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Old 08-20-2019, 07:11 AM
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mchell mchell is offline
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Default Forged rotating assembly’s

Building a new bottom end.

What forged rotating assembly’s are preferred these days.........3” mains, 70 400 block, 4.25 or 4.5 stroke.

Any reason not to run a 4.5 stroke over a 4.25?

Scat
Ohio
Eagle
Molnar

Others?

Looking for the best bang for buck if possible.

Also, Would you build a forged bottom with studded two bolt, splayed or 4 bolt caps.

600hp is goal for this build....90% street 10% strip, run hard either way

What’s your opinion?


Thanks

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71 GTO, 463, KRE 295 cfm heads ported by SD Performance, RPM intake, Qjet, Dougs Headers, Comp cams HR 246/252 ...11 to 1 , 3.55 cogs, 3985lbs.....day three- 11.04 at 120mph ....1.53 60', 6.98 1/8 mile

Last edited by mchell; 08-20-2019 at 07:16 AM.
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Old 08-20-2019, 07:26 AM
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The best as you say bang for the Buck is the 4.250" stroker if your going to use your KRE heads on top!

The added low end torque of the 4.500" stroke does not bring much to the table in terms of street performance, but it does allow either a better idle or allow you to run a bigger Cam without any down side.

2 bolt studded caps will hold up to 600 hp when prepped right, but I would be darn scared to make 600 hp with them with 4.500" stroke trying to rip them off the block as the 2 bolt caps are not the made like the factory 4 bolts caps with better nudular / Armasteel cast iron!

Even running 2 bolt studded caps with a factory 4.210" stroke up to 6000 rpm is starting to get risky to me!
Are you assembling the short block yourself?

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Last edited by steve25; 08-20-2019 at 07:36 AM.
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Old 08-20-2019, 07:38 AM
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Hi Mchell: I run 2- early 455 4 bolt blocks, .030 . Both made 600 hp 600 torque with KRE D-ports. 4.25 Scat forged cranks, Forged Ross pistons, Forged eagle rods. Both blocks use ARP studs. Both are Hard blocked. We run Torker 2 with 950 quickfuel. No issues to date.

In your signature it looks like your combo would or should be somewhere around 600 hp already.

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Old 08-20-2019, 07:57 AM
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I would recommend the 4.25" stroke over the 4.5". It's just easier on the bearings, less piston speed, wrist pins aren't clear up in the rings, and easy to make 600hp.

Unlike the cast steel cranks all the forgings we've done have been excellent dating clear back to the first one we did when they became available. That particular engine is a 505, KRE high ports, solid roller and made 735hp on the dyno. In all these years it's had one set of rod bearings "rolled in", valves springs and a rocker arm upgrade.

I quit doing these engines a couple of years ago, but up till that time every forging we'd used was from Star Galaxy and spot on everyplace. Not sure what's out there now, but like many other parts it's likely they are all from the same source no matter who is marketing them here under their label......Cliff

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Old 08-20-2019, 08:06 AM
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Bang for the buck leader might be the SGI forged rotating assemblies. $495 plus shipping for the crank. Seem to be ok pieces, comparable to an eagle or Ohio.

4 bolt main with 600hp my pick do the 4.5 in a street car. Two bolt the 4.25 crank would be my pick, bang for the buck, hard to beat the 4.25 with some main studs.

SGI seems to be the star galaxy supplier


Last edited by Jay S; 08-20-2019 at 08:08 AM. Reason: Edit
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Old 08-20-2019, 08:20 AM
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Here’s what I did to my N crank.

Look between #3 and #4 mains.

WAS a 455 block, 6.6.35 (2.2 journal)eagle h beams, 2 bolt mains, BRC pistons. 11 to one

Bearings were excellent, no signs of detonation etc....appears that crank just snapped

Don’t want to mess with this again...$$$$$

Thought the N cranks could hold moderate power...
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71 GTO, 463, KRE 295 cfm heads ported by SD Performance, RPM intake, Qjet, Dougs Headers, Comp cams HR 246/252 ...11 to 1 , 3.55 cogs, 3985lbs.....day three- 11.04 at 120mph ....1.53 60', 6.98 1/8 mile
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Old 08-20-2019, 08:32 AM
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Better pic
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71 GTO, 463, KRE 295 cfm heads ported by SD Performance, RPM intake, Qjet, Dougs Headers, Comp cams HR 246/252 ...11 to 1 , 3.55 cogs, 3985lbs.....day three- 11.04 at 120mph ....1.53 60', 6.98 1/8 mile
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Old 08-20-2019, 08:50 AM
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mchell, how much HP\TQ in that cast motor? dang hate to see this. How hard was that motor been run? a lot of track time? street? Always thought you had an excellent combo, runs hard. Love the 71s, make it better this go round.

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Old 08-20-2019, 08:58 AM
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Mchell. .. That rod bearing was fretting by evidence of the rod = pre'ignition / detonation.

Anytime you use a part that is marginal for the job conditions need to be a little more perfect

That crank was getting hammered

.

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Old 08-20-2019, 09:07 AM
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Quote:
Originally Posted by Formulas View Post
Mchell. .. That rod bearing was fretting by evidence of the rod = pre'ignition / detonation.

Anytime you use a part that is marginal for the job conditions need to be a little more perfect

That crank was getting hammered

.
The bearing is not on the rod in the pic...the crank broke, snapped the rod bolts and wedged the cap and bearing into two cylinders seizing the rear half of the engine

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71 GTO, 463, KRE 295 cfm heads ported by SD Performance, RPM intake, Qjet, Dougs Headers, Comp cams HR 246/252 ...11 to 1 , 3.55 cogs, 3985lbs.....day three- 11.04 at 120mph ....1.53 60', 6.98 1/8 mile
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Old 08-20-2019, 09:17 AM
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Its obvious the bearing is not in the pic, the fretting was between the bearing and rod, which means the bearing was dancing around from high frequency as in detonation / pre'ignition (crank killer)OR the bearing was loose in the rod to start with but that typically doesn't break a crank, that typically turns into a spun bearing

Your engine call it what you want

.

