Guys could some of you enlighten me on how the main shops/vendors on here balance there rotating assembly for built bottom ends??

I have researched balancing race engines, and understand how to work out the reciprocating mass and the rotational mass but every thing i have read says that to balance the rotational mass you need to remove/add material from the piston/ rod set up.

This is ok with ruff cast parts but it states that new forged light weight pistons rod's there is no place to remove metal with out weakening the part?

Most piston's/rod's come in mached +/- 1 gram sets and the crank is balanced from the factory.

Im unsure of how you balance this set up ?? Any one have a the inside story on how this is done??

Thanks. Tim.

 

bump!

Bloody hell Tim, are you going to do THAT yourself?? respect!

 

Hahaha. Hi Val! Im not looking at trying it my self im trying to find out what and how the shops do it??

I have a access to a very high speed balancing machine in work that is used for turbine's that do 50,000++ Rpm so a crank would be cake but its getting the piston/rod's set up sorted im unsure of! If you can't take metal off or add to it?? What can you do other than ensure the cross weight's on the rod/piston's match?

 

when you are talking about a rod, or piston, they come fairly well matched from the manufacturer - it's literally grams that you are talking about shaving so it's not great amount of material. The idea is simply to take the lightest one of each and then adjust the other components so they equal the weight of the lightest one.

the crank can have more substantial differences in weight across it's plene - holes are drilled to take material away and allow for more even balancing. Yes it is balanced, but only to a certain degree of accuracy. As you ask that crank to bear more substantial load, either via rpm, piston/rod weight, or both, more finite adjustments are needed.

Anything that rotates can be balanced - the cam, the clutch, the flywheel, the pulley, etc etc - all depends on how involved of a build you are doing, and the skill level/attention to detail of the actual person responsible for assembling the engine

here is a decent general article

http://www.custompistols.com/cars/articles/balanced.htm

 

^^^^

Nice Article.

I thought that "blueprinting" also includes recording the measurements of the individual components, for example each of the weights, the piston diameters, the cylindar bore diameters, the rod journal diameters, etc.

Is this correct, or is there another term that is used when everything is to be documented?

 

documenting it would be part and parcel of any build from the machinists standpoint - whether or not he shares that info with the end user, or the shop reselling his work, etc, will vary

 

"Blueprinting" is more of the little stuff, like chamfering oil holes, radiusing hard edges, smooth out oil passages for better drainback, etc.
As you stated, recording of values is very important for when it comes to an inspection of a motor.

 

Thanks for the replys guys. I fully understand the idea of weight matching parts eg, pistons and rods. I can only assume you would file metal off from under the skirt of the piston but i dont see where metal can be removed from on the rods??

I have used high speed balancing equipment in work before on large turbines, shfts ect but all of these items can be spun on the machine where as the crank, rods, pistons bullt up can't be spun on the machine? This is the part where i dont understand the balancing part?

Assume the crank is perfect you then weight match you pisto/ rod combo and fit the closest wieghts opposite from each other on the crank. Can you then balance the WHOLE assembly or do you just have to assume the thing is in balance because of the weight matched parts??

Thanks again.

 

I would of thought to reduce the amount that needs to be removed you would weigh the piston and rod assemblies together and match accordingly ie:

Piston 1 = 360.5 grams
Piston 2 = 360.3 grams

Rod 1 = 500.3 grams
Rod 2 = 500.4 grams

So you would match P1/R1 (860.8g) and P2/R2 (860.7g) therefor you would only need to remove 0.1g from P1/R1

 

no.. you'd want to make all the pistons as light as the lightest one.

then, move on to the rods, and make all the rods as light as the lightest one, too.

 

thats the way i was taught as well.

 

Wow I love this thread, learning alot. Keep up the awesome responses!

 

I forgot something very important on the rods.

YOu need to weigh both the big and the small ends of each rod and adjust each in accordance. a tool like this is what is used:

 

Thanks for the clarification.

When weight matching then what is the excepted variance. For example my Wiseco pistons come pre-weighed but the largest variance is 0.5g

 

Good post.

It is my understanding as well that the rods themselves need to be balanced to have the same C/G.

 

Why did this thread die??????????? any chance of a wake up??????

 

seems the question was answered, not many people have the ability to balance a rotating assembly so a step by step isnt very useful.

 

there are areas on the rods that can easily be machined to lighten them. on the bottom cap you can either file or use a small bit and drill a tiny divot in it. Like people are saying, if it's down to .1 gram that is a very tiny amount of material to be removed.

 

Thats how my machine shop did the balancing with the rods and removed excess weight from under the piston dome.