Hi,

im still in proces of building a 383,
so far i put all the pistons and conn.rods and rings together.

i checked for enough clearance between conn. rods versus block
and conn. rods versus camshaft.

after some grinding to get the clearance i made all the pistons/conn.rods exactly the same weight

1306 gr. that is.

At first i thought that the crankshaft (Eagle steel) was in balance of it self, so adding the pistons and stuff that are all the same weight would keep it in balance.

BUT i read in CARCRAFT that the counterweights of these crankshafts are 1855 gr. (bobweight they called it).

so the pistons and stuff should have the same weight (1855gr).

so all the counterweights should have 549 gr. less weight, right?

but not every piston has its own counterweight, does it?

So, does 1 counterweight have to keep 2 pistons in balance? Then it should be that every piston/conn.rod should weight half of 1855gr (=927.5gr.) So ill have to take from every piston/conn.rod 472.5gr. to get 1855gr together to match 1 counterweight?

If every piston have to match 1 counterweight, ill just have to drill 549 gr. of material out of each counterweight...


The reason i want to do it myself is that all those machine shops over here plain out sucks and way Expensive.

thank you..

RR

Robert, it is a ton more complicated than that!

There is a formula they use to figure bobwieght based on percentages of reciprocating wieght and rotating wieght.

Reciprocating wt = pistons, pins, locks, rings & samll end of rod (suspend the rod by a string throu the big end and weight the small end)

rotating wt = big end of rod (suspend it by the small end and weigh the big) and bearings.

We can assume all the bearings, pins, rings, and locks weigh the same, so wiegh one set of each.

Make the 8 pistons weigh the same, ditto for the rod small ends, and rod big ends.

Bring all these numbers and the crank to a balancing shop.
The tech will plug your numbers into a formula to determine the bobweight.

Then he'll make the 4 bobweights, strap them to the crank's rod jounals, and balance the crank to zero imbalance.

You'll save maybe 30-40% by weighing and balancing all the lil pieces yourself.

You need a special machine to balance the crank, you can't do it yourself.

Proform sells a balance beam for matching small end rod weights for a hundred bucks at Summit. (part number PRO-67596). With it you match two rods against each other until you find the lightest then you carefully grind off weight from the top of the rod (small end) until all rods balance. Determining big end rod weights requires another fixture and it's use is more art than science. As mentioned you can use a postal scale to get all of the piston assemblies the same weight, but it will take a competent machinist with equipment dedicated to balancing a crank to get your reciprocating weights exact. It makes life easier if you start with an internally balanced crank, but that is just my opinion as external balance cranks can be balanced as well.

You may not like it but this isn't something that the average shade tree mechanic can do in his garage, unless your garage has a fleet of NASCAR race cars or is sporting a bunch of NHRA Pro-Stocks. Most average racers pay the machinist and hope for the best.


Larger Dave

I agree internal balance is nicer in the long run, like if you have to change flywheels or harmonic dampner. Ive also heard theres other benefits like not having wieght swinging around the ends of the crank but I dont know how true it is.

Robert,
To get the "bobweight" you add up two of each rotating weight, then add one of each reciprocating weight. The total is the "bobweight" that you attach to the crank while it is on the balancing machine.
Here is an example:

Connecting rod large end ............. 400 grams
Connecting rod large end ............. 400 grams
Connecting rod bearing ................ 50 grams
Connecting rod bearing ................ 50 grams
Weight for oil ............................. 4 grams
Piston and pin ............................ 600 grams
Piston pin locks .......................... 4 grams
Piston rings ............................... 50 grams
Connecting rod small end ............. 200 grams
-------
Total Bobweight ...................... 1758 grams

The balancing machine tells you how much to remove from the counterweight, and where to remove it.

So as you see, it is not all that simple. One thing is, even though you have all of your rods at the same weight, you might not have all the big ends the same , and all of the small ends the same. The easiest thing to have done, especially considering your location, would have been to get the crank balanced by Eagle before they shipped it to you. I think your best choice now is to pay someone to do it properly, or risk losing all of your investment :(

Hope this helped,

so, if i just bolt the stuff together, the risk is big that the engine will be ruined?

NO, but it sure won't be the best!!

I ran my old UNBALANCED, made out of junk parts junk301 for quite a while fine even though I could hear her go up rpm-wise through three harmonic's until it was above 7,000!

Sure not the best, but like an old Timex watch, it took a licking AND kept on ticking!!

pdq67

" it took a licking AND kept on ticking!", thats the best thing i heared in years!!!