Stan.Shaw@Excell.Net

I took my rotating assembly in to the local machine shop to be balanced. The rotating assembly includes:
Scat six counterweight crank (928 Int'l design)
Oliver rods
968 pistons
stock piston pins

The problem I have is that the machine shop's calculations indicate 500 grams of weight must be added to the crank (which is quite abit) and there really isn't any place to put it because the crank counterweights are so small.

I am presuming my crank is an unmodified version of the 928 Int'l design, so am wondering what others have done.

The machine shop suggest balancing externally and said there is no performance difference in their @20 years of experience.

The image below is of the numbers recorded by the machine shop.

BC

I have had some ideas of recent on this, as I have been researching some engine build options for my stoker crank:

Custom pins inside your stock 968 pistons
The skirts on the 968 pistons can be cut down a bit if you make sure to coat that edge with PC-9 from Swain Tech (which is on your side of the country)

Also on the pistons, there is a BIT of material at the botton of the pin boss that can be used to lighten - it was used on m y pistons by the machine shop 928intl suggests in SoCal.

Mallory Metal is expensive. At 500 grams, I think you are talking about some big bucks.

FlyingDog

I don't really have a thorough understanding of how crank balancing works for a V engine. Can somebody explain why you can't just subtract weight from the opposite side instead of adding weight? I guess that's basicly what Brendan is saying with weight shaving on the pins and pistons, but what about the crank? Couldn't you rebalance at a lower mass?

Stan.Shaw@Excell.Net

Mark A. suggested the lighter piston pins, but the machine shop didn't think it would matter enough.

My pistons have already been HPC coated, so that probably isn't an option.

Yes, the cost of the mallory would be quite high.

How have others used this crank? Titanium rods perhaps?

Stan.Shaw@Excell.Net

Matt,

My limited understanding:
You can take weight off the piston/pin/rod components or add to the crank.
There are some ways to reduce weight on the piston/pin/rod components
Adding weight to the crank is done by drilling out steel and replacing with Mallory, so the net effect is the weight of the Mallory mass over the steel mass.

Mike Frye

Stan,

No help to offer, I'm just trying to understand the process better. (I like this kind of school).

Does that mean the crank is out of spec or is it a non-standard application and the specific combination of parts you listed that caused the net lack of weight on one side?

Stan.Shaw@Excell.Net

Hi Mike,

What no help!

It is a matching of components issues, not a crank out of spec. My simplistic view is that the counterweights should offset the piston/pin/rod weight, and in this case the counterweights are coming up short.

It just seems to me that a number of these cranks were made, so I wonder how others using them have done so.

I would think the components I am using are not uncommon components for this type of build.

BC

There aren't some thinner-walled pins, or similar out there that can drop less than 50g a piston? I find that hard to believe. It would COST, but so will the malory metal, and as you mentioned, I don't see alot of placed to add it, other than drilling out axially into the side of the fillets, which doesn't bode well for harmonics. Not that I know anything about any of that. *Ouch* - just poked myself with the screwdriver again.

Mark Anderson

On my new motor we used some JE pins that were much lighter than the stock 968 pin.

Stan.Shaw@Excell.Net

Any chance you know what the weight of those were, Mark?

If I could determine what is possible, then it would make it easier to determine my course.

Fabio421

I have had some pistons lightened before. They were weight balanced the old school way by drilling small holes into the underside of the piston. Although 500g is alot of weight to lose. I would think you would get there easily with the Ti rods. The Ti rods would also pay dividends in a race engine I would think. The load on the rod bearings would be substantialy lessened, especially at high RPM. This should help to alleviate the 2/6 failures as well, right?

BC

Ti Rods and Lighter Pins and you'll be SUBTRACTING weight from the crank, me thinks.

Stan.Shaw@Excell.Net

I am going to see what JE can provide for lightened piston pins to start with.

Stan.Shaw@Excell.Net

Here is the crank, doesn't look like I will need to turn down the counter weights for clearance.

Fabio421

Even better