Hello all, I'm new here and new to Evos. I just purchased an Evo 9 with a blown head gasket, and looking for some advice on which one to install.

I took the head off to have decked, and they took .008 off. The engine has Wiseco E85 HD 10:5 Pistons (85.5 bore), Manley rods, and GSC S2 came and springs, with the stock Evo 9 turbo for now. I originally purchased an OEM head gasket from Mitsubishi, but the technician doing the work is recommending that I use a 1.8mm gasket with an 86.5mm bore. He said that since it has aftermarket pistons and cams, and he doesn't know how many times the head was decked prior to this time, plus the .008 that he took off, that he is concerned the car will not run right with the OEM gasket. I wanted to get the community's opinion on this, as this is my daily driver and I want to make sure the car runs correctly with no issues. Thank you all in advance.

 

His concern isn't that it won't run right, his concern is that the pistons will hit the valves since he doesn't know the history of the engine. And that's a very valid concern. Claying the pistons-to-valve clearance is a big job, and it sounds like he's trying to get you up and running in the cheapest way possible. Your compression ratio is already quite high, I don't think you'd notice a difference going to a slightly thicker head gasket. I'd do it for the peace of mind, knowing it wouldn't have to be torn apart again to fix bent valves.

 

Just use an Evo9 gasket. Those Wiseco pistons have DEEP valve reliefs. You're not anywhere near having an issue with only .008" off the head.

 

Ok so he just got back to me on this and he says "We're not doing this for clearance issues, we're doing it to lower compression. I measured the head and it looks like about .015-.018" has been taken off the head in total, so it appears that it has been decked once before now. Here is the issue. You have 10.5:1 pistons which are already too high for pump gas. Now add in a head that's been decked 2 times and you're approaching 11:1".

 

You're not gonna be able to run a thick enough gasket to make it pump gas safe, you'd need a .080" thick gasket to get it just under 10:1, and even 10:1 is questionable on pump. A .120" gasket would get it down to 9:1 for pump gas safe compression, but either of those gasket thicknesses are ruining the squish and it isn't going to run well.

Change pistons or move to E85/race fuel.