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Old 01-21-2023, 03:22 PM
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The 235 ft-lbs bolt torque and slight press-fit that Mike mentioned is how we could run a 14-71 blower at 42 lbs boost without a front crank support. We worked a deal with RCD to get Ford crank hubs prior to their final hone (which would have made them loose on the Pontiac nose).

Eric

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  #102  
Old 01-21-2023, 04:56 PM
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Thanks Eric, good engineering information that shows the reasoning behind the statements that Paul K made in an earlier post about a slight press fit was preferred, to a loose slip fit.

I've always liked the design of Pontiac slip fit balancers, over Band X press fit balancers, but that hinges on constant torque on the balancer bolt holding all the pieces together so the snout on the crank won't flex. Your explanation shows how the press fit may be superior, especially for severe side loads, such as driving a supercharger.

Some of the old front engines drag cars would mount the blower directly in front of the engine because they had the room, and I suppose it also side stepped the side loading of the crank snout. The only problem would be the ability to change drive ratios without some sort of a planetary gearbox. in between.

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Old 01-21-2023, 05:06 PM
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As a side note here the balancer goes up against the crank gear and if your crank gear is made right ( some are not ) the end of the crank snout will be right at the end of the hole in the balancer as it needs to be.
The thick washer should not be made to bend when torqued up.

I recall helping a buddy years ago put the front of his motor back together in the car after replacing the chain and gears and then not having his belts line up right.
The reason we later determine was that chamfer on the rear of his new crank gear was not machined deep enough!

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  #104  
Old 01-24-2023, 12:00 AM
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Probably 20 years ago I took my 428. Tank to get checked out and turned if needed they nagged it and found a bunch of small cracks in the radius’. That’s the only factory crank I’ve had to take out of service. I think I went to an Eagle cast crank and ran it for quite a long time till I decided I “needed” to go to forged crank for piece of mind. I bought an Ohio forged 455 crank. It lasted a season and cracked near the thrust main. So much for a forged lasting longer. I did buy another forged and used it with aluminum rods and it has lasted a long time. I figured I lost the first crank because of banging on the 2-step. It could have been a junk balancer too. It’s been so long ago I don’t remember what I used at the time. I only use ATI balancers now days.

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Old 01-25-2023, 06:59 PM
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Default put me down for 1 broken 455 N crank at the front rod throw

My last Iron Dport combination met its demise due to a broken factory 455 N crankshaft that broke at the 1/2 rod throw arm the same exact way another is pictured in this thread.

The crankshaft let go after I had been cruising a sustained 2600 rpm on the freeway for 20 minutes or so.

An interesting thing that happened within 6 months prior to the crankshaft breaking; and I'd be interested to hear you guys' thoughts on this as to whether it may be an indicator there was an issue with the crankshaft

Many of you may have seen or experienced an OEM crankshaft accessory pulley (damper pulley) crack and break off of the damper while leaving the flat mounting portion of the pulley still bolted to the damper. I had this happen twice within a 6 month period and in as many months later the crankshaft broke while cruising on the freeway.

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Old 01-26-2023, 07:25 AM
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In my opinion you can't blame the Crank with what you have posted about here!

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Old 01-26-2023, 10:02 AM
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Quote:
Originally Posted by RBAILEY View Post
My last Iron Dport combination met its demise due to a broken factory 455 N crankshaft that broke at the 1/2 rod throw arm the same exact way another is pictured in this thread.

The crankshaft let go after I had been cruising a sustained 2600 rpm on the freeway for 20 minutes or so.

An interesting thing that happened within 6 months prior to the crankshaft breaking; and I'd be interested to hear you guys' thoughts on this as to whether it may be an indicator there was an issue with the crankshaft

Many of you may have seen or experienced an OEM crankshaft accessory pulley (damper pulley) crack and break off of the damper while leaving the flat mounting portion of the pulley still bolted to the damper. I had this happen twice within a 6 month period and in as many months later the crankshaft broke while cruising on the freeway.
That's an interesting story. .. Did your crank pulleys have the factory support plate in place when they broke?

Do you recall if the balancer bolt was still tight after the crank failed?

