Balancing pistons - material removal

msoultan · Post by **msoultan** » Thu Mar 19, 2009 10:31 am

I've already searched the forums and read up on all the posts regarding piston balancing. It seems to be the consensus to remove material from under the crown. I also saw a post from Jake that says "I never go past .120 deep and don't like to go past .080 deep", so that gives me a limit on how deep to go. What I don't know is how close to the edges to go? Should I start at the center and move outward from there with incremental passes, or do you start at the thicker part towards the edges and move inwards?

The reason I ask is because I have a machinist friend that can do the milling no problem. I also have a local automotive machine shop that can probably do the work, too. My worry is whether the machine shop will know the proper area to remove material from. If anything, I'd like to guide them on where to cut...

I can also bring my pistons to Rimco, but they're a bit farther away. I brought my current set of pistons to them a while back for balancing, but I can't remember where they took off material and won't know until I pull the engine. I can't remember if they took it from under the crown or from around the wrist pin (which I read is the wrong place to remove material).

I don't think I need to remove too much material. My pistons weigh in at 504, 505, 506, and 509.

If anyone has any pictures of the machine work done on a balanced piston, that would be great to see!

Thanks!
Mike

typ4 · Post by **typ4** » Thu Mar 19, 2009 1:01 pm

Have you swapped wrist pins around to see if they will balance out?

There should be a balance pad on the underside of the piston or in the case of the 96's I am doing,there is a machined ring the factory used to balance.
Also some removal of the inside of the wrist pin has been done.

msoultan · Post by **msoultan** » Thu Mar 19, 2009 2:51 pm

typ4 wrote:Have you swapped wrist pins around to see if they will balance out?

All of the wrist pins are the same weight

There should be a balance pad on the underside of the piston or in the case of the 96's I am doing,there is a machined ring the factory used to balance.
Also some removal of the inside of the wrist pin has been done.

According to other posts, there are no balance pads on the mahle pistons. I probably should have mentioned that I have the 94mm pistons.

In my case, I need to balance the pistons themselves because everything else is in balance (rings, pins, etc). I don't want to be mixing parts up to make things balance unless it's going to help dial things in at the very end (+/- 0.5 g)

fusername · Post by **fusername** » Thu Mar 19, 2009 2:52 pm

how off are they? mine are 4 + on two, dead on the other two. I figure once I balance the big end of my rods i will use them to balance the small ends.

msoultan · Post by **msoultan** » Thu Mar 19, 2009 2:57 pm

It's not a question of whether or not I want to balance them - I will get them balanced to each other as everything else has been balanced accordingly. I really am looking for the exact place to remove material under the crown so that I can bring the heavy pistons to spec. As I mentioned above, they are 504, 505, 506, and 509.

Thanks,
Mike

fusername · Post by **fusername** » Thu Mar 19, 2009 3:04 pm

oops, missed it in there. I assume the crank clutch and rods is all balanced as well? I wish i had the money for all that fun stuff.

msoultan · Post by **msoultan** » Thu Mar 19, 2009 3:06 pm

Yeah, I had everything balanced a while back. I'm just doing a top-end rebuild and needed to get the pistons in balance.

thanks,
Mike

fusername · Post by **fusername** » Thu Mar 19, 2009 3:13 pm

I am curious how you are gonna remove material? ie what tools and such? My concern would be stress focus points from scratches and such, but I really don't know much about this process. I am just finnaly working out how to do Rods on a bench sander.

Clatter · Post by **Clatter** » Thu Mar 19, 2009 10:30 pm

For most motors, you can peck with a drill to make divots on the underside of the crown.

A serious 7000RPM motor with a nice set of J&Es or something might need a lathe, but your average motor will be fine with the drill method.

Make a few reverse cone shaped holes real small spread out all around the underside of the crown, and check your work as you go.
If it ends up needing more, you can deepen them.
It's rough cast under there, so it really doesn't matter what shape it is...

So many serious motor guys have been doing it that way for years, your average motor with cheap pistons could care less..

Ok, Edited to show a couple of (sorry) type 1 pistons i did..

Cima 94:

sorry so dark, but you can see how a higher quality piston needed just a few small pecks to come to balance, this was the heaviest one..

Cofap 87:

This one was the heaviest from a real crappy set, it took something like 15g or more to get this heaviest one to spec.

This motor has been limited to 5500 or 6000 sometimes (stock crank)RPM, but abused badly, detonated sometimes for several years and has held up fine.

A friend (local well-known engine builder) spaced out late one night balancing a set for a big stroker 2-liter. he kepy going not knowiing that he was lightening the wrong one... By the time he figured it out and got them all balanced they looked like swiss cheeze! Can't believe how much got hogged out...
That motor is still together and fine after 6-8+ years of hard abuse...

Post by **Type 4 Unleashed** » Fri Mar 20, 2009 12:16 am

I get my balance work done at Revco Balancing, over by you in Long Beach, It's on Cherry Ave between South St and I think a block before Harding, right before I think it's a burger joint on the corner. On the right side of the building all the way at the back.

msoultan · Post by **msoultan** » Fri Mar 20, 2009 1:00 am

Type 4 Unleashed wrote:I get my balance work done at Revco Balancing, over by you in Long Beach, It's on Cherry Ave between South St and I think a block before Harding, right before I think it's a burger joint on the corner. On the right side of the building all the way at the back.

Do they mill the pistons on the underside of the crown to balance them out?

thanks,
Mike

Post by **Type 4 Unleashed** » Fri Mar 20, 2009 8:21 am

msoultan wrote:
Type 4 Unleashed wrote:I get my balance work done at Revco Balancing, over by you in Long Beach, It's on Cherry Ave between South St and I think a block before Harding, right before I think it's a burger joint on the corner. On the right side of the building all the way at the back.
Do they mill the pistons on the underside of the crown to balance them out?

thanks,
Mike

They remove material from anywhere safe to do so. And if there is any place you would prefer for them not to remove material from, then tell them.

Here an article I found about some one using them.
http://www.highperformancepontiac.com/t ... stons.html

typ4 · Post by **typ4** » Sat Mar 21, 2009 7:03 pm

Do you have a good picture of a piston. I know there is a balance pad on them somewhere.
I am doing several sets right now so I have run into some interesting places to remove material.

msoultan · Post by **msoultan** » Sat Mar 21, 2009 7:06 pm

I don't off-hand.. but tomorrow I'm going over to my machinist friend that has a mill and we're going to pull material off from under the crown until we get the pistons within spec. That's what jake and others suggested, so I'd venture to guess it's the right way to go. Jake also said that what someone referred to as balance pads (area near wrist pin) weren't actually balance pads...

These are the standard 94mm mahle pistons, btw.

Mike

jan72 · Post by **jan72** » Sun Mar 22, 2009 1:13 pm

OK, not balance pads... What are those, then? Same width, different height on a set of rods coming from one engine... did they fit in some fixture for boring or something and were used for balancing after that? Is there a ratio to keep between rotating and reciprocating masses? I imagine that, on a flat engine, it's important to lower the piston/small end mass for gravity/friction reasons...