Engine builders' advice req'd - removable piston weights? - Page 3 - PeachParts Mercedes-Benz Forum

Stretch · #31 10-22-2013, 11:33 AM

Quote:

Originally Posted by vstech

as long as total recriprocating weight is equal, (small end of rod, wrist pins, and piston/rings) and rotating weight is equal (large end of rod, and rod bearing, cap screws)
you should be just fine.

Yeah but there's a weight concentration at the ends that it is disproportionate to the weight of the connecting rods.

My connection rods range from 754 grams to 758 grams with big end bearings fitted. I estimate that at least half of that weight is centered about the crank journal. The little end is much littler than the big end.

vstech · #32 10-22-2013, 11:36 AM

you are not supposed to weigh the rods as a total, you are supposed to weigh the big end, and the small end... since the large end is rotating mass, and the small end is reciprocating mass... the numbers are rotationally specific to the balance of the crank.

vstech · #33 10-22-2013, 11:37 AM

like this...

Stretch · #34 10-22-2013, 11:38 AM

Quote:

Originally Posted by vstech

the next question is...

are you balancing the weights to the original weight of the original parts? or are you balancing the crank too...

I'm not going to touch the crank - it is staying in the block - I am not going to remove it no no no no no no no no!!!! The crank is good. I'm not worried about it. I just need to get the combined weights as close to each other as possible. At the moment I think the maximum difference is about 20 grams. The bits that came out had a max difference of 6 grams and I was able to whittle that down to 4 grams without too much arsing about. I'm not going to accept a max difference of 20 grams now.

vstech · #35 10-22-2013, 11:39 AM

EAA - EAA Experimenter - Connecting Rod Balancing
here's a great pictorial on balancing pistons/ rods/ etc...

Stretch · #36 10-22-2013, 11:39 AM

Quote:

Originally Posted by vstech

like this...

I know but an over all weight measurement is a good place to start. If I have a set of pistons that are heavier than the specs in the FSM that's a bad start.

Stretch · #37 10-22-2013, 11:40 AM

Quote:

Originally Posted by vstech

EAA - EAA Experimenter - Connecting Rod Balancing
here's a great pictorial on balancing pistons/ rods/ etc...

Thanks

vstech · #38 10-22-2013, 11:50 AM

here's a pelican thread on it.
Where/how to weight rods, pistons, etc? - Pelican Parts Technical BBS
My point is, if you get the pistons AND the small end of the rod balanced withing spec total, you'll be just fine unless this motor is hitting 10KRPM...

1project2many · #39 10-22-2013, 12:22 PM

Quote:

when building race motors, we always balanced the assembly as a whole. balanced the small end with the piston, balanced the large end too... not sure why the pistons themselves would all need to be balanced separately...

For future repairs, and / or to allow the balancing operation to progress when all parts are not available. If pistons are balanced to each other and rods are balanced to common weight, then a replacement piston can be installed without re-balancing the the rod/piston assy.

An additional reason to maintain consistent piston weight (and to match the original weight) is to maintain the crankshaft counterweight or "bob weight" effectiveness. The crank essentially unbalanced to counter the force applied as the pistons peak at TDC. Changing piston weight changes the magnitude of force being applied to the crank.

You can (and I do) "home balance" rods by using a fixture that allows fastening two rods to a common pin and allowing the pin to roll on a level surface. The heaviest rod always falls. The greatest challenge was making the fixture consistent when balancing the small end. Turns out it produces better results than the chain fixture above.
http://www.beardmorebros.co.uk/Images/Project%20images/engine/5%20balancing%20rods.jpg

Simpler=Better · #40 10-22-2013, 12:54 PM

Quote:

Originally Posted by Stretch

I'm not going to touch the crank - it is staying in the block - I am not going to remove it no no no no no no no no!!!! The crank is good. I'm not worried about it. I just need to get the combined weights as close to each other as possible. At the moment I think the maximum difference is about 20 grams. The bits that came out had a max difference of 6 grams and I was able to whittle that down to 4 grams without too much arsing about. I'm not going to accept a max difference of 20 grams now.

You're not replacing the main seals?

Stretch · #41 10-22-2013, 01:07 PM

Quote:

Originally Posted by Simpler=Better

You're not replacing the main seals?

Front one is done. Back one isn't - there's no evidence what so ever it has been leaking so I'm risking it.

Stretch · #42 10-22-2013, 01:09 PM

Quote:

Originally Posted by 1project2many

For future repairs, and / or to allow the balancing operation to progress when all parts are not available. If pistons are balanced to each other and rods are balanced to common weight, then a replacement piston can be installed without re-balancing the the rod/piston assy.

An additional reason to maintain consistent piston weight (and to match the original weight) is to maintain the crankshaft counterweight or "bob weight" effectiveness. The crank essentially unbalanced to counter the force applied as the pistons peak at TDC. Changing piston weight changes the magnitude of force being applied to the crank.

You can (and I do) "home balance" rods by using a fixture that allows fastening two rods to a common pin and allowing the pin to roll on a level surface. The heaviest rod always falls. The greatest challenge was making the fixture consistent when balancing the small end. Turns out it produces better results than the chain fixture above.
http://www.beardmorebros.co.uk/Images/Project%20images/engine/5%20balancing%20rods.jpg

That is very clever - simple and clever

Stretch · #43 10-22-2013, 01:11 PM

Update

Seller says I can have my money back if I fill in the form etc.

I'll probably send them back but now is the time to try and make contact directly with Nural.

97 SL320 · #44 10-22-2013, 09:11 PM

I'm thinking that the seller mixed piston versions. Sometimes pistons have version differences and are intended to be replaced in sets.

I'd return them for a closer matched set.

However, you can easily pull weight from the heavy ones. Chuck it in a 4 jaw lathe chuck with the crown towards the chuck. Run a cutter from the skirt side towards the crown. Have a look at the inside diameter from the skirt side , you will see a cut ring. When removing weight cut only towards the crown and don't increase the diameter.

vstech · #45 10-22-2013, 09:18 PM

Quote:

Originally Posted by 1project2many

For future repairs, and / or to allow the balancing operation to progress when all parts are not available. If pistons are balanced to each other and rods are balanced to common weight, then a replacement piston can be installed without re-balancing the the rod/piston assy.

An additional reason to maintain consistent piston weight (and to match the original weight) is to maintain the crankshaft counterweight or "bob weight" effectiveness. The crank essentially unbalanced to counter the force applied as the pistons peak at TDC. Changing piston weight changes the magnitude of force being applied to the crank.

You can (and I do) "home balance" rods by using a fixture that allows fastening two rods to a common pin and allowing the pin to roll on a level surface. The heaviest rod always falls. The greatest challenge was making the fixture consistent when balancing the small end. Turns out it produces better results than the chain fixture above.
http://www.beardmorebros.co.uk/Images/Project%20images/engine/5%20balancing%20rods.jpg

this makes sense, but IF he cannot resolve the issue with the vendor, balancing the rod small end to the piston would be an alternative...