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All aluminum engine vs cast iron (Piston to head)

General engine tech -- Drag Racing to Circle Track

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All aluminum engine vs cast iron (Piston to head)

Post by CGT » Mon Jun 17, 2019 3:58 pm

How much block growth is there on aluminum and how much would it affect hot piston to head clearance?
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Re: All aluminum engine vs cast iron (Piston to head)

Post by PackardV8 » Mon Jun 17, 2019 4:06 pm

How high is up?

Are we to answer about a Subaru block 6" tall, a SBC 9" tall or a Ranger 18" tall?

Steel or aluminum connecting rods?

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Last edited by PackardV8 on Mon Jun 17, 2019 4:13 pm, edited 1 time in total.
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Re: All aluminum engine vs cast iron (Piston to head)

Post by CGT » Mon Jun 17, 2019 4:09 pm

PackardV8 wrote:
Mon Jun 17, 2019 4:06 pm
How high is up?

Are we to answer about a Subaru block 6" tall, a SBC 9" tall or a Ranger 18" tall?
assuming roughly 9 inches
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Re: All aluminum engine vs cast iron (Piston to head)

Post by Caprimaniac » Mon Jun 17, 2019 4:37 pm

Just as important; bore size.
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Re: All aluminum engine vs cast iron (Piston to head)

Post by Brian P » Mon Jun 17, 2019 4:42 pm

Based upon http://hyperphysics.phy-astr.gsu.edu/hb ... thexp.html or https://www.engineeringtoolbox.com/line ... -d_95.html

Aluminum (nominally) has a coefficient of thermal expansion of 24 x 10^6 per degree C. (Could be 21 - 24, let's use worst case)
Steel is around 12.

What's relevant is the differential expansion between the cylinder block and the steel bits inside (connecting rod, crankshaft).

I will call your block height 200mm and I will call the temperature difference to be 100 C for easy calculations and make a simplifying assumption that the crank, rods, and block, and the oil splashing everywhere inside and all over these bits, are the same temperature.

200 mm x 100 C x 0.000024 = 0.48mm (that's the amount the block "grows")
200 mm x 100 C x 0.000012 = 0.24mm (that's the amount the steel bits inside "grow")

So the block "grows" 0.24mm (or about 9 or 10 thousandths of an inch, if you prefer) relative to the steel bits inside with a temperature difference of 100 C. Piston to head clearance increases as the engine warms up (because the aluminum block is "growing" faster than the steel bits inside, for the same temperature difference).

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Re: All aluminum engine vs cast iron (Piston to head)

Post by Kenny M » Mon Jun 17, 2019 4:51 pm

I just built a all aluminum 9.4 deck ford engine. I set the valve lash at .000 cold when it gets to temp. The lash is .015. not sure if this info help you or not. I had the same numbers on a old ZL/1 and AMC all aluminum engines.

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Re: All aluminum engine vs cast iron (Piston to head)

Post by MadBill » Mon Jun 17, 2019 11:10 pm

To slightly de-simplify (some would call it nit-picking), some other points:
o For longer run time than drag racing, the steel parts are likely to run hotter than the block, maybe + 20-30°C., reducing the relative expansion by ~ 0.10 mm or 0.004".
o ~ 1.5"/38 mm of the internal parts stack-up is the piston CH, which expands at twice the rate as the crank and rod, but also runs much hotter, possibly 200°C. or 100° more than than the block, 80° more than the above revised internal parts temperature so a further reduction of ~ 0.036 mm or 0.0015".
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Re: All aluminum engine vs cast iron (Piston to head)

Post by twl » Tue Jun 18, 2019 11:01 am

For anecdotal comparison, our vintage all aluminum motorcycle engine with 3.54" stroke and 3.4" bore has .012" thermal expansion vertically when hot.

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Re: All aluminum engine vs cast iron (Piston to head)

Post by MadBill » Tue Jun 18, 2019 12:07 pm

Is that measured or computed?
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Re: All aluminum engine vs cast iron (Piston to head)

Post by pamotorman » Tue Jun 25, 2019 11:30 am

when I built all aluminum BBC with steel rods I set the pistons at .010 out of the block at piston rock both sides with a .042 head gasket. I told the owners not to run at full throttle till up to temp.

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Re: All aluminum engine vs cast iron (Piston to head)

Post by Caprimaniac » Tue Jun 25, 2019 5:41 pm

That would make 0.032 P-H clearance, Cold. If we say the Growth is 0.01+, it clearance goes to 0.045 at temp, which is in the ballpark for a BB?

A smallblock 4-4.15" bore.... 0.035" at temp, so 0.020- 25 Cold would be OK? Assuming steel rods.

There was a thread on this subject some years back, With good info. Discussing aluminum rods, too. Maybe quench- related discussion.
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Re: All aluminum engine vs cast iron (Piston to head)

Post by twl » Tue Jun 25, 2019 6:34 pm

MadBill wrote:
Tue Jun 18, 2019 12:07 pm
Is that measured or computed?
It is both measured and computed.
The lash gap is set zero cold, and grows to .012" hot.
This is factory specs, and it bears out when measured, although being air cooled it can vary very slightly depending on how hot it is running.

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Re: All aluminum engine vs cast iron (Piston to head)

Post by MadBill » Tue Jun 25, 2019 9:25 pm

Not trying to be A-R here, but the rocker ratio is 1.0:1.0 then?
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Re: All aluminum engine vs cast iron (Piston to head)

Post by twl » Tue Jun 25, 2019 11:29 pm

MadBill wrote:
Tue Jun 25, 2019 9:25 pm
Not trying to be A-R here, but the rocker ratio is 1.0:1.0 then?
Yes.
Old Brit bike.

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