Date registered: Apr 2006
Vehicle: W111 Coupe
Location: Washington DC
Mentioned: 1 Post(s)
Quoted: 19 Post(s)
Pistons shape the cylinders in an ovalized pattern. This is part of the break in process and increases with mileage. This is more true for iron lining than Alusil. So if you change the stroke you need to rebore the cylinders and use oversize pistons so that pistons break in a new pattern corresponding to the new stroke.
I know Alusil requires specific rings, but I'm not sure the pistons per se are different. May be you could use 420 pistons and conrods, and 3.5 rings, or whatever rings for iron lining that would fit the 420 pistons. Rings are available in all kind of shape, diameter and thickness.
MB modifies the compression ratio by making the cup in the center of the piston more or less shallow. But the clearance for the squish area (crown of the piston) remains the same whatever the compression ratio. May be you could use some US 8.8:1 CR 4.5 pistons, shave the crown to get the correct clearance and increase the CR a bit at the same time as it will make the cup in the center a bit shallower. You may then have to make some cuts on top of the piston for valve clearance, especially if you use hotter camshafts.
Honestly, after sourcing another crankshaft, conrods, oversized pistons, rings, rebored the block plus all the work involved you will have paid for a 450 5.0L engine which has more displacement and is lighter. If like me you fit a 4.5 Djet intake manifold and keep the 3.5 cam covers and air filter housing and paint the block in black, only specialits will be able to tell it's the wrong engine, as it will be 1cm wider on each side. Other than that, it will look just like your 3.5 (the block is basically a 4.5 alloy block rebored to 96.5mm).
As you don't seem too concerned by weight an easier option is to source a good euro 4.5 engine. Your trans would bolt straight.
Last edited by GGR; 01-28-2011 at 04:22 PM.