Which Aluminium grade for Main caps?

la360 · Post by **la360** » Thu Jun 23, 2005 11:38 pm

Which grade is preffered, I haven't had a change to research it at all. 6061 or 7075?
Thanks
AL...

OldSStroker · Post by **OldSStroker** » Sat Jun 25, 2005 7:23 pm

What kind of engine for the alum main caps? Why aluminum?

7075-T6 is stronger than 6061-T6, but just about the same strength as low carbon (mild) steel.

Matey79 · Post by **Matey79** » Sat Jun 25, 2005 8:17 pm

I also wonder why some manufacturers go to the aluminum cap design. I know I did a Mopar Race Block some time ago that was an Iron Block/Alum. Main Caps. Scary to think what happens with heat and things moving around!

la360 · Post by **la360** » Sun Jun 26, 2005 7:34 am

Small block Chrysler , factory block. Aluminium absorbs bottom end harmonics better than steel????
AL...

OldSStroker · Post by **OldSStroker** » Sun Jun 26, 2005 8:50 am

la360 wrote:Small block Chrysler , factory block. Aluminium absorbs bottom end harmonics better than steel????
AL...

Isn't sufficient strength to keep the crank in the engine at max rpm/power Job One? Aluminum caps on an iron block seem to me to be the weak link.

I'm not familiar with "botom end harmonics". Please elaborate.

crazycuda · Post by **crazycuda** » Sun Jun 26, 2005 8:22 pm

the 7075 is more commonly used for main caps. To my understanding the alum mains help because they are not as brittle as cast/steel main caps. Where the alum cap will flex the steel one has a better probablilty of cracking

la360 · Post by **la360** » Mon Jun 27, 2005 2:18 am

I think this begs the question Why engine builder's fit aluminium caps to Blocks , over a set of 4340 steel caps? It is obviously more common in the Mopar engine building circles than anywhere else. The common reason given for doing this is that Aluminium absorbs the harmonics/vibrations produced during the engine's operation better than steel or cast iron. I think alot of us have just taken this as gospel, myself included , without really understanding the ins and outs of the whole situation.
Alot of guys running factory Mopar B and RB blocks, will run the Aluminium caps as apposed the factory caps, which crack, or a steel cap, because the common thinking is the block lives longer in a highly stressed situation, and I guess this is backed up by the number of varying Brands of aluminium caps available for them currently. I was following the same thinking with my small block, I was curious to see what affects and possible benefits the Aluminium caps may produce.

Any Thoughts?
AL...

OldSStroker · Post by **OldSStroker** » Mon Jun 27, 2005 8:44 am

la360 wrote:I think this begs the question Why engine builder's fit aluminium caps to Blocks , over a set of 4340 steel caps? It is obviously more common in the Mopar engine building circles than anywhere else. The common reason given for doing this is that Aluminium absorbs the harmonics/vibrations produced during the engine's operation better than steel or cast iron. I think alot of us have just taken this as gospel, myself included , without really understanding the ins and outs of the whole situation.
Alot of guys running factory Mopar B and RB blocks, will run the Aluminium caps as apposed the factory caps, which crack, or a steel cap, because the common thinking is the block lives longer in a highly stressed situation, and I guess this is backed up by the number of varying Brands of aluminium caps available for them currently. I was following the same thinking with my small block, I was curious to see what affects and possible benefits the Aluminium caps may produce.

Any Thoughts?
AL...

Maybe it's the Coriolis effect of running a North American designed engine below the equator.

Thoughts:
With only a couple thou main bearing clearance, distortion of the main bores needs to be minimized as much as possible iln order to keep from wiping out bearings and then the crank.

If the blocks are weak enough to move around and crack iron caps, aluminum caps will allow things to move around more, but to me that means wiping out bearings, and sooner or later running over you own crank.

One of the cap's duties is to stiffen up the lower end of the block. 4-bolt, and cross bolting and girdles or structural oil pans all lead to this.

I was not familiar with aluminum cap conversions for Mopars, and still don't see the benefits. Aluminum has a shorter fatigue life than iron or steel, so if they "absorb(s) the harmonics/vibrations produced during the engine's operation better than steel or cast iron" they should have a very short lifespan.

FWIW, cast iron is about the BEST metal for absorbing vibrations. It is used extensively for machine tool bases just for that reason.

Someone else might be able to shed more light on this.

My $.02

Post by **SchmidtMotorWorks** » Mon Jun 27, 2005 10:48 am

Maybe the reason for Aluminum is that they are easier to make?

OldSStroker · Post by **OldSStroker** » Mon Jun 27, 2005 11:11 am

SchmidtMotorWorks wrote:Maybe the reason for Aluminum is that they are easier to make?

While that does apply to beer cans, I'm not sure easier to make (read cheaper) applys to main caps.

shawn · Post by **shawn** » Mon Jun 27, 2005 1:14 pm

We put them on to save weight, and they ran just fine in a 900hp small block.
shawn

la360 · Post by **la360** » Mon Jun 27, 2005 2:42 pm

What kind of Small Block was that Shawn??
Isn't running Aluminium rods in a Nitrous engine done for similar reasons? Or is it just a case of the Aluminium allows things to move around a little more without breaking?
AL...

shawn · Post by **shawn** » Mon Jun 27, 2005 3:46 pm

It was one of the chrysler prostock truck motors.
shawn

Matey79 · Post by **Matey79** » Mon Jun 27, 2005 7:16 pm

Shawn- The motor I did was an Ex-Pro Stock Truck block. It runs 930 HP @ 9600 RPM. I was surprised it would survive with the aluminum caps. The Main Housing bores measured .001 tight from the tight side of the factory spec. The clearances where on the tight side too. But has survived the 1/4 mile for 3 years.

shawn · Post by **shawn** » Mon Jun 27, 2005 7:25 pm

did it have anything stamped on the caps?
shawn