4032 Pistons????

Now that I have solved the issue of sleeving/overbore I'm on to pistons. I thought my options were going to be limited to traditional cast or forged examples. In my internet travels I discovered pistons made from 4032 aluminum alloy that the designer claims need only a .003 to .004 skirt clearance in a Norton 750 engine. Appearing to be the best choice, by far, makes me very skeptical.

Is this a believable claim?

The most common alloy for aftermarket forged pistons in the US is 2618, a fairly low silicon alloy. That's what I have my JE pistons made in, and that's their default material for most of their pistons. They do offer 4032 as an option in some of their piston designs. 4032 is a higher silicon content alloy, and pistons made in that alloy typically do use a smaller piston-to-cylinder clearance. The appropriate clearance also depends on the piston design, but .003" to .004" for a Commando piston in 4032 sounds reasonable to me.

Ken

Some comments on the two alloys from a Mahle designer.

McFarland: These two alloys are similar in many respects. In terms of overall strength the 2618 edges 4032 out by a small margin. Silicon content makes up the main effective difference with the 4032 alloy containing 12-13 percent silicon compared to 2618 alloy having 0.2 percent or less. Silicon reduces heat expansion, while being hard increases wear resistance. The decrease in expansion allows for tighter clearances, reduced wear on both the piston and bore, also resulting in quieter operation. The hard silicon element greatly helps to increase the number of heat cycles the piston can endure before ring grooves and skirts start to distort. This makes the 4032 alloy well suited for a wide range of applications from street performance to upper level sportsman racing. The 2618 alloy is more malleable, allowing it to flex and move under extreme loads further and more frequently before reaching the point of fracture. This gives the 2618 alloy a greater resistance to the shock loads of detonation. The compromise is that the alloy softens at a much faster rate, allowing the piston to distort more rapidly. This makes the 2618 alloy best suited for extreme-duty race applications where the engine will be serviced on a regular schedule. Mahle uses the 2618 alloy for extreme-duty applications or those that have a high likelihood for experiencing aggressive and or frequent detonation. Due to the increased wear resistance and longevity characteristics of the 4032 alloy, Mahle uses this alloy on a wider scale.

Ken

"Two of Ford, one of Chevy" - Burt Munro

I'm running 4032 pistons in my motor right now.

But if I had a choice I probably would have chosen a heavy wall cast. Jim

Hi Jim,
Can you elaborate on "heavy wall cast"? What constitutes a heavy wall as opposed to not heavy wall? Supplier?

Thanks,
Ed

Pretty much any cast piston designed for a Norton [or other air cooled motors] will be a heavy walled cast piston.

Look at a cast piston designed for a water cooled engine with tight piston to wall clearance and you will find very thin skirts.
When the piston to wall clearance is less than .001 inch like most water cooled applications then you can get by with thin skirts because they don't get hammered as much.