GregBBRD

Joseph's 8 counterweight Scat crank had to have the counterweights machined down for the 5.850" rods and pistons to clear the crank (it appeared that the crank was made for 6.000" rods.) We used J&E pistons and Oliver rods for this engine. It took a whole bunch of Mallory to balance this combination....so it would make sense that the 6 counterweight version might require more Mallory to get it to balance. Interestingly enough, with the Carrillo rods and 968 pistons, the last Moldex crank we had spun needed no Mallory at all. Kind of strange....seems like the two cranks should have similar needs in terms of weight. Anybody got any thoughts on this?

gb

Vilhuer

Obviously further away from center metal is in crank better it counters pistons weight which is even further away. If piston and wrist pin are 600g together and 6" away from center, crank would need 1200g 3" away from center to cancel out pistons mass. Relationship might not be linear and crank could need 1800 or 2400g at 3" but I think thats general idea how it works.

In order for two same design cranks to function differently only explanation is one has to be made from heavier material. In case counterweight design is different it could easily explain several 100 gram differences on what weight pistons work without added mallory. Easiest variable is how thick those weights are.

8CW heavy Moldex


Greg's 6CW Honda Moldex


6CW Scat in Stan's picture also has thick looking first and last weights but four weights in between could be thinner than in Moldex versions. Maybe solution to Stan's problem is not just drilling crank hollow to allow as much space as possible for mallory. Instead problem could be solved by making counterweights thicker by adding more material on their sides. It looks like 968 pistons would allow some thickness added in Moldex crank counterweights on rod side of the weight.

svp928

Most counterweight thickness is dictated by journal spacing and proximity to the main webs and rods on each side. Erkka's pic above only shows one rod, but if you looked at the other one, you'd see there isn't much space there. So, you can't add much to the sides. I notice that Greg's crank has a large hole drilled in the rod journals- can't tell if the Scat is that way or not? That is common practice on most racing cranks, and makes a big difference in balancing.
About the only place you see a difference in counterweight size is out on each end of the counterweight, where it is cut for 'knife edging".
So, Greg BBRD, if Joes crank had been made for the shorter rod length originally, the counterweight would have extended out farther on the sides, and its maximum OD may have been larger, to make up for the loss in material in the middle to clear the piston.

Vilhuer

There is little more space between another rod and counterweight as weight is thinner. Not much but it could help along with drilling rod journals as much as oil holes allow.

John Veninger

Mine is like that.

Fastest928

DEVEK 3rd gen cranks are gun drilled down the center, have journal lightening and bull nosed...not knife. Too much debate...

Cheers,
Marc

slate blue

Marc how much weight is saved by this feature and what is the diameter of the hole? I thought about this feature, it is a good one, not essential but good none the less.

Thanks in advance

Greg

Fastest928

Alot of weight is taken off the center.... quite a few pounds.....Translation, I don't remember...I can calc it though. Takes a std cam plug for the rear seal.

But the main purpose was for crankcase ventilation....the best blocks for reduced resistance to crankcase air movement is the GTS block, but it is also the weakest for high hp applications.

I have always wanted to follow the NASCAR folks and build a really high output two valve, but it really is easier to get high hp form the 4 valve. A bit lazy I guess

Cheers,
marc

Stan.Shaw@Excell.Net

I would be interested if you will take a used set of 968 pistons in trade, say for $1k?

svp928

The Nascar guys are now sealing off each cylinder pair in the pan, and scavenging each set to produce a vacuum... but thats off-topic...

Fastest928

Steve, we will talk about dry sump once we have aproven concept...yea, we are go9ing to get rich making dry sump pans, right

You azre right...off topic.

Stan, will think about it....we might be close to a deal....

Marc

SwayBar

Just for reference, here's what a set of LS7 titanium rods went for on eBay:

http://cgi.ebay.com/ebaymotors/ws/eB...m=260131808818

Stan.Shaw@Excell.Net

I am presuming the set Marc is mentioning is new, so the value can be determined easily enough. He had also mentioned they couldn't be used with the 968 pistons, so I would want to trade/sell 1 of the sets (of 8) I have

Fastest928

Stan is correct...Newin box.

What tol group are the pistons?

Marc

Mike Simard

Don't get your hopes up, that set was from a motor that spun a rod bearing and at least one is visibly trashed. You might find a good used set for something like 1000-1500 but it would be better to just come up with enough to buy them new and not wonder what kind of life they lived.