount of oil sloshing around inside the rotor has MUCH more of an effect, and the amount of oil in the rotor will depend on how fast the engine is turning and how aerated that oil is.

That's basically why I never bother getting a rotating assembly balanced.

 

you also have to take into account the weights of the things. with a piston engine, they balance to the gram, but a piston is like 300 grams, or something. with a rotary the rotor is 4500 grams.

so one gram is such a teeny percentage that it isn't going to matter much, especially when you unevenly cover it in carbon and then sometimes fill it with oil and slosh it around.

or good enough, is good enough. the exception would be if you're road racing AND trying to turn some exceptionally high rpm.

 

yes