Engine builders' advice req'd - removable piston weights? - Page 2 - PeachParts Mercedes-Benz Forum

Dan Stokes · #16 10-21-2013, 04:05 PM

I wonder if your internet seller is getting rejects? Factory seconds - that sort of thing. Just a thought.

Dan

1project2many · #17 10-21-2013, 06:03 PM

Quote:

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.

That would not be the best method. Rod balancing is done in two steps. The large end is considered as an assembly that rotates around the wrist pin then the small end is considered as an assembly that rotates around the crank pin. Simply picking a heavy rod and mating it to a light piston discounts the rod balancing procedure and doesn't resolve the piston imbalance.

A careful guy with an end mill in a drill press might be able to remove some material from the crown. Additional careful work around the pin boss with carbide burr then a sander in a die grinder will finish the job. Since these are not diesel pistons and they are presumably not subject to positive intake pressure they're likely to be plenty strong enough.

Adriel · #18 10-21-2013, 07:07 PM

Built a air cooled V.W. engine within 0,5 grams, and took a lot of fiddling, which I love. Need to do it again as used too much cam lift... Other than that, sounds and performs great!

First step is to weigh all your wrist pins separate from the pistons. I.I.R.C. they were five grams out, so every bit helps.

Machine shop said to take off a smidgen on each corner of the skirt. 0,25 on each corner isn't much and adds up fast.

Hope this helps, good luck, and most important, have fun!

Stretch · #19 10-22-2013, 10:53 AM

I think a difference of 20 grams is too much - the FSM says 10 grams max. Obviously I'd like them to be as close as possible.

The seller got back to me with a suspicious cut and paste bit of information from technical support saying that 20 grams was OK. I know it isn't. I'm not getting a good feeling about this seller - and I've never known anyone in technical support to reply that quickly. (Within normal working hours the reply must have been done within two hours) I smell a rat.

vstech · #20 10-22-2013, 11:07 AM

are all the wristpins the same weight?

I'd certainly grind off the bumps, and thin out the skirts and any flash inside the heaviest ones...

vstech · #21 10-22-2013, 11:08 AM

have you considered that the pistons are actually knockoffs?

vstech · #22 10-22-2013, 11:09 AM

try contacting Nural directly...

Stretch · #23 10-22-2013, 11:13 AM

Quote:

Originally Posted by vstech

are all the wristpins the same weight?

I'd certainly grind off the bumps, and thin out the skirts and any flash inside the heaviest ones...

Sorry I forgot about that bit...

...yes...

Wrist pins are all the same weight at 108 grams.

I can't see 10 - 20 grams of weight difference in the rings which are still fitted. I don't want to remove them 'cos knowing my luck one or more will break and then I can't send them back.

I could indeed remove the obvious bumps but the inside of all of the pistons have been nicely finished - there isn't anything obvious to remove. The knobbly bit in the pictures on page one that was discussed previously has a weight about 2 grams (I know that from arsing about with my older M102.910 pistons).

{Please don't make me explain why I've bought new pistons!}

Stretch · #24 10-22-2013, 11:18 AM

Quote:

Originally Posted by vstech

try contacting Nural directly...

I think that will show that the seller hasn't been in touch with them and so it will start a Dutch "I mustn't loose face" situation which will mean that I will probably get shafted. At this stage I have learnt that it is best to go with the flow and only start the full on whinging when it is necessary.

I've shown the seller proof that what he has sent isn't within the specs in the FSM. I'm waiting for a response to "if you can't change them for other pistons that gives me a set of four within spec then I'd like my money back please"...

...tedious...

...it is always a people problem...

vstech · #25 10-22-2013, 11:18 AM

how about the small end of the rods? is there a grinding pad on them?

vstech · #26 10-22-2013, 11:19 AM

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...

vstech · #27 10-22-2013, 11:30 AM

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.

Stretch · #28 10-22-2013, 11:30 AM

Quote:

Originally Posted by vstech

... not sure why the pistons themselves would all need to be balanced separately...

Because they are a heavy part that moves very quickly and stops very quickly at the end of each stroke!

vstech · #29 10-22-2013, 11:32 AM

Quote:

Originally Posted by Stretch

Because they are a heavy part that moves very quickly and stops very quickly at the end of each stroke!

see post# 27

vstech · #30 10-22-2013, 11:33 AM

the next question is...

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

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