Dishing pistons

[Lukej]

I need to find 5cc.
50cc chambers
1mm deck height
AA slipper skirts with 6.5mm crown.
82 x 94
42 x 37
Web 86b
1 5/8"

Full street car.

Currently 11:1
5cc in the piston would be 10.2:1 heads have been worked and ported so would rather not butcher the CC out of them unless a piston dish is specifically a bad idea.

Theory suggests a dish can centre the burn and make it more efficient. Anyone have any experience of this?

Round dish? 75mm diameter 1.3mm deep would do it

Luke

[Martin S.]

Here's a pic of mine in the rough machining stage. they are cut in a bathtub shape to try to match the chamber shape. The idea is to keep the two squish bands intact and focus the burn in the center of the chamber, while creating turbulence at the same time.
Mine also incorporated a flame port in the head to further spin the mixture around which makes a more efficient burn.
I couldn't be happier with the result! The heads run cool and burn everything that goes in. Valve adjustments have been minimal and it has a huge cam in it.
Engine now has 29,500 km on it and still seems brand new 

[Lukej]

Thanks Martin, that's exactly the kind of feedback I'm after. How many CCs did you get in that dish?
Do you have a picture handy of the finished piston? Also the chambers?
Looks exactly like what I am picturing

[Martin S.]

Look closely and you can see it written on the top left piston in red marker 

[Lukej]

Ah ha! derp... Thankyou...

I'll weigh things up, not sure I have the tooling to machine a slot like you have...

[Martin S.]

I think Steve made most of his own tools. Here's a pic of it being dished on the mill.

[brian e]

I do it all the time.  I try to keep it less then .070" deep, and I make sure the diameter of the circle does not over hang onto the squish flats.  I have done a couple oval shapes, but a regular circle in the lathe is far easier and way quicker.  Your 75mm x 1.3mm deep looks about right. 

Brian

[Martin S.]

Got any pics Brian?

[brian e]

These were the ovals I did.  I will be doing a set of circles next week.  I will get some pics of my fixture and the dish measurements while I am doing it. 

Brian

[Martin S.]

Looks almost exactly like mine, nice!
Someone should CNC these and sell them already done.

[Lukej]

I'm going to go for 63x1.55mm, basic round dish.
Should get me to 10.2:1

[Martin S.]

Why round, because it's easier?
Are you going to have the edges like what is shown (with a flat bottom), or more of a gradual parabolic shape (like what VW did with some factory pistons)?

[Taylor]

I had to have some swimming pools machined in mine to get the compression down as well.  This is 17CCs

[spoolin70]

Same scenario as Taylor for me - made by JE

[ Attachment: You are not allowed to view attachments ]

[Lukej]

Taylor, spooling, thanks for sharing! they look awesome.

Martin, yeah flat bottomed. Purely because I have the means to do that myself.

[brian e]

Here are some quick pics of my easy piston dishing fixture.  It registers in the machined step in the bottom of an AA piston.  Slip fit.  The pin is scrap nylon tapped for whatever bolt you have around.  Tighten the bolt into the pin, and suck the piston against the fixture.  Chuck on the round aluminum fixture and check the run-out just above the top ring.  I made this one to work for 92 and 94mm AA's.  I have another for T4 stuff. 




Brian

[Martin S.]

I wouldn't want to lose the squish bands like that. 

[Lukej]

Martin are you referring top Spoolin' post?

[Lukej]

[K-Roc]

These were the ovals I did.  I will be doing a set of circles next week.  I will get some pics of my fixture and the dish measurements while I am doing it. 

Brian

This is exactly how I do them as well,  My thinking is that if you are going to do a full circular dish you have lost your squish band and tight deck... you may as well just shim the barrel out or use super thick copper shims, you will have the same result.

[Taylor]

These were the ovals I did.  I will be doing a set of circles next week.  I will get some pics of my fixture and the dish measurements while I am doing it. 

Brian

This is exactly how I do them as well,  My thinking is that if you are going to do a full circular dish you have lost your squish band and tight deck... you may as well just shim the barrel out or use super thick copper shims, you will have the same result.

That's true, but then your pushrods are longer.  😛  made good power.   We'll see if it overheats.

[Lukej]

Highlighted in red is 70mm diameter, my dishes are 65mm specifically to avoid losing the squish

[Martin S.]

Is it really that much more work to do it properly?

[Torben Alstrup]

Not if you have the time and money to scan and/or program the cnc, or do it manually in a Bridgeport. But the 65 mm dish as shown above works well. The tiny loss in squish on the non plug side is so little that it doesnt matter.
How do i know that works well, - because these engines do not need much advance to perform well, and the emissions are fine. Very little unburned gasses emit the pipe.

T

[Martin S.]

And you don't need the extra cc's.
These are not one-off customs are they?
You'd think it would be easy to set up for one size piston and crank them out with different depths.

[Lukej]

Is it really that much more work to do it properly?

Was that a question for me? If so then YES, it would be much more work seeing as I don't have a suitable radiused end mill to machine a bathtub... The round dish was the best with what I've got to work with.

Thanks for your Input as always T

Luke

[Martin S.]

If it still has the squish intact, it will be great! 

[Lukej]

I've maybe lost 1 or 2 mm on the none-plug-side at most... >95% of the squish is still intact.

Thanks for your input

[Taylor]

Is it really that much more work to do it properly?

Do what properly?

[Fiatdude]

JE made these for me