Skimming Pistons

[Doug_RS2]

How much should I take off the standard pistons to reduce compression? I'm using a metal head gasket but have a head that has been skimmed a couple of times

Any suggestions Mihnea?

Is there any special way these should be done? Can a normal machine shop do this? What should I be asking to see if they know what they are doing? Should they be polished?

[audiman]

Im gonna do the two head gasket trick by taking two apart and making one that is thicker. The squish area in the crown is an important shape, dont mess that up

[Pisobiker]

Doug, you could have the piston's crowns machined down in the area where there already is a dish (original is about 1.2mm deep) and maybe have valve pockets reworked slightly as well. But this way crown thickness is reduced and might not give enaugh strenght on high boost applications.

Tom, you are right being concerned about squish area, but if you use a taller head gasket, effect will be much the same as taking off same thickness from piston crown surface. If instead you make a deeper dish (given it is possible, due to strenght issues) squish phenomenon will stay as OEM.

Marco

[Mihnea]

Doug, you should be fine with skimmed pistons, as AFAIK this is also what MTM/Dahlback/Lehmann do in order to decrease CR.

I guess Marco could do the right maths (he's better than me for this ) to tell you exactly how much they need to be skimmed in order to drop CR to around 8.7~8.4:1...

Man I really feel for you, I guess you're getting used to taking the head off, aren't you?


I'm leaving for France tomorrow and I should be back next week, no idea when but I'll try to log in a few times to check what's up with you guys!


Cheers,


Mihnea

[Pisobiker]

Doug, how much is "8 thou" ?
How much would you think it has previously been taken off?
If you wish I can do some math for you to calculate how much your pistons need to be skimmed for lowering CR.

Marco

[porkkis]

I had to ask the same. Now I know
1 thou is 0.001 inch so 8 is about 0.2mm.

[Doug_RS2]

0.2mm was taken off the last time - I think it had been off once before - and it is off again - I suppose we could assume 24thou taken off - 0.6mm off from standard.

There is a measurement that can be done - from the headbolt mounting surface through the head to the machined face - the mimium this can be is 118.1mm according to the audi manual - but I don't know what iis standard - I will measure this tomorrow though.

And if I get the pistons machined - what area is getting machined? Do I want to keep the top lip and the recess part and machine an equal amount off each?

[Pisobiker]

Doug, i think 0.2mm each time is a little too much.
I think 0.1 first time, 0.2 last time you did it (but head was a little warped) and now no more than 0.05 (but lets say 0.1 again).
This will be 0.4mm total.

If we consider same area as bore area for combustion chamber where material is taken off (but it should be about 95% of it) CR should raise form the originally 9:1 to ~9.31:1.
Even considering the 5% reduction of combustion chamber area it will then go to 9:29:1, so difference is negligeble.


Piston already has a dish in it which is ~Ø59mm and ~1.2mm deep which makes ~3.28 cc of volume. Valve pockets should be in the range of 0.5cc both.

A) If you have pistons machined down only where OEM dish is then:

- for 8.8:1 you'll need 1.26mm deeper dish than OEM;
- for 8.6:1 you'll need 1.81mm deeper dish than OEM;

B) If you have pistons crown machined down you'll have;

- Bringing crown down where OEM dish is (which means 1.2mm) you'll get ~8.91:1;

- for 8.8:1 you'll need ~1.38mm pistons crown machined down (0.18mm below OEM dish );
- for 8.6:1 you'll need ~1.69mm pistons crown machined down (0.49mm below OEM dish).

Those numbers are not 100% precise due to not having exact volume and volume-vs-depth of valve pockets, but error should be very small, maybe in the range of +/- 0.02:1 CR difference.


I feel best way would be working only on the already existing dish area, but that way crown thickness on most critical part (center of combustion chamber) could be too thin.
If somebody would double check my numbers and also give an indication on best place where to cut material off, that would be nice.


Marco

[Doug_RS2]

Thanks Marco - I know Habib has had his machined and I think he had 1.4mm taken off - I'll PM Habib.

[RSCoupe]

If someone has an old piston they don't need anymore, they could cut it in half, thus creating a cross section of the piston. This way you can see the thickness of the crown at various points and how much you can safely remove. What the minimum safe thickness is, I don't know.

[Stick Insect]

Marco and our other metric readers a simple rule of thumb

1mm = 40 thou ( thousandths of an Inch )

8 thou = .2mm

HTH

David

[Pisobiker]

Ah ha !!

Thou=thousandths of an inch...

now that does make sense



Thanks for the info (@ Porkkis too)

Marco

[Stick Insect]

Glad to be of Help Marco.

I'm sure that you know 25.4mm = 1 inch.

David.

[Pisobiker]

Sure I do

[A6 AVANT]

Now now boys stop getting personal!!!