dgruenke

I was browsing Scat's website and ran into this:

Precision ground & micropolished, balanced to factory specs (except as noted) and ready to install right “out of the box”


383 SMALL-BLOCK CHEVY 350 MAIN 2-PC REAR SEAL

9-103750 5.700" Rod Length, 3.750" Stroke, Factory Balance, external.



How in the world can this crank be "Factory Balance" and "ready to install right 'out of the box'" when they don't know what rods, pistons, etc. are going to be used?

Guru_4_hire

Probably can send them an email asking.

dgruenke

I just went a step further and called them. I asked them how the crank could be externally balanced and ready to install out of the box. He said, "I wouldn't do that".

I couldn't help but laugh because that is exactly what their site says you can do. He says that they probably only balance it to about 1300 bobweight, and that you will need it balanced at about 1750 bobweight.

Looks like they need to change their site.

Guru_4_hire

That sounds like there is a disconnect between sales/marketing and the engineering staff.

68/70Vette

I think the Factory Balance statement just assumes that you use factory rods, pistons, etc. If you vary, then balance is up to you.

My personal opinion. If you're going to build your own engine, you should pay the extra money to internally balance the engine with the pistons, rods, etc you select. The 383 cranks, "Factory Balanced," are externally balanced as far as I know. If your crank is not internally balanced, my understanding is that the bearings and the crank itself take a lot of stress at high rpms.

dgruenke

Yeah, but what car came with a factory 383? There isn't one. Therefore, you cannot use the factory rods and pistons.

TotalC1971

Dont waste your time with a cast crank,definately will not hold up. I just picked up the forged pro-lite from CNC Motorsports in S.D. for $900 , and that included shipping! Internal balance is the way to go, they might be getting by with a 400 balancer & Flex/Flywheel.

dgruenke

I'm not going to be pushing enough horsepower to need a forged crank. It would just be overkill.

cardo0

I believe the SCAT cranks were balanced well enough to use as stock replacements or even hi-po engines with the proper parts/weights selections. That was thier economical advantage as a budget performance crank. SCAT was known for high quality control performing all the same inspections as the big national race engines used. Every crank saw non-destructive crack inspection and generous journal radius for stress reliveing then of course balanced for a specific rotating weight. But i think u were asking to install a crank that was made for different rotating assembly weights than what u want to use.

Well don't take my word for it but try David Vizard's "How to build Hi-Po Chevy sb on a bugdet".
And most street performance applications running <450hp and @<6,000rpm don't need the internal balance <2 grams.
IMHO u can select parts well enough to install without balancing at all and still build a reliable hi-po engine.

Good luck.
cardo0

bluegtp

for a street car don't worry about it, ext balanced cranks are just as good as internally balanced cranks.....as long as you have it balanced correctly with your setup.......I'm running a scat 383 9000 cast crank, ext balanced. I believe it's a "light weight" one too.....I had my whole setup balanced (rods, pistons, flexplate, balancer).........even if i would've bought an int balanced crank, i would have still had to get it balanced cause I had to clearance the rods for the stroker. you change the weight, you throw of the geometry of the "factory balance."


now, as for scats quality control....hhmmm, that's a different story.....the guy that balanced my bottom end said when he put it on the machine and started spinning it, it was shaking the machine.....the little bit off the rods couldn't have thrown it off that much.....anywho, he did the necessary taking away weight and it was smooth and good to go........

jackson

1300 no way! more like 1900. I've called scat at different time asking avg mass of two different rods ... got bs responses. For the price, I like their stuff ... but their tech support is miserable & totally unreliable. Bottom line, expect to have any crank/rotating assembly balanced.

dgruenke

That's kind of what of thought when I was talking to them. Although, they also have completely balanced rotating assemblies. I have seen some forum members that said that it was right on. Have you had any experience with their balanced assemblies? I do have some reservations about getting a balanced assembly and then having to cam clearance the rods.

Manuel Azevedo

1300 is in the ball park for a small block. you only use the rotating part of the rod in the BoB weight. The formula goes as list below, and I have done these for a lot of years this and works well. As far as internal or any other way makes no difference as long as you bring the meters on the machine within .0001


1-piston
1-set rings
1-piston pin
1-set locks or buttons if used
1-small end of rod
2-big end of rod
2-complete bearing for two rods
These are the weights you would use to make up your BoB weight. This is only for a 90 degree v engine, all changes with different degree engines such as a 60 degree V6.
You are only useing one set of recipacating weigth because in the V engine they are completely countered by the other piston assm. on the same journal. Hope a didn't make this sound to confussing.

Pete K

For what it's worth, I normally run 1900 (give or take) bobweights. Heavy stuff.
I typically install 1 slug of Mallory in the front of the crank. This allow me to run a neutral balance in the front and an external rear flexplate. Works pretty weel. Standard balancing touch up is always needed.

ratflinger

My Eagle forged kit was factory balanced & my machinist checked the whole assembly out. He said the factory balance was right on. I don't know about Scat though.

68/70Vette

WRONG,WRONG. THERE IS A FACTORY 383. Its in the General Motors Performance Catalog. The GMPP engines are factory engines. This is the engine that a lot of people have bought, and it's also the I wish I had bought instead of my ZZ4. (Didn't know at the time.) The 383 Hot Burn engine is a factory GM engine targeted to sell to people that have older cars that don't have to comply with the smog conventions.

dgruenke

Were the rods already cam clearanced or did they clear without machining?

dgruenke

No, I asked "what car came with a factory 383". The answer is none.

68/70Vette

I think this is pretty much a true statement. Actually, many production street engines are externally balanced. However, if I build an engine, I'd prefer that it was internally balanced. At each piston throw in the crankcase, I'd prefer that the balance at that point was correct. Balancing a long crackshaft and all the rods, pistons, etc, just by adding a weight on the flywheel doesn't seem right for an engine were you'd like to do everything correct. I think internally balancing really reduces stresses on the crankshaft bearings and this probably results in additional power from the engine. I would be very surprised to find that NASCAR engines, F1 engines, etc were externally balanced. As a notion, i'd bet they were all internally balanced.

jackson

Your formula is correct ... plus about 4g of parasitic oil. But I dunno what sbc (titanium rods?) you're building with 1300g bw? I've built a few LIGHT ct motors and they aren't close to 1300. A 355 w/ JE Superfly (lite!) & 5.7 Carrillo ProA LTD (lite!) has a bw just under 1500g ... do the math ... if I'm wrong I'll buy the beer!