Fly-cutting pistons to prevent the 'money shift'?

[Tippy]

I DAFS without luck so.......

Has anyone done this? By the scars on my 930 pistons (on a C3.2), the depth seems to be not too bad from the looks of the valve 'love taps'.

I thought the pistons were trash until I realized the impressions were carved out by the edge of the crown where the piston has the most 'meat'.

Then I got to thinking about chucking up a bit on the Bridgeport and cutting out a little more than the depth of the scars and just a little wider than the valve face preventing this from ever happening again. The material remaining should be adequate IMO but I'm not sure.

My last thought was if this would work, why wouldn't everyone with a 930 or 3.2 configuration I have do this?

The only explanation I could come up with is valve float is too unpredictable and even with fly-cuts, the valves may still hit the piston. But, the depth on mine seem fairly consistent. Of course, that also can be explained with basic physics and metal properties I'd assume.

Thoughts?

[Flieger]

You could just get some new JE pistons or Mahle MFI/carb pistons with valve pockets already in them, and not have to worry about the possible micro-cracks.

[MBruns]

If you run more camshaft than normal it has to be checked and machined if needed, all the clearance in the world won't help if you dont have enough spring. At the closest point the intake V to P should be .040 in min., the ex. .080 to .100 min., with some of the newer more agressive cams the stock springs don't have enough room for proper pressure without coil bind, the thickness of the 930 mahle should have plenty of room to cut some valve reliefs for most street turbo cams.
Mike Bruns JBRacing.com

[Tippy]

I should have stated I'm using the factory C3.2 cams.

[JFairman]

Fwiw, around 20 years ago I did the same thing on my 1975 BMW R90S 900cc motorcycle motor.
I had it bored out for some new BMW R100/7 mahle pistons I got cheap that have lower compression and lower domes than the origonal R90S pistons.

A buddy who owns the trickest machine shop in town, Micro Tool Engineering, Inc. - Products and Services made a jig to spin the heads in a lathe and he milled the heads down .030" for more compression and it it worked really excellent.

There were valve pockets in the cast pistons from mahle and I cut them a little deeper and wider by hand with assorted wheels and a diegrinder becasue the valve to piston clearance was closer after milling down the heads.

I'm patient and good with hand tools and after hand polishing the crowns and valve pockets and balancing the pistons you could not tell they weren't done on a Bridgeport.

I know there is no correlation between an air cooled 2 cylinder horizontally opposed 1975 BMW motorcycle engine and yours, but the piston manufacturer is the same, the hemi combustion chamber shape, piston crown dome shape, squishband, and valve angles are practically identical ...and that motor redlines at 7300rpm. It uses 2 Del Orto 38mm accelerator pump slide carbs rejetted a little richer and raised mixture needles in the slides.
With dual plugs and CC products lightened flywheel and headers it flies.

One very nice difference is the old air cooled BMW boxer twins used 4 head studs per cylinder that are thicker than what Porsche ever used and they NEVER EVER break or pull out of the case.. that kind of crap is a non issue.

[911pcars]

Have you measured the thickness of the crown on Mahle pistons? I believe it's thinner than JE's; also not designed to be machined for deep valve clearance pockets; one reason why they're lighter.

Sherwood