Trying to match bob-weight.

I am in the process of building a forged bottom end for my 383 and I am in the clearancing stage before it goes to the machineshop for all the fun stuff.

I had to remove a little from my rod to clear the cam. (at this time I also clearanced the bottom of the cylender and the pan rail for the rods as well.)

My problem is I need to know if my math is wrong or right.

2 X piston end weight and 1 time big end weight = bob weight , right?

My cranks Bob-weight is 1876 and I am falling well below that at 1722

Piston weight x2
Rings weight x2
Pin weight x2
Small end of rod x2

Big end of rod x 1
Bearings x 1

Is this right?

How much should I add for oil?
Of course I'm going to have my machineshop final balance it, but I just wanted to make sure I was very close to the cranks advertized bob-weight.

Thanks in advance :)

Ya got to get it balanced anyway so don't worry about bob weight.

I think it's actully:

Bob weight = 2 x (rod big end + rod bearing) + (rod small end + pin + piston + rings + locks) + 4 gm for oil

Just balanced mine last Saturday . . . to within .3 grams. :D

Mike

I just got my rotating assembly back yesterday. Interesting to see that my neutrally balanced flexplate was out 10 grams. The first 2 spins were without the flexplate then with the flexplate. I should have taken in my dampener also. Oh well.

Mine was spun 10 times for a final balance weight of .00 gm on the left and right plane.

According to my balance sheet, oil is 3.9 gm

Piston w/pin 805.6
Locks 4.7
Rings 68.8
Small end 223.2
Total 1102.3

Pistons/throw x 2
Reciprocating weight 2204.6
Balance factor x 50.0
Reciprocating weight 1102.3

Insert 50.3 (rod bearings?)
Big end 563.3

Rods/throw x 2
Oil 3.9

Rotating weight 1231.1
Bob weight 2333.4 (I have no idea how he came up with that number)

He explained a bunch of the spins. He spun it up and it said to take out weight in a place where there is no place to take out weight so he adds or removes weight to another part of the crank to balance it up. There's lots of little holes drilled into the crank and only one plug welded up. The last crank I took in needed 3 plugs welded.

Bob weight 2333.4

That sounds awfully heavy for a bob weight. Is that for a BBC? Or maybe TRW pistons?

Mike

That sounds awfully heavy for a bob weight. Is that for a BBC? Or maybe TRW pistons?

Mike

I thought the same thing, as most small block stuff I see is 1750-1850. Then I looked in his sig and saw something about a 540ci Alcohol mill. That has to be what this is for.

Also mike has the correct numbers for calculating bob weight.

540 BBC, JE pistons with a huge dome for about 13:1 compression (biggest dome I can get for this bore), Eagle H-beam rods, internally balanced Eagle stroker crank.

Although everything is heavy, there's a lot of lightweight parts being used. Can you imagine the weight if I was using non race parts?

I also spin my engine up to about 7000 so perfect balance really helps.

Can someone enlighten me as to what bob weight is?

Can someone enlighten me as to what bob weight is?

A crankshaft by itself is not balanced - nowhere near it. Since you have rods, bearings, pistons, etc. . . attached to the crank you have to simulate the added weight on the crank when you balance it. A bob-weight is an adjustable clamp-on weight set that you attach to each journal when you spin it up to balance it. You have to adjust the bob-weight to match the specific internals you have since rods and pistons can be drastically different weights.

The engine shop where I balanced mine showed me a set of pistons that weighed 700 gm/ea (TRW forged) and another set that weighed 350 gm/ea (super lightweight dirt-track stuff). My Ross blower pistons were 500 gm.

Mike

My PSD puller motors are about 4400 on the bob weight depending on which rods we use:eek:

...

Thanks for the info.

I'm using a Speed Pro Forged piston (VMS-75), floating pins, and a 12cc dish. They weigh 476 Grams a peice.

where does the

50.00000000000000000
0000000000000000000
percent
ratio between the big end and small end
come from?

Well now you had to go and ask the difficult, but obvious, question. . .

Imagine a single cylinder engine. If your crank alone was perfectly balanced, then the piston going up and down would cause quite a bit of vibration. Now, if you made the counterweight totally offset the weight of the piston, then you would have zero vibration up and down (Y axis), but then you would have all of your imbalance in the X axis since the counterweight goes side to side, as well as up and down.

The theory is that the 50% thing cuts the vibration in the Y axis in half, but gives you the same amount of vibration in the X axis.

Now, add in 7 more cylinders and my pea brain isn't capable of comprehending the interactions. . .

BTW, I used to work for Poulan Chain Saws in the Engine Development Group.

Mike

'mike, that's not the way I heard it.

50 percent come from...
Harley-Davidson people use 50 percent,
or pretty close to that. 48, 52 51, ya whatever

a cross plane V-8, is 4 vee-twins, arranged to cancel
2nd harmonic

my assumption is that the OEMs have
'total engine stand, vibration analysis',
so they will do whatever is needed to balance.

but aftermarket,,,,,,well, ..............
a pretty good guess at a balance, is better,
than an engine 'test stand external balance'

my point is, 50 percent, is not fundamentally the same
as what you would get on a engine test stand.
but 50 percent is close enough,
when you build one of something.