Pistons for m20 Stroker

[digger]

can you send a stl file to me?

[Coyote_ar]

while we are on the subject for off the shelf stroker pistons.

Does anyone know if someone sells a piston for a 84mm stroker, with 135mm rods, and low compression (9:1 or under)? For a turbo setup, so having the proper dome shape would be a must to avoid knock.

Thx for any suggestion.

[digger]

i'm not aware of any. shelf is not really cheaper than custom anyway lol

[downforce22]

Yea racetep.com can get you what you want probably out of california but they are expensive

[digger]

It is 900-1000 roughly for a Set of custom ones

[apostate]

4032 is the weaker piston and it is only good (poorly) for a stock replacement.
Expansion difference between the two alloys is only 0.0008" at a 84 mm. diameter.
4032 doesn't have titan in it. It is much more brittle and fragile, at least on paper because of the high silicon percentage and in the absence of a stiffener like titan.
4032 will take less temperature abuse and is much more prone to detonations and knocks.
Sorry that I intruded.

[digger]

to be honest if i could get a cast piston from an OEM the same shape as what i can a custom forged i would use the cast piston as it is every bit as strong as a piston needs to be for a naturally aspirated street engine. there are plenty of OEM using cast pistons on turbo engines and many OEM use forged (4032) very few use 2618 for OEM its rarely necessary to require the full mechanical property benfits of 2618

while it's only 0.0008"-0.001", but 0.0035" vs 0.0025" is a 40% increase (numbers come from the manufacturer) the ring lands last better and you dont need any fancy coatings on the ring grooves to make them last over 100k miles

its always a case of it only needs to be as strong as it needs to be. i dont see the point of a piston stronger than i need if it starts to comprise other aspects of what a piston is supposed to do. its called bad design and engineering. i only need forged because i cant get cast, so a slight upgrade in mechanical properties is not a bad thing and the compromises are not too bad.

if i was building a high boost turbo engine it is totally different story, in a way like everything it is horses for courses

[apostate]

Here is a part of an e-mail exchange with TopEnd with regard to the piston material:

For a High Compression Street Car like that I would only use the 2618.
The 4032 is for when we are making stock replacement pistons.
The 2618 is Stronger and all I use here for my Street engines…
I make less than 10 sets a year with 4032 and over 500 with 2618

For your Big Bore 89.6mm Stroker my recommendation is to use the 2618…You will be very happy…

Ready to go when you are…

Thank you for these clarifications!
By the way what is the thermal expansion coefficient of 2618 compared to 4032 at 86 mm diameter?

Cheers.

It is less than .008” in clearance difference…
.0022-.0025” vs. .003” for the 2618
Just use the 2618 for your application…They are quiet and they are stronger and better…

[digger]

Here is a part of an e-mail exchange with TopEnd with regard to the piston material:

top end use JE, here is what JE say, no need to ask steve as you get the printout of the relevant pdf below in the box lol

https://www.jepistons.com/PDFs/TechCorner/SCPDrawings/piston_instrc2618.pdf

https://www.jepistons.com/PDFs/TechCorner/SCPDrawings/piston_instrc4032.pdf

0.0020-0.0025" compared with 0.0030-0.0035"

be careful when you trust people who are trying to sell you something, its always better to seek the advice of an impartial person if you don't know enough about the subject to form your own opinions ;)

if you cant tune then 2618 is better, but you will soon realize that even that isn't enough and detonation will still kill them, and the guy down the road who can tune is probably still running on stock cast pistons making more power. for what most people here need the stronger forging doesn't offer any additional performance they actually need and they infact will wear out quicker.

[apostate]

I don't need TopEnd to make up my mind, it was just a quotation of what they recommend also. The price is the same for both alloys so it doesn't make any difference for them which one I shall choose.

The decision of which alloy to be used depends on the application and should be based on the contents of the two alloys. As stated above, the lack of titan and the high silicon content of 4032 are huge disadvantages for a high CR engine piston.

[ForcedFirebird]

There are always advantages and disadvantages when you start comparing materials. 4032 is a better choice for a mixed street/track engine that is expected to last several miles/years, and 2618 is more suited for an engine that will be torn down periodically (every race or weekend for drag cars, seasonal for road racing), as they will definitely wear much faster.

...and while saying 2618 "only" needs .0008" more clearance on an 84mm bore, that is actually more than the stock clearance. Forgings are already over, with the 2618 sounds like marbles in a can until they warm up (and that was in a v6 with a much better 1.7:1 rod/stroke ratio). You add that to a high compression engine like an m20 with a very short rod/stroke ratio (R:S ratios get worse on stroked m20's).

I was surprised I saw no mention in your questions of the intended purpose of the engine, as that should be the true determining factor in piston material choice. That's the first thing I as every engine we build or consult on.

[digger]

4032 Pistons are used on many turbo engines and high compression street engines. The metric mechanic forged Pistons for their 3.2l 11:1cr m20 engines that have been around for well over a decade use a 4032 alloy piston made by wiseco. Those engines will last over 100,000 miles. They are perfectly suitable alloy for such an application.

[apostate]

CR will be 11.5-12:1 (depending on how much the block and the head need shaving. The car barely sees 1.5-2K miles per annum and only in the hot summer months. Mainly used on winding roads and three to four track days. Not a daily driver.

@ForcedFirebird - piston to cylinder clearance in M20 for new OEM pistons is from 0.0004 to 0.0016. Both alloys exceed that specification. Cylinder wear limit is 0.0047. Pretty sure that you know very well that what seals the cylinder are the piston rings, not the piston itself. Aluminium has greater linear expansion than steel and what is important is how long it will take for the rings to close the gap rather than how long does it take for the piston to expand. In both cases (2618 or 4032) the piston rings are the same, i.e. they will close the gap for approx. the same time, may be 2618 rings will be a little bit faster due to the higher piston to cylinder clearance. At that point the rings stabilize the piston.

Edit: By the way the clearance specs given above are from Bentley Publishers E30 manual. These are not genuine BMW specifications. I really doubt that piston to cylinder clearance can be 0.0004.
IMO 0.0016 is the bare minimum.

[digger]

Having the piston sit without rock in the bore means the rings will have better conformity during warmup. This is one reason why OEM use low thermal expansion alloys. If the piston rocks excessively it's not good for the rings or ring lands. When you have a short compression height and skirt length it gets magnified.

When it's upto temperature they should be the same.

[ForcedFirebird]

Edit: By the way the clearance specs given above are from Bentley Publishers E30 manual. These are not genuine BMW specifications. I really doubt that piston to cylinder clearance can be 0.0004.
IMO 0.0016 is the bare minimum.

Never owned, nor consulted a Bentley manual, measured them first hand many, many, times. ;)

BMW i6 engines, and especially the m20, have tighter piston clearances than most other makes I have ever worked with. Just broke one down for a rebuild that has .0012" minimum total clearance with who knows how many miles.