Hi

Does anyone know about rotor balancing.

I am rebuilding my 3rd gen motor (TT) after a nasty surprise when I opened her up. Both rotors were damaged and eccentric shaft so both have been replaced. At the same time I wanted to change the flywheel to a lighter one.

I have been told after trying just about every RX7 tuner in the UK that balancing is rare in the UK. Will it matter if the engine goes back together with new rotors and Flywheel unbalanced? (Rotors were like for like)

Also can anyone suggest a good weight for the Flywheel and what counterweight will be needed on the other end.

Many thanks

Gregg

 

welcome to the board.

aftermarket flywheels usually include the automatic model rear counterweight, so as long as the two rotors that you get are within 2 letter weights, additional balancing should not be necessary. if you happen to buy a flywheel that does not include the rear counterweight, then just order one from a dealer or see if you can score one from a local private party.

 

As diabolical mentioned, the engine is balanced to the counterweights, not the flywheel. Now, the flywheel itself could be out of balance, but most companies do a good job of balancing those.

For race motors, balancing is more important. If you have the time and money, it would be good to balance the engine assembly; however it's not exactly required.

 

Thank you, much appreciated

 

are all the auto rear counterweights the same then????

 

no,, auto counter weights look the same but are in fact matched to the rotors and the front weight
and so as rotor masses have evolved over the years,, each generation has its own unique front ( and rear auto ) counter balances

mazda made things easier by making turbo and non turbo rotors to the same masses for each generation

there is no identity marks on later ( post 1985 ) rear masses ,, you need to measure the outer lip surface width
and on the post 1983 front weights you measure the height of the main part of the bob

generally all 13bs pre 1985 use the same masses as the same weight tolerances are built into all the early rotors
( the rear one is stamped 1029A,, or just 29 in later ones ,, and the front is marked 1757 )
- however 1983-1985 RESI/GSL-SE engines have unique front mass ,, as the 6p runner under the timing cover clashes with the 1757 mass
( the new mass with a ground lip on the edge can be used to retro fit to earlier rotors as the eccentric balance is the same )

1986- 1988 s4 rotors ,, turbo and no turbo have there own front and rear mass
and 1989- 1991 rotors have there own different massses
( of which the front one is narrowed to suit under the timing cover )



1992 onwards till rx8 share same mass rotors,, but detail differences in FD manual versions for the front mass
( IE a larger torrington bearing area suit that FD clutch )
so all of these will swap front and rear masses to some extent with the 1989-91 models

 

great info. thank you.

 

As bumpstart mentioned, if you fit a light flywheel you need the corresponding auto rear counterweight. An FD auto rear weight will do. RX Motors has them available.

If you still want the engine balanced I understand that it's about £400 (+/- depending upon whether heavy metals are required to be added) and I have a friend that gets them done. He's in Hampshire.

 

I'll help elaborate on the above. The extent is the differences in rotor weights (A-F). Example, if your Fd engine has the heavy "A-B" rotors, you can use matching counter weights from an S5 NA or Turbo of the same rotor weight. You do NOT want to use matching counter weights from and assembly that had lighter "E-F" rotors. Your engine will be far out of balance.

 

When exchanging the factory heavy flywheel, I personally would never just add any basic undrilled auto counter weight from a vendor to my engine knowing very well that it wont correctly balance with my rotating assembly. A lot of people forget that these rotors themselves still have a weight range of (A-F) inside their engines. Mazda drills every counter weight to match that particular rotating assembly. So it's best to find the RIGHT auto counter weight and not some generic one.

 

So you recommend pulling and disassembling your engine to verify rotor weights when installing a flywheel?

 

If your removing the heavy factory flywheel that has the counter weight built in, how else are you suppose to know what auto count weight you will need? That's your choice if you want it done right other wise you have absolutely no idea of the balancing of your assembly with an unknown counter weight. Those are just the facts! There's a reason why Mazda always ship remans with counterweight/flywheels already attached.