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Old 08-20-2019, 09:33 AM
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Quote:
Originally Posted by Formulas View Post
Its obvious the bearing is not in the pic, the fretting was between the bearing and rod, which means the bearing was dancing around from high frequency as in detonation / pre'ignition (crank killer)OR the bearing was loose in the rod to start with but that typically doesn't break a crank, that typically turns into a spun bearing

Your engine call it what you want

.
Right.....would that translate to more bearing wear on the actual bearing surfaces?...looking at the bearings that came off, I couldn’t find evidence of wear.

One other question, could the crank have been cracked for some time and causing harmonics to this area?

Thanks

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71 GTO, 463, KRE 295 cfm heads ported by SD Performance, RPM intake, Qjet, Dougs Headers, Comp cams HR 246/252 ...11 to 1 , 3.55 cogs, 3985lbs.....day three- 11.04 at 120mph ....1.53 60', 6.98 1/8 mile

Last edited by mchell; 08-20-2019 at 09:39 AM.
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Old 08-20-2019, 09:48 AM
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What kind of balancer up front? Possible to over RPM a stock balancer as RPM increases the frequency changes the balancer might not of been able to dull the vibrations enough.

Just saying looks like fretting and usually a crankshaft has a contributing factor to break.

A forged crank is the way to go tho.

.

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Old 08-20-2019, 09:56 AM
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Hp made and the loads on the rotation assy in terms of G force loads are two different forces .

The internal loads go up with the square of the rpm.

The loads at 4000 rpm for example are twice as high at 6000 rpm, so concider that going up from a 4.250" stroke to a 4.50" stroke is a increase in loads of 6% which is a minimum of a 12% G force increase at 2000 rpm .

So with the square of the rpm factor thing going on it's not hard to see why what was a good cast Crank gave up on you even with just a 4.210 stroke.

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Old 08-20-2019, 09:59 AM
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Quote:
Originally Posted by Formulas View Post
What kind of balancer up front? Possible to over RPM a stock balancer as RPM increases the frequency changes the balancer might not of been able to dull the vibrations enough.

Just saying looks like fretting and usually a crankshaft has a contributing factor to break.

.
Appreciate your input.

I’m running a professional products balancer....nothing great by any means.

I admit, I did run it up past 6k at times.....got 100”s of 1/4 mile passes.....it gets run, street or strip

Never exhibited a hint of detonation....bearings Faces were in excellent condition, even with the amount of metal flying around after it broke

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71 GTO, 463, KRE 295 cfm heads ported by SD Performance, RPM intake, Qjet, Dougs Headers, Comp cams HR 246/252 ...11 to 1 , 3.55 cogs, 3985lbs.....day three- 11.04 at 120mph ....1.53 60', 6.98 1/8 mile
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Old 08-20-2019, 10:37 AM
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Quote:
Originally Posted by steve25 View Post
Hp made and the loads on the rotation assy in terms of G force loads are two different forces .

The internal loads go up with the square of the rpm.

The loads at 4000 rpm for example are twice as high at 6000 rpm, so concider that going up from a 4.250" stroke to a 4.50" stroke is a increase in loads of 6% which is a minimum of a 12% G force increase at 2000 rpm .

So with the square of the rpm factor thing going on it's not hard to see why what was a good cast Crank gave up on you even with just a 4.210 stroke.
Agreed .........thanks

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Old 08-20-2019, 10:59 AM
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I would be interested in seeing what the face of the broken off piece of the crank looks like. Kind of looks like there may a been a flaw in the front side of the throw. Hard to see in the pictures though.

Not that it really matters. Forged crank will take care of any worries. Go for a forged and get it going again. Looked like you have it running good. Nice et and mph.

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Old 08-20-2019, 11:31 AM
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I went with a 4.375 Molnar crank in my last build and 6.7 rods. Got the pin out of the oil ring so no supports needed.

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Old 08-20-2019, 03:31 PM
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2 bolt block, not drilled, and have 4 bolt caps fitted, can be straight bolts, or splayed, but if you lean towards splayed, you need a good machine shop that has done Pontiacs before. Fill it to the bottom of the freeze plugs and you will be fine at those power levels.

Stud the mains and heads, unless of course you go splayed, then you have to run bolts on the outers.

I say 4.25 too, and certainly forged. That broken crank looks like it's a cast one.

Eagle and Ohio have been what I've used in the past, may have used a Star, can't recall.

Your choice of rods and pistons, see what deals you can get on kits, and see what options the vendor you choose has for pistons/rods.

I've been leaning towards the round port E-heads for a while, and they have served me very well. The KRE D-ports need work to get to 315cfm or so, E-heads not much at all. And personally, round ports are much easier to install headers on.

68-72 A-bodies can squeeze a Victor under the hood with minimal effort, or if you want, an RPM will be fine at a 460-something engine. RPM manifolds fit easily.

Def only run SFI balancers and flexplates/flywheels.

I say solid roller, at most you only have to check lash twice a year, depending on how many miles you drive.

.

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Old 08-20-2019, 03:43 PM
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The 70 block I’m planning on using is drilled for 4 bolt caps so splayed caps are out.....it’ll have to be studded 2 bolt or the pro-gram 4 bolt caps at this point.

already own prepped kre d port heads, so I’m good there. Planning on using a roller this time.

Thanks everyone for the info you have provided so far...

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