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My 550 is being relinered with Darton DI liners by Steve Demirjian, and I'm running the numbers on whether I need to move the piston pin heights to compensate. Reason being of course that the stock Ferrari system has the liners sitting proud of the deck by 0.03 to 0.08mm and with the Darton's the liners will be flush with the deck. Too, the deck surfaces will be machined down just a tad to make them equal (currently the left bank deck is about 0.005" higher than the right), and to ensure the liners are flush.

Original "as found" block deck heights w/o liners:

Left:...........7.915"........201.04mm
Right:.........7.910"........200.91mm

Rod length:...5.157"........131mm
Stroke:........2.953"..........75mm
Pin Height:...1.299"..........33mm

So figuring what the theoretical assembled height is of the stock piston at TDC, I have 7.933" or 201.5mm, which is taller than the actual deck heights w/o liners, and still a bit taller than the deck with liners. So for the stock setup my piston is "out of the hole" by about 0.5mm.

Once the new liners are installed, we're shooting for these final block deck heights:

Left:.........7.905"........200.79mm
Right:.......7.905"........200.79mm

I'm not changing the rods or the stroke, so the only dimension I can change is the pin height, and on the old Mahle pistons, the top of the pin was nearly at the oil ring groove...... So if I want to limit my piston to being no more than 0.5mm out of the hole, I'm going to need to raise the pin about 0.21mm, and possibly squeeze the distance between ring grooves or squeeze the ring heights. That or go with thicker head gaskets.......or just leave the pin height as is and take a chance

Thoughts?

Who's making the pistons for you?

What I normally do is tell them what I what for basic dimensions and let them recommend the specific design.

Meaning, give them the deck height, rod length, desired CR, chamber cc, gasket thickness, and see what they have to say. On my engine I wanted the squish band to be about .035-.040 so the pistons pop out of the bores .015"...that change a lone would fix your problem.

I've always had good experience with Wiseco, but I haven't bought any in several years, and both my buddy at PolyDyn and Steve D. feel CP now make a superior piston, so it'll probably be CP's in this engine.

The squish band on the old Mahles has some half moon cutouts in 4 places that sorta remind me of diesel pistons. I've meant to ask the Mahle guy about them in the past but kept forgetting. Wonder why they didn't have a full circumference squish band around the crown with some tapering into the dish? I find it hard to believe that these full depth cutouts needed to be there for valve clearance. I could understand some scalloped areas opposite the valves, but these cutouts go right down to the bottom of the dish, (or top of crown).

What's worse, on the outboard exhaust side, these cutouts are the low spots of the piston crowns and "channeled" condensation at this area to drain onto the top rings, which started rust and corrosion forming below these locations. Too, the top rings were welded together in these locations on 2-3 of the pistons:

strange.......I wonder if the purpose is exactly to act as drains? That's the only thought I have on it.

That would have made "some" sense for the cutouts on the outboard (exhaust) side, which are lower than the inboard ones due to the Vee angle, but if so, why include those on the inboard side?

It's definitely a strange looking piston crown and squish ring, but once I begin discussing these with CP I'll get their thoughts on why these were put there.

The presence of these also kept Mahle from moving the ring packs much further up the piston. Notice there's a substantial amount of space between the top of the piston to the top compression ring, and when I pulled them, there was a substantial "ridge" of carbon in the tops of the liners. Again, this sorta reminds me of a diesel setup or possibly a turbo piston.

Is that combustion condensation or standing condensation?

Looks odd.
Not familiar with diesel pistons but that sure does look like a turbo piston

Are those Mahle pistons MMC?

Yes, those are very wide top ring lands. If you ditched the clearance notches, you could move the ring up and reduce crevice volume (GOOD!) while move the dish around to compensate for the increase in compression and keep the ring lands thick enough (neutral).

Personally... i'd just get a couple of quotes for having shops mill 0.010" to 0.020" off the quench rings on the pistons. If you only have one head gasket thickness to chose from, do a trial assembly before hand to measure the piston deck clearance, then specify individual cut amounts for each piston in order to end up with a consistent 0.035" of piston to head clearance.

The shop will have to have something like these:
http://www.summitracing.com/parts/krc-1335





http://www.summitracing.com/parts/pow-pow351090

After having issues with Wiesco and JE getting too expensive, toss in that our ferrari's use small bores and most all custom piston mfg use larger forgings. We've jumped over seas to an Italian place that makes pistons that look like jewelry and use forgings that are specific to the bore size we need, helps that they do Ferrari and Lambo pistons .

some pics of the render screen shot and installed in engine. This set is for the 4v 4L. narrow pin boss cross forging, short skirt.

Everything was sounding pretty good about them until that part

in what way? they are not the oem of course. here's their webpage.
http://www.pistonispeciali.com