How much taper on a 4032 piston skirt?

[the_crack_fox]

How much taper would you expect to see on a piston made from 4032 alloy?

From what I've seen in cast pistons, the taper in the skirt is a similar number to what the clearance is likely to be. So if the recommended clearance is 0.002", there's probably about 0.002" taper in the piston.
I'm aware it gets more complicated than that with barrel shaped skirts, but as a baseline generalization would this be similar for 4032?

[F120]

Ask the manufacturer.

[the_crack_fox]

Yeah, that's where it gets tricky. I am the manufacturer...
I'm making a set 90mm billet 4032 pistons for an FA20. Machining the O.D. is the next step and I need a bit of insight into skirt design.

[F120]

Yes, you are the manufacturer, just turning down, pistons machined from solid.

Good luck.

[jsgarage]

With the same problem on two-strokes back in the day. one tuner set a std top clearance (as you suggested) and just guessed at the taper. He Dykem-ed the skirts and ran the engine at fast idle for a minute or two. Pulled the pistons & looked at the skirts. Wherever the Dykem was worn off, he sanded a few thousands of aluminum off, re applied Dykem and repeated. until finally running it 5 full minutes or so gave a nice even wear pattern. Packed up his stuff and went to Daytona. No piston problems that year.

Note- assembling/disassembling a 500 cc two-stroke twin is likely much easier than what you face. Good luck.

[the_crack_fox]

Yeah, its gonna be a challenge. Once the engine is in the car, its not coming apart again unless it fails. I will err on the looser side for clearance.
I was hoping someone might have a 4032 piston on the bench that they could measure and post some info. Diameter at top and bottom of skirt of a commercially available piston would be handy information

[In-Tech]

Hiya,
At 90mm and a lathe you will want at least .0008" -.00012" more at the pin height using a home made dual groove cutter for oil retention, compared to the ~.003" at the skirt. The skirt doesn't care. Then taper from there to the top. There's a reason why most all pistons are "cam" machined.

Loose is fast if you think that piston stays round

[modok]

I would look at it the other way.
Figure you need .002" per 1 under the rings,
Then decide how much it tapers -bigger- at the bottom of the skirt, based on how long the skirt is, how hot it will get down there, and how springy the piston skirt is and how much cam grind.

[BCjohnny]

Yeah, that's where it gets tricky. I am the manufacturer

You couldn't make this up ......

So you're trying to make something you can't design ?

Lordy .......

[mt-engines]

Put it on a hot plate and measure it

[the_crack_fox]

Put it on a hot plate and measure it

That's probably a good place to start.
Drawing and machining a complex profile isn't a problem. I have solidworks and a cnc milling machine. I can put any cam/barrel/taper on them I choose.
I can accurately plot the taper profile of an existing piston on the mill with a dial gauge. My problem is that I only have access to cast pistons. The missing information that would be helpful is an approximate figure for skirt diameter at the top and bottom of a 4032 piston.
I'm sure my finished product won't be suitable for nascar racing, but making something that works without immediately seizing shouldn't be impossible.
Even if it does fail, I will still have learned a lot for the next project.

[modok]

Then just buy some pistons, measure them on your machine, and then sell them again.

[the_crack_fox]

Took some measurements today.
Blue line is the taper profile of a subaru FA20 stock piston. Red line is a toyota 2AR-FE piston. There's a bit of noise on the 2AR line, the piston hasn't been run and has a bit of texture on the skirt.
Also tried to get a hot measurement of FA20, 2AR and my billet piston. Difficult to do, they cool down quickly! All 3 measured 0.12mm (about 0.005") bigger when heated to 100degC (soak in boiling water for 5mins). Interesting... I was expecting more growth from the 4032.
I'm thinking if I go with the 2AR skirt profile, and run about 0.003" piston clearance I should be good. Stock FA20 clearance is 0.0008" to 0.0013".
Feel free to tear my theory to shreds, I am here to learn!

[Geoff2]

My understanding is that 4032 alum expands less than 2618, hence usually fitted with less bore clearance. 4032 is not as strong as 2618, which is why 2618 is used with high HP engines. The cam ground into the piston is also going to have affect on expansion.

[novafornow]

Send a PM to D-cal on here. I think that He made pistons for a living.