Corey_68

Ok here's the deal I have the oppertunity to buy a NEW 4340 forged 4.25 stroke crank for my Big Block really cheap (friend owes me a favor). I have .210 dome on the pistons and was curious if I would run any problems running this crank? I'm also worried about the piston to head clearance. The motor is in the car and I hate to pull it apart and measure it for nothing.

Anyone know without pulling the motor??

It's a Gen V 454 H.O. block
stock 7/16 rods and 118cc open chamber heads 2.19/1.88 valves


[Modified by Corey 68, 8:08 PM 5/22/2003]

Pete79L82

No, you have to either change the rods or the pistons or the pistons will hit the heads.


Big blocks have 9.800 deck height. You have to add up stroke/2 + rod length + piston compression heght + quelch height and have it equal the deck height.

A stock 454 is 2.0" (4.0" / 2) + 6.135 + 1.645 + .020 = 9.800

With a 4.250 stroke crank you will need a combination of rod length + compression height + quelch height to equal 7.675"

The best combination with stock rods would be:
stock rod length: 6.135
zero deck height: 0.0
Piston compersion height: 1.540




[Modified by Pete79L82, 8:15 PM 5/22/2003]

bence13_33

No.

At the very least you are going to have to change the pistons otherwise you'll be smacking valves off of them. The reason you have to change pistons is because you need a different wrist pin height.

The rods can be used, however you have 6.135" rods. If you use the 6.135" rods you will have a rod/stroke ratio of 1.44. The lower the number, the higher the amount of stress on the rotating componants. Most 496 kits use 6.385" rods. The 6.385" rods will bring the rod/stroke ratio up closer to that of a 454's. Your rod/stroke ratio will be 1.50 with the 6.385" rods. A 454 has a rod/stroke ratio of 1.55.

So there you have it. If you want to do it right, you'll need the proper pistons and slightly longer rods.

FYI I have a 4340 4.00" stroke Eagle crank in my basement :) . You should also note that if you go with the 4.25" stroke crank you will have to clearance the bottom of the cylinder walls to accept the longer stroke.

Keep us posted.

Corey_68

Damn that suck because I can get this crank for $100 (plus my stock crank) BRAND NEW, never been turned.

The motor all ready in the car, plus I'm looking at another $600 for rods.

Just my luck... :cry

Flareside

Stock rods are also an issue with most 4.25" cranks, because the pistons will contact the crank weights on the downstroke. Lots of labor to make everything fit.

SCAT has just released a cast 4.25" crank that does fit correctly with stock 6.135" rods, and it sells for a little over $200 brand new.

Corey_68

Good price, but my entire rotating assembly is forged. I rather have less cubic inchs and a bulletproof bottom end than a cast crank and 28ci.

Maybe next time. :sad:

bence13_33

Corey:
Actually Corey, a 4.25" stroke crank with a .060" over bore makes a 496. You would definitely be gaining more than 28 cubic inches :) . For what it's worth my motor has forged pistons, 4340 H-Beam rods and a stock LS4 cast crank shaft. It has held up very well thus far. I regularly rev it to around 6500RPMs and the motor has seen 7000RPMs before.

I don't know the bore on your motor so just for the heck of it we'll say it's a 427 +.060" . That makes 440 cubic inches. With a 4.25" stroker crank your motor would be a 496. That's a gain of 56 cubic inches! That's A LOT more cubes. Heck my block was a standard bore block that could have remained a standard bore. I wanted the extra 13 cubic inches so I had it punched out .060" .

Just thought I'd throw that out there :) . But considering your motor is in the car and ready to rock, I don't think I'd go that route now. However, I would (if I were you) probably consider picking up the crank anyhow. That way whenever you pull the motor out some day you can increase the cubes.



[Modified by bence13_33, 10:56 AM 5/23/2003]

Corey_68

Shane, it's VERY temping to pull the motor out. It's a Gen V 454 H.O. + .060 so it's a 468. My dad has told me time and again that I built this motor as overkill. He said in the late 60's and early 70's he's spun his cast crank 427 7K and even to 8K on occasions with no problems. I built this motor with intent to spray so I will a piece of mind. I would still buy the crank, but the deal was to give him mine and he'll give me the stroker crank plus $100. I can't affor $700 for a new crank when the money could be used elsewhere. After all of that I still wish I had those 28 cubic inches. :D

bence13_33

Corey:
I'm sorry, for some reason I thought you were building a 427? I see where you are coming from now.

In hindsight I kind of wish I had gone the 496 route. However I didn't have the money to get the forged crank. I recently picked up an Eagle 4340 Forged 4.00" stroke crankshaft for $375 brand new! I'm going to keep it until I pull the motor to hop it up a little more in the future.

In your case, I would say stick with what you have I'm sure it will serve you just fine :) .

Corey_68

I'm sure it will sufice, I'm just ready to get this thing fired up and take it to the track. :reddevil

marky mark

Corey, with the Gen V block you will need to be sure the crank is of the one piece seal variety. Unless, of course, you have already switched over to a 2 piece seal adaptor kit.

I'm contemplating a similar build up. Estimate about $2,000 in parts just for the forged rotating assembly and various other items needed. Stroker cranks forged (425) can be internally balanced. Cast cranks (400, 425) are externally balanced. If you go internally bal. you need a new harmonic bal and flywheel/flexplate. It just keeps adding up... these darn Big Blocks!!!

Good Luck!!

Regards, Mark