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iron block, aluminum main caps

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iron block, aluminum main caps

Postby dan miller » Mon Jan 10, 2011 12:30 pm

I'm building a JrFuel engine - 4.125" X 3.825", 409 cid early hemi, N/A methanol, filled block, 900 hp.

We put aluminum caps (five, center three splayed 4 bolt) on a previous engine, and the block cracked after just a few passes. The #2 main web is the weak spot on early hemi's, and that is where it cracked.

We had bored out the cam tunnel (55mm as I recall), but that's a long way from the main bearing, and we figured the cracked block was probably due to the aluminum main caps. We had also bored this engine out to 4.125" which is very large for a 354 hemi.

We then bored out the cam tunnel on our original 4.035" bore/steel main caps JrFuel engine, which had been running for four (maybe five) seasons. It ran well for three passes, then started slowing down. After the tenth pass, we pulled the pan, and observed that the #2 main web had cracked in exactly the same place as the aluminum main cap engine.

Bottom line: The aluminum main caps save 10 pounds, and I have one set of aluminum and one set of steel main caps. The block is headed for the machine shop tomorrow, and I need to decide on steel or aluminum caps. If steel, and no issues, we'll never know what would have happened had we utilized aluminum. If aluminum, we'll probably learn, but it will be an expensive lesson.

I don't see any SBC JrFuelers (the only other engine currently running in JrFuel) running aluminum.

At 3.5 pounds/cid, 10 pounds equals about 3 cubic inches, which at 2 hp/cid equals about 6 horsepower.

I'm sure wanting to do it, but scared. lol

Whatcha think?

Thanks, Danny
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Re: iron block, aluminum main caps

Postby governor » Mon Jan 10, 2011 12:47 pm

I would think you would want to use metal of simular expansion rates, maybe the aluminum was creating stress due to it expanding furture.

There will be other with more metal knowledge than I, but it sure seems to make since to me to use the steel main caps.

I have never seen or heard of aluminum caps on an iron block

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Re: iron block, aluminum main caps

Postby Keith Morganstein » Mon Jan 10, 2011 1:33 pm

Not unusual to use aluminum main caps on the B / RB Mopars. In those engines it works. Your block???
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Re: iron block, aluminum main caps

Postby governor » Mon Jan 10, 2011 1:34 pm

All SBC's

Hey it was worth getting out of bed, putting my feet on the floor and sucking wind as I learned something today. You see I live in the sheltered SBC world and was not aware that was a common practice.

Thanks,

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Re: iron block, aluminum main caps

Postby Unkl Ian » Mon Jan 10, 2011 1:38 pm

Any chance the Aluminum caps are not stiff enough, and flexing under load ?
The bolts transfer the flex to the block.
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Re: iron block, aluminum main caps

Postby Stan Weiss » Mon Jan 10, 2011 2:05 pm

Danny,
No experience on these blocks. But the conclusion I would draw from what you have posted is the bored out cam tunnels is the only thing both block had in common when they cracked.

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Re: iron block, aluminum main caps

Postby Dick Gazan » Mon Jan 10, 2011 2:31 pm

Aluminum caps were common in the 60's in Top Fuel, ran a few sets myself with no problems. I'm curious about the overbore however. As these blocks got hard to find before the Donovan's a lot of guys tried to bore them over .040 and just ran into a nightmare. Some ere even 8 sleeved and maybe lived 4 rounds. I've never overbored cam area but the blocks were so fragile that I would say that's where your problem is.....

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Re: iron block, aluminum main caps

Postby dfree383 » Mon Jan 10, 2011 2:53 pm

Maybe try Cryogenicly treating the block? or a different firing order?
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Re: iron block, aluminum main caps

Postby PackardV8 » Mon Jan 10, 2011 3:55 pm

X3 on boring the cam tunnel most likely being the source of the problem.

Guess I quit looking at Top Fuel before the days of aluminum main caps. All I remember working with was at first chrome-moly straps across the top of the main caps and later a Crower one-piece billet steel main girdle which weighed at least forty pounds. Talk about a bitch to get all five caps and the pan rail cap screws to fit your block. They were hell-for-stout. I saw one blown out onto the track, but the girdle took the bottom half of the block with it.

You mentioned boring to 4.125". Back in the bad old days, a guy I helped spent a ton of money trying to run a 4.125" x 4.25" = 454" Top Fueler because he heard all the California guys were doing it. Whatever secret they had, he didn't and it was just a 500' hand grenade. He finally went back to 392", ran faster and kept it together enough to win an occasional race.

In those days, running a top fueler meant having every used car guy and wrecking yard guy you knew scouting the country for '57-58 Chryslers and Imperials. On 98%, the blocks had a finite life span, but exactly how long was unknowable. Most serious guys built a fresh used block for every major event.

FWIW, most of those early '50s blocks are weak up the middle, including the Cadillac, Oldsmobile and Packard V8s. Lots of beef in the cylinder walls, but easy to crack up the webs. The Olds was particularly bad; the stress of a 6-71 blower drive could be too much. I remember Tom Beatty using six V-belts on an Olds, because that way he didn't have to run them with heavy tension on them.

Dan, one thing I like to do is custom fit a set of ARP main studs. Take a bottoming tap and cut the threads all the way to the bottom of the hole. Usually, this will give 1/2" more thread depth than OEM main cap screws. Measure to the bottom and order appropriate length studs from ARP. When you are ready for final assembly, hand install the studs in the block with Loctite and immediately torque the main caps into place. My theory is this extra thread engagement spreads the forces over the greatest possible area of the main webs.

One other main cap technique I saw a few years back was machining the main cap and block mating area for longitudinal square keys. The demonstration consisted of installing the center main keys in the block, tapping a main cap into place and then hoisting the block by the cap. Just the interference fit was all holding it together. Anyone else have any first hand experience with this?

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Last edited by PackardV8 on Mon Jan 10, 2011 4:22 pm, edited 1 time in total.
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Re: iron block, aluminum main caps

Postby robert1 » Mon Jan 10, 2011 4:05 pm

It's not going to help you decide but, I got in a 350 to freshen up that had steel splayed caps. The block was broke in the thickest part of the main webbing. I've seen aluminum blocks crack where this one did but not iron. Just goes to show no matter how hard we try to keep these things together it doesn't always work.
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