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I'm trying to get my Brazilian Mahle pistons weight-matched for a stock rebuild ('78 Bus 2.0).

Unfortunately, my high to low range is 10g. Where can I get 10g of material out of the heavy piston?

How deep a cut can be milled on the bottom of the piston?

Here's what I'm starting with. I numbered the other different areas that may be candidates for having material removed.



Thanks!

the shop we use for balancing removes the piston weight from the pin. they use a very hard tool to machine a small amount of material from each side of the pin centre. denser material, and type 4 pins are very thick, so no issues with making it too weak.

Scott,

Do they remove material from the end of the pins, making them shorter in length?

Or do they do a lathe cut on the outside diameter of the pin? (This is my interpretation of what you're saying.)

Or do they drill to enlarge the hole through the center of the pin?

Thanks!

lathe cut the inside diameter of the pin, an equal amount on both ends. it isn't much material in most cases, you have to really look to even notice it.

Gotcha! I'll see if my local shop would be willing to try that. If not, then my pistons and pins may be making a trip to Toronto!

HHHmmmm.

Here is a notion for you.

I assume what you are trying to do is to get each rod / piston combo to weigh the same.

So if you look at the rod, piston, pin, rod bolts, rings as an assembly. You have several places you can remove weight to create your end result.

In a perfect world. ALL the rods would weigh the same, As well as the pins, pin locks, bolts, nuts, rings and big end bearings for that matter.

In the real world. That rarely happens.

So you may want to weigh out each item mentioned above separately.

Then assemble each ASSEMBLY to get the closest match you can.

It is much easier to take a bit of weight off the rod than the piston or the pin.

Unless you are building a very high end race engine that needs to be absolutely perfect as humanly possible.

Ya night want to try the above and simply belt sand the rod to remove about 10 grams or so of the big end or a combination of places on the rod.

You can also MILL the underside of the pistons, cut the skirts, bore or radius the pin, remove casting/forging parting marks on the rods, radius bearing edges to match the fillet on the crank, grind some off the rod bolts, flycut piston domes, etc.... etc....

Just food for thought.

Clayton

I never reduce balance differential by removing pin material.. Typically the balance shops that do this are of the same caliber as those that don't index any of the parts for assembly as they were fitted during the balance process, totally compromising the processes.

The main reason for this is the fact that IF the pins are evr mixed between pistons the balance differential is even higher.. This is a big concern if you have forethought to consider the next guy that tears into the engine and remember that could be you..

The max differential of balance I even try to work with is 7 gr on stock pistons and even at that an enormous amount of material needs to be removed.

But thats just me-

I'd prefer the weight come out of the piston too.

Unfortunately, my rods have already been matched to one another, and balanced end to end. So, I'd hate to undo that by removing rod material to compensate for the pistons.

Where do you remove material? How deep a cut can you mill on the bottom of the piston?

I was taught that you peck the bottom of the crown with a drill.

Make small divots with a drill, spread out all around the bottom...

Was kind of freaked out about it when i first saw it,
but then a guy mixed them up, and lightened one way too much.
He had like, what looked like almost holes in the crown. before they balanced...
Way, way more than you would ever take out...

And the thing is still running hard - never came back like six years later..

Been pecking ever since...

Those bus pistons take a lot more to balance than any others i ever seen...

Don't let it freak you out, those crowns are thick...

Here are a few videos of John Edwards showing some piston / rod balancing basics for those of you that like seeing videos. John is one of my engine mentors and a great guy to hang with.

He does most of my machining. As I always want a one off nut case experiment. And he is kind enough to indulge me.
And I get to use his equipment when needed.

http://videos.streetfire.net/video/4055b140-fdd4-4c47-be0d-99cb000e71c9.htm

And another video of lightening pistons. Watch the last half to see one of the ways to lighten a piston.

http://www.youtube.com/watch?v=R48c0Z4E5-s

And here is a link to some of his other short videos.
There is about 30 videos that are engine machining specific.

http://www.youtube.com/user/fiatnutz
(Look towards the bottom right of the page for the videos)

I will try to talk him into doing a engine balance video soon. That will hopefully help some of the fellas that need to VISUALLY see the process, to get their head wrapped around it. (Like me)

All of these videos are rudimentary. And they don't go into detail. These are just so you can understand the basics.

Oh... And he will be at the STF annual dinner just like last year.

Clayton

Yo Tuna or Doc. Can you guys make the links in the above post work. I am batting zeros......

Clayton

I only mill pistons to remove weight, having seen stock pistons come apart when all the "dots connected" I don't do it that way and haven't for years...

Its also never good to create rods and pistons that MUST be matched, it gets confusing to assemble and if you ever have to tear into it things must go back together the same way. If you are unlucky enough to teatr into an engine that has been balanced with this methiod and don't realize it you'll put it back together unknowing iof the fault..

Always have forethought and always balance like items to the closest tolerance possible without using associated components to make up the difference..

That said, taking more than 7 Gr out of a stock piston is hairy.... It requires a .120 deep mill pass across the floor of the crown at best. I never go past .120 deep and don't like to go past .080 deep-

That should fix them.

typically? i guess the shop that we use is atypical.