Pistons for m20 Stroker

[apostate]

Come on, man.
Fairy tales are not my speciality.
I give up.

Edit: If you measured 0.0012" cylinder wear, this is a totally different animal compared to piston to cylinder clearance.
0.0012" is normal for cylinder wear. But not relevant (at least not directly) to the discussion.

[ForcedFirebird]

Just broke one down for a rebuild that has .0012" minimum total clearance with who knows how many miles.

Come on, man.
Fairy tales are not my speciality.
I give up.

Edit: If you measured 0.0012" cylinder wear, this is a totally different animal compared to piston to cylinder clearance.
0.0012" is normal for cylinder wear. But not relevant (at least not directly) to the discussion.

Reread my post. Where did I say piston wear? Total piston to bore clearance is .0012" on this high mileage engine I broke down for rebuilding. I build these engines often, not just pulling numbers from thin air.

[digger]

isn't nominal clearance 0.02mm ? Most Pistons say 83.98mm so with 84.00 nominal bore that is 0.02mm?

[digger]

Reread my post. Where did I say piston wear? Total piston to bore clearance is .0012" on this high mileage engine I broke down for rebuilding. I build these engines often, not just pulling numbers from thin air.

I think he means that whatever wear is there is included in the measurements. This reinforces that it came tighter from the factory though.

[ForcedFirebird]

isn't nominal clearance 0.02mm ? Most Pistons say 83.98mm so with 84.00 nominal bore that is 0.02mm?

Correct, =/-.009mm allowed piston variance which almost half that but I have found the Mahle to be very stable. I used screen shots of software to make a listing of typical BMW engines and their respective piston clearances. Not only the m20 has tight jugs.

[zaq123]

you guys are jumping from metric to english..... adds some more confusion to this discussion. :)
I think 84mm oem bore is a nominal value. Standard pistons are spec'd at 83.98mm. Standard new clearance is spec'd at 0.01 -0.04 mm. This makes bore diameter anywhere between 83.99 and 84.02mm.
FF measurement was 0.03mm (0.0012"). He didn't specify if it is a bore or piston wear. He simply stated that his high mileage engine was still tight. Looks to be still within a spec for new assembly.

If anyone cares, My IE/Mahle M142 Alloy pistons are spec'd at 0.051mm PtoB clearance.
M142 is recently designed alloy that shares benefits of 4032 and 2618, that's what they use in Z06 corvette.
4032 will have smaller clearances but less malleable, higher chance for fracture under extreme load. On the other hand, 2618 will distort much sooner, not a good choice for the long term reliability. IMHO, between those too, 4032 is the way to go unless one is building extreme duty dedicated race engine that will be in and of the car a lot.

[ForcedFirebird]

you guys are jumping from metric to english..... adds some more confusion to this discussion. :)

As you can see from the pic I posted, one has to be on their toes ("in inches unless otherwise noted"). Even the manuals jump back and forth unfortunately.

[digger]

here are some data on the Mahle alloy published by Mahle themselves

M124P is forged 4032 alloy
M142P is the "mixed" blend used on the IE pistons
M-SP25 is the 2618 alloy

chemical composition


forged


cast


typical crown temps are in the order of 250-300C
skirts much cooler maybe 100-120C

lets compare cast to forged.

"strength" forged 4032 is a step up from cast regardless, others offer slightly bigger incremental steps
"ductility" forged is better and 4032 is only the same as cast at room temperature which doesn't matter in a running engine. 4032 is actually superior at high temps.

forged vs forged

M142 is slightly stronger than 4032 and 2618 is stronger again especially at temp.
4032 is more ductile than M142 other than room temp and above 150 is superior to 2618
both 4032 and M142 are superior to 2618 by >30%

it should start to become apparent where you would use certain alloys. if the pistons going to get very hot, see very high loads / knock, and doesn't need to cover alot of miles then 2618 is the go. for everything else there might be better tradeoffs.

[apostate]

Regardless of the "trade-offs" in any case I wouldn't take the risk of having a brittle and fragile piston in a high CR and a high revving 89.6 stroker engine.

I wouldn't much like to see a similar picture like this:


https://www.r3vlimited.com/board/sho...+piston&page=9

[SkiFree]

Regardless of the "trade-offs" in any case I wouldn't take the risk of having a brittle and fragile piston

Yikes, please remember that engineering IS ALL ABOUT "trade-offs".

For example, the negative "trade-off" of using the 2618 alloy on a 89.6 high-rev NA build means that with the standard 135mm rod (Stock,M52/S52,aftermarket, etc.) your wrist pin is heavily encroaching on the oil control ring, which coupled with the larger piston/bore clearencing needed with the 2618 alloy means high rates of blow-by and general wear while you are waiting for the engine to get up to temp. Even with the best of care, that engine will not last past two active race seasons or 30k miles street-driving (that's me being generous).

This is experience coming from speccing hundreds of 2618-alloy M20 pistons.

As for your link, I'm unclear where that is a material failure or a design failure... for that matter it doesn't even say what alloy was used. I'd be happy to post a couple examples of 2618 failures.

[digger]

Andrew, it is a 2618 piston if its JE and it's turbo charged imo I base this on the forging they used

Je diagnosed it as inorrect install saying the locks came out of groove and pin moved out of position. It doesn't look like good fi pin imo looks very thin wall so the pin flex works the piston bosses.

Regardless of the alloy on any part the geometry has to be appropriate

[apostate]

Turbo charged or not, doesn't matter. What matters is the revs.
It is the revs that put an enormous load on rods, pistons and pins. Not the boost. Period.

@SkiFree - while you are being generous about the 2618 alloy, how come you never mentioned this before (30K miles)? I guess you sold quite a lot of pistons of this alloy while working for IE?

[digger]

The engine revs same as a stock engine with chip and cast Pistons have no issues even st 7k and they are a lot heavier with the same 5mm pin boss wall thickness . Compression loads flex the pin and piston bosses have to withstand that it's a very common problem with the crowd that thinks lighter is better.

I've done about 100000 miles on my 4032 pistons revving over 7k many many times but no can't be done....lol

[apostate]

Wrong, Digger.
It's all about RPMs.
Nothing loads the whole piston-pin-rod assembly like the RPMs.
You can boost an M20 engine as much as you want, but if it stays at 6k RPM with the right tune, it's all good.
Try a N/A engine at 7.5K and you'll see the difference in how all that iron starts to surrender.
That's why everyone boosts M20 and a few go for a high revving N/A M20.

[digger]

ive crunched the numbers and built fea models with piston pins and rods so I couldn't care less what you think but maybe someone else reading this might pay attention to the details rather than look at everything simplistic or black/white when the world is mostly grey