Pistons for m20 Stroker

[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]

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]

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]

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

[downforce22]

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

[digger]

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

[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]

can you send a stl file to me?

[bmwmaster81]

I have got laser scan *.stl files from m20 pistons and amc head. Sadly with *.stl files its not possible to construct something. When soemone is interrested and can convert stl in to something useful for cad, please pm me.

https://drive.google.com/open?id=0B73WY9YlBFqRVWdMVHVrLUNtMDA

[LateFan]

which loosely translates to "Three ball turbulence chamber"

Funny but logical!

I've had bad luck with these - I think it's that the nose keeps digging in to the cyl wall...I wouldn't recommend them..

[ForcedFirebird]

Now it makes sense!!!

Drei Kugel wirbelkammer is what they called the m20b25 design, which loosely translates to "Three ball turbulence chamber". The piston is one ball, chamber is another, then the dish is the third ball.

[digger]

any CAD guys wanting design their own piston dome

this is how the m20 8.8:1 style piston dome seems to be made

spherical dome (not a hemisphere) approx R100





subtract a spherical dish approx R80mm

the centre of the sphere is offset in two directions from the bore axis (towards the spark plug) as well as being offset in the piston axial direction






section view showing dish (9.7:1 style pistons have bigger radius making a shallower dish)



SO in actual fact the squish is not a chamfer its slightly radiused. Same with the head the machined part is radiused (concave) to match the piston



if you look at the spark plug side the head there is a decent amount of potential area for squish, however the OEM pistons dont utilize much of it as possible because its removed when the dish is added. you could design a custom higher compression set of pistons to use more of the area if you wanted...

add some valve reliefs



get a sample 3d printed and mock it up to check you didnt fuck it up...

[digger]

Do they have the proper dome on file ? If not you need to send a sample.

Cp are every bit as good if not bettter the JE in a general sense this is based on professional opinion I've been told from people who have been getting stuff made by both companies and others

[dustyperez]

Why not use cp pistons? They are what i have with the 89.6 crank and 135 rods. They were the first custom I believe for that crank and have the dimensions on file. I just gave them bore, stroke and rod length. they did the rest. It was about $1200 but they are pretty.

[ForcedFirebird]

This is a piston that TopEnd designed for my engine build (E36 2.8L crankshaft and rods, 885 head,CR 10.7:1, Schrick 280/272 camshaft) using the 4032 forging

Can't wait for numbers on this. Your head out-flowed the previous local head that made good power, but needed more cam than the 272/272 used (even with 9.5:1). Curious to see if that cam/combo also runs out of steam....