Engine builders' advice req'd - removable piston weights? - PeachParts Mercedes-Benz Forum

Stretch · #1 10-21-2013, 04:40 AM

G'day Folks,

Google couldn't help - well it gave lots of information I didn't want.

I have four new pistons for an engine. I want to them to weigh the same.

At the moment wrist pin rings and piston weights combined come in at

614, 617, 633 and 634 grams

From experience I know that the wrist pins and rings are usually within 0.5 grams of each other - but I will check.

I noticed that on the underside of the pistons there are four steel spring clips fitted to the inner piston wall.

These are Nural pistons for a gasser M102 engine

Are these bits of steel added with the intention to be removed if necessary?

Sorry for the newbie question but if you don't know - you don't know!

1project2many · #2 10-21-2013, 06:44 AM

Are those clips really detachable? Pistons are often cast around a steel core to control expansion. The idea is to cause the piston to be round when it's at operating temperature. This may be a different way to achieve the same result. Balance pads are generally made of thicker aluminum.
Nural | ENGINE PARTS (UK) LTD (bottom of page)

Since you have to balance all pistons to the lightest one, you've got some work to do. 20 grams is something like 7cc of piston material. I'd be studying the pistons to see if I could spot where material is missing from the lightest ones. Any area with a forged surface has the potential to show piston - to - piston variances so I'd measure crown thickness under the pin, distance between pin bosses, thickness of pin bosses, and thickness of skirts. Seven cc seems like a fair amount to me so I'd be looking at everything. How close is the weight of these pistons to the originals? If MB balanced the rotating assembly with original pistons, are you going to re-balance it or just hope for the best?

Interesting reading:
Eaton Balancing » Engine Balancing, Part 3

t walgamuth · #3 10-21-2013, 06:57 AM

There is not an obvious place to remove material on those.

benedict · #4 10-21-2013, 08:02 AM

Not an engine builder..just my own building experiences.

Can you mix and match the wrist pins?

You can probably machine 5 grams off either side of the wrist pin boss's but that still leaves 10 grams to go for the heaviest of the four. If you don't have valve pockets cut into the top then you could divot the underside of the crown. Shame there is no balance boss there.

You are right, 0.5 grams within each other is smooooth.

That's a bit rude to send you four so out of spec of each other. You are wanting to loose as much weight as the picture below.

Dan Stokes · #5 10-21-2013, 09:40 AM

Yes, that is a lot of variance. I'd call Nural and ask what they recommend. Seems unusual that they're so far off and maybe they'll send you some that are closer.

I'm used to seeing balance pads on the pin bosses. It's hard to say where you could cut without dangerously weakening the pistons. Again, I'd let Nural tell you where to remove the weight.

Good luck!

Dan

1project2many · #6 10-21-2013, 09:43 AM

Quote:

Seems unusual that they're so far off

I've found a few threads in other forums that described similar issues.

Stretch · #7 10-21-2013, 10:31 AM

Thanks for the replies and information folks - I've elected to get in touch with the seller to see if he'll sort me out with either a new set of pistons within the weight specification or a set of two pistons that are closer to either the group of 614 / 617 grams or the 633 / 634 grams. I think that's the easiest solution - to be continued...

barry12345 · #8 10-21-2013, 11:34 AM

To me that tang sitting on the left side wrist pin boss seems to be a possibility. Enough correction seems to be the issue if machined.

The steel piston reinforcers obviously should remain unmolested. Contacting the supplier or manufacturer cannot hurt as was mentioned.

Stretch · #9 10-21-2013, 12:17 PM

Quote:

Originally Posted by barry12345

To me that tang sitting on the left side wrist pin boss seems to be a possibility. Enough correction seems to be the issue if machined.

The steel piston reinforcers obviously should remain unmolested. Contacting the supplier or manufacturer cannot hurt as was mentioned.

Oh I didn't think they were reinforcing pieces. They look like they might be persuaded to flip right out of the side of the walls. I'll try and get some better pictures to show what I mean.

Mölyapina · #10 10-21-2013, 01:29 PM

Quote:

Originally Posted by Stretch

gasser M102

I saw that

!

1960mog · #11 10-21-2013, 02:24 PM

It's very unusual to have such a high weight difference.
Did you buy this pistons separate or as a set of 4?

Do not remove the steel pads, they control the expansion of the piston.

You could weigh the connecting rods and pair the heaviest piston with the lightest rod.

Otherwise try to get a set of pistons that is closer on the weight tolerance.

Stretch · #12 10-21-2013, 02:32 PM

Quote:

Originally Posted by Jooseppi Luna

I saw that

!

Let's face it DD is the place to be for these types of questions

Stretch · #13 10-21-2013, 02:38 PM

Quote:

Originally Posted by 1960mog

It's very unusual to have such a high weight difference.
Did you buy this pistons separate or as a set of 4?

Do not remove the steel pads, they control the expansion of the piston.

You could weigh the connecting rods and pair the heaviest piston with the lightest rod.

Otherwise try to get a set of pistons that is closer on the weight tolerance.

So they are more like springs than reinforcing then?

Even matching the heaviest rod with the lightest piston I'm still miles out.

And yes I did buy them as a set of four. The seller hasn't responded yet - I hope to get some sort of a response tomorrow. I don't think I can remove 15 grams of material from the heavier castings. Removal of the little knob of metal next to the wrist pin on the left in the pictures above gets you about a reduction of 2 grams...

...not enough...

Mölyapina · #14 10-21-2013, 02:53 PM

Quote:

Originally Posted by Stretch

Let's face it DD is the place to be for these types of questions

Ah, I was afraid that the pot would notice that the kettle is black, too

. I get what you're saying, though, Tech Help is a bunch of rich guys

.

1960mog · #15 10-21-2013, 03:55 PM

I don't believe that Nüral would allow a set of pistons with that high a weight difference to leave the factory.

The pistons of the 616/617 engines can have a max weight difference of 10 grams.

On a gasser it should be less.