Piston Balancing Question

[Dave Taylor]

I've taken my engine apart down to the case and had to replace one piston & cylinder (#1) due to a broken ring. Before reassembling I thought I'd weigh the pistons to see how close they are. Here are the results:

#1-568g
#2-596g
#3-594g
#4-586g
#5-582g
#6-586g

I weighed them with rings and pins. I was told that they should weigh within about 5 grams of each other or even closer if possible. The difference between the lightest (#1) and the heaviest (#2) is 28 grams.

Should I send them to a shop to have them machined? If so, who would you recommend?

Thanks, Dave

[HawgRyder]

Dave:....28 grams = 1 oz.
I would think that 5 grams is the most that should be allowed when building an engine.
Personally I would try for 3 or less if using a used set of pistons and 1 or less with new pistons.
Remember...the closer the weight...the smoother the engine.
Bob

[sammyg2]

Measure the pistons without the wrist pins, then measure the wrist pins. Match up the lightest pins with the heaviest pistons and see if you are closer. Ordinarily i wouldn't recommend swapping parts around in an engine because they wear together, but if you have significant wear patterns in your wrist pins or rod bushings they should be re-done anywho.
Either that or take a dremel and carefully and evenly remove weight from the inside of the wrist pins on the heavy pistons. No rocket surgery required.

How are the weights of your rods (small end weights and large end weights vs. total weight)? that's where it gets a bit trickier. The small end weight is reciprocating, the large end weight effects both the rotating and reciprocating balance.

If I were to get serious about balancing an engine that is where my time and money would go. There and the crank/flywheel/pressure plate.

RTOFLMAO, I just went back and read the previous post, the one just before this one. While I agree completely whith the post, I just found it incredibly humourous that someone who is so concerned with balancing the internals of an engine rides a harley! LOL....

[ChrisBennet]

From the looks of it, the replacement piston (#1) was not in the same/correct weight group as the others. I would see if I could find a P&C from the correct (piston) weight group and a matching (cylinder) height group.

(See Carrera Specs for a description of the the groups. It's for a 3.2 but you'll get the idea.)

If you pair up the matching weight pistons on a 911 motor so they are across from each other, the weights will cancel out. Ideally, you want a piston that weighs the same as #5 (582gr).

If your piston #'s refer to the actual P&C position I'm kinda surprised that they were arranged the way they were. I would expect the motor to have 3 pairs pistons matched by weight.
Like this:
596 and 594 on #3 and #6 (heaviest toward on flywheel end)
586 and 586 on #2 and #5
582 and ??? on #1 and #4

-Chris

[HawgRyder]

Sammy...it may interest you that when I build a Harley engine I balance it at a 55% wieght value.
This means that in the course of balancing...I install 1 piston on the rod assembly (both rods are on the crank pin) complete with rings and clips...then place 4 pennys in the bowl of the piston and go ahead and slow balance the flywheels.
This turns out to be the 55% factor.
When running one of these engines, there is a definate difference in vibration verses the un-balanced unit.
No slight on you...I know that Harleys are called Milwaukee Vibrators...LOL
Just a little insight on how the other side does things.
Bob

[Dave Taylor]

Thanks for the input guys!

I think I'll find a piston that's more closely matched in weight, then match up the pairs as you suggested, Chris. I'd rather start over with a whole new set, but right now I just want to get it back on the road.

Dave

[911pcars]

How much are the shops in your area charging to balance the reciprocating weight? Might be cheaper, $ and time, than locating a piston.

Sherwood

[sammyg2]

Bob, thank you for taking my little jab with a sense of humor.
I meant no offense or anything about how you build em, it's just the design of the engine, a v-twin with both rods on the same throw and very big pistons.
They just shake by design.

[HawgRyder]

Sammy...shhhhh....keep the secret.....girls LOVE the vibrations!!
All kidding aside....I've been riding Harleys for a long time...Wow
that many years eh?
Time flies when you're having fun.
You might be surprised how much of the vibes can be eliminated with the proper balancing.
With horizontally opposed engines (porsche, etc) the engine is normally in balance.
Years ago we built a VW engine with 85mm barrels on one side and 92mm barrels on the other just for fun.
We were shocked to find the engine ran amazingly smooth...go figure....so the bottom line is: balance it if you ...but even if you can't....it will still run.
Bob