Now just because I'm saying that your engine will be unbalanced with a generic counter weight, doesn't mean you still can't get many thousands and thousands of miles out of that engine. Unbalanced engines will need new bearings during a rebuild. Perfectly balanced engines don't. The bearings on my original Fd engine that had 108k are in perfect condition an will easily go another 100k. Even Mazda will tell you, perfectly broken-in bearings are something you always want to keep during a rebuild.

My advice, if a light weight flywheel is a must (can't think of any reason why someone would think they are THAT important) and you don't know your internal weights, wait till rebuild time so you can do it right.

 

you know, i never thought of this. it makes sense to me. thank you for the insight.

however, it raises the question (to me, at least) if Mazda gives you the choice when buying automatic counterweights?

 

I don't believe they come pre-drilled from the factory or matched to any particular set of rotors.

 

Wouldn't you think that they would come matched to 'C' rotors as they are in the middle, just like all new rotors are C?

 

^ No they come full weight so they can be drilled down to match the rotors. If Mazda only did them for "C" rotors, then you could never balance the assembly for heavier A-B rotors. Remember "E-F" are the lightest. Make since?

 

Ah yes, that would seem to be the case.

 

So, in summary, if you didn't remove all the rotating assembly from a factory built engine, so that you know it was balanced as an assembly, send it off to get it balanced before you assemble it. It's good insurance.

Also, a good balance job shows up in a smoother engine over even a factory balanced assembly. Most shops that do balancing hold a lot tighter tolerance for balancing than the factory. Only use someone who has rotary balancing experience. CLR, Atkins, RB, Mandeville, etc.

 

one question: if the counter weight is built into the OEM flywheels, and you wanted a lighter flywheel with an auto weight, couldn't you weigh the OEM flywheel and carefully drill the auto weight to close to the OEM flywheel weight minus the new flywheel weight? e.i. OEM flywheel=light flywheel+auto weight. I was told by engine builders that if you are running stock-350hp that balancing was unnecessary as long as what I stated above took place. may last a few thousand miles less then it could have, MIGHT have to replace bearings at rebuild time, but you'd still get 100-150k miles out of it (if not beating hard all the time).

balancing is mostly for race motors that have high hp and run at constant high rpms. If you got the coin and doing a rebuild then I'd balance the assembly but I would not do it if you're just replacing your flywheel and not cracking the motor open. The method I stated above should be more then efficient. that is unless you are running over 400hp in your motor, then get it balanced. my .02

 

1st off, balancing a rotating assembly reduces the stress on the e-shaft and stationary gears. It has nothing to do with how much HP your making. Balancing is all about rpm's and not power. An unbalanced engine can rear it's ugly head even during no load revving. Also trying to match the weight of the the OEM flywheel wont work. When you balance a rotating assembly, you are removing weight in specific areas of the counter weight. It's sorta like if your balancing a tire. The machine spins, and the computer tells you exactly how much weight to add to a specific area of the tire however on a engine, your drilling holes to remove weight.

 

Exactly!

 

basically I am mostly speaking from discussions about rebuilds. I understand the principal of balancing the rotating assembly for the counter weights but at least getting it to around the same weight at that end of the motor is better then nothing is what I understood. I just know of people that have done this, right or wrong.

I was just reading through a bunch of lighter flywheel threads and it seems that as long as you have a rear counterweight from an auto the same series/generation as your engine then you should be good as log as you put the counterweight in the proper position concerning the front counter weight. i.e. if you had an s4 engine you would need a rear counter weight from an auto s4 car.

I skimmed somewhat thoroughly the mazdatrix flywheel page and this is the only page even referring to balancing. its talking about if you are in doubt you have the right counter weight on your motor or for high rev aplications. if you needed to balance the assembly for a flywheel that is light then they would have said something about it. they are pretty thorough. so balancing isn't necessary unless it is high revving, you don't know what your counter weight is at all, or you are doing a rebuild and the engine is apart anyway and you have the extra $ for balancing. that is the answer so we can end this thread. just use the auto weight for your series motor.
EDIT: here's the link to mazdatrix site that I was reading http://mazdatrix.com/flywheel.htm

 

Yeah and you would be surprised on how much of an improvement can be made over the factory balance assy. Remember, these engines were not designed to blast up to 10k rpms like everyone is doing.