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  #108  
Old 01-26-2023, 10:41 AM
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When the second dampener failed on the Rbailey example it would sure seem something was up. Did it break the crank in front of the number 2 main on the rod throw?

This is a little fuzzy to me. But for some reason I recall 4” cranks having more of a tendency to fail mag tests from cracks like Probird described? Am I thinking of 421 cranks?


Very interesting on the Ohio crank breaking down at the main thrust. That is about the only example (so far) with a failure there. I think Steve C’s had a crack there, but it was welded there to fit the 455 N crank in a 3” block. Pontiac is in kind of there own strength category there with the larger diameter mains, it isn’t a place you expect to see a crank break. That must have been a bad crank to fail like that, or they welded up to sell it and didn’t get something right on correcting a issue with the thrust. I don’t think it would fail there from bad harmonics, if it did it would be from more torsional harmonics. We have broke cranks there at the thrust on some diesels, for those it was because the chassis frame on was putting a twist on the block. Broke them in 2 pieces.

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Old 01-26-2023, 03:52 PM
RBAILEY RBAILEY is offline
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Quote:
Originally Posted by PAUL K View Post
That's an interesting story. .. Did your crank pulleys have the factory support plate in place when they broke?

Do you recall if the balancer bolt was still tight after the crank failed?
Yes on the support plate; both times it had a support plate.

And all the bolts were tight ... nothing loose at all. It appeared to be a fatigue failure at the point the factory pulley makes the bend/transition from the flat to the start of the dished area. I might be able to find one of the broken pulleys and post a pic, but Im not sure I have it as I've been reading a book titled "How to throw schitt away"

Jays, the damper did not fail in any way...it was only the pulley that broke.

25Steve, I'm not making any statement about fault of the crankshaft failure etc ... only telling of a sequence of events that led to the crankshaft failure and the fact that I experienced a crankshaft breakage where it seems to be the common place to break them when they do break, that is the 1-2 throw.

Hoping to gain insight into how 2 perfectly good used OEM pulleys broke apart in the way that they did and how that may be a warning to others if they experience the pulley breaking apart that it may be a clue that catastrophic failure could be lurking. After the first pulley broke I replaced it with another identical OEM 2 groove crank pulley. Some 6 months later the 2cnd pulley broke in the same exact way, so then I bought a March aluminum billet pulley and 6 months later the crankshaft breaks.
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Last edited by RBAILEY; 01-26-2023 at 04:44 PM.
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  #110  
Old 01-26-2023, 05:14 PM
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I follow now….RBailey, The harmonic balancer was bad on that engine. There is more to it than it fitting tight on the crank snout. The inertia ring can look like it is in good shape, and feel tight, but the rubber connecting the hub and inertia ring can get old and let the inertia ring on the outside move just enough that it is bad, making the harmonics on the crankshaft worse. It broke in the exact place I was asking about. Breaking on the rod throw in front on the number 2 main seems to be from a bad STOCK balancer.

We had an engine that no matter what we did we could not keep the belts on in. Everything was perfect on belt alignment. We eventually figured out the harmonic balancer was bad, replaced it, then it was fine. Belts lasted forever after that. There was nothing at all that indicated the balancer was bad on that engine. If your lucky you will have the timing marks shift to indicate it is going bad. Many occasion’s though, it can be bad and the timing marks still line up. Having the belt whip excessively when you rev the engine up can be a sign the harmonic balancer isn’t doing it’s job, so is having issues with the crank pulley’s breaking.


Last edited by Jay S; 01-26-2023 at 05:19 PM.
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Old 01-26-2023, 05:50 PM
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I cracked a 455 crank once. It didn't fail all the way. I noticed the outer ring of the factory balancer had moved about a 1/4" to the rear. It was time to pull the motor anyway to freshen it up. The main bearings were pounded up pretty bad and the crank failed the mag test. Its kind of a chicken or the egg thing. Did the balancer fail and caused the crank to crack or did the cracked crank flexing around cause the balancer to fail?

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Old 01-26-2023, 06:02 PM
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Hmmm yea: cranks cause the damper to fail, then dampers cause the crank to fail.

Some of us have done a cam change and failed to tighten the big bolt: damper hub cracks around the crank hub, usually when driving at peak torque rpm. I know that 1st hand.

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