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I’m building a 1987 Mercedes 190e 2.3-16v engine for track use and I’m seeking input on piston-to-head clearance aka squish.
First a brief background on the project: What started as a routine rebuild has snowballed into custom JE pistons + Pauter rods to bump compression ratio to around 10.5:1, following a formula advised by a reputable shop called Top End Performance(https://www.racetep.com/). I’ve also opted for the installation of double valve springs from the later 2.5L version of the same engine. I worked with a machinist who decked/bored the block and installed the pistons, rods, crankshaft and bearings, but after completion of the block I decided to find a different machinist for reasons I won’t go into here. I’m happy with the new machinist, who’s more used building engines for performance. He installed the double valve springs, performed a valve job, and checked the work of the previous machinist. Everything looked good. He’s also encouraged me to do as much of the assembly work as I want to do, and offered to provide guidance over the phone if needed.
Now onto the subject of this thread. The old machinist advised me of a negative deck height of .033” with the supplied JE pistons. In other words, the pistons protrude from the block .033.” Sources online say that the stock Mercedes head-gasket measures anywhere between 1.3-1.5mm (or 0.05118”– 0.05905”) when compressed. If my math is right, this gives me a Piston-to-Head clearance range of .01818”–.02605,” depending on compressed head gasket measurement. I understand this is tighter than the commonly accepted minimum piston/head clearance of .036”. I called Top End Performance and he said the amount of negative deck height is typical for this piston, but he more commonly sees .028”–.030”. I’m at 3 thou over with .033”. He also mentioned that this tightness in clearance is particularly typical of other Cosworth engines, and the engine I’m working on has a head by Cosworth (but I cant help but wonder if he's just trying to cover his ass!) The JE pistons have valve reliefs for higher lift cams than I’m installing, so I’m less worried about the clearance between valve and piston. I’m mostly worried about the piston kissing the head.
My question is this: Are there any other steps I should take to better understand the risks of this little squish? I’ve breezed over a lot of details just to keep this initial post brief, but I’d be happy to delve into any details that would help me find a path forward.
P.S. Pic of pistons and rods just for attention
The amount of clearance you can get away with will depend on how much the piston will rock at TDC which is affected by the bore diameter as well as your piston to cylinder wall clearance. 0.020" is a little tighter than I'd like to see personally and I like to be in the range of 0.035-0.040" for peace of mind. Can you get a thicker gasket?