Cylinder to Case Gasket

[DRACO A5OG]

PAG tech manual states: If heads are machined to use 0.50 MM gaskets with the assumption the machining was 0.25.

What if machining was only 0.18MM? Would you still use 0.50MM or go with 0.25MM?

TIA,

Jim

[VFR750]

I guess it depends on your piston to head clearance and the deck height.

My heads were machined .008" (.2mm) and I still used a .25mm base gasket because the clearances were acceptable with the thinner gasket.

[DRACO A5OG]

Thank you for the post, yes, I will verify deck height and p/h clearance. Just wondering what other have done after machining the heads.

[VFR750]

I bought the .5mm gaskets thinking I'd need them. just worked out.

Plus I had verified my head volume and piston dome volume. Gasket will also affect compression ratio.

[DRACO A5OG]

Yeap, I have a buret on the way

[kclem19]

I had my heads machined by 0.13mm and I also replaced my original T Pistons with JE 9.5:1. My deck height was less than 1mm with 0.25mm base gasket. I'd have to go back in my records to check exactly what it was and I ended up going with 0.5mm base gaskets. I think I ended up around 1.1-1.2mm after that.

[DRACO A5OG]

Awesome, Thank You!

[KTL]

I would do a "dry" mock up of the piston and cylinder after you get your case assembled to see what your deck is with the typical 0.25mm gasket. By dry I mean nothing gets sealed up. You definitely want to lube the cylinder & rings (to avoid scoring the bore) and lube the main bearings rod bearing with assembly lube since you'll be rotating the crank to bring the piston up to TDC in the cylinder. It's wise to use the two compression rings in the piston because the rings "square" the piston in the bore. W/out the rings you will have the piston rocking in the bore.

BTW 1mm is just a good safe rule of thumb for deck. There's nothing written that says 1mm is the absolute bare minimum or else you're screwed. So if you mock it up and find your deck is 0.92mm, I don't think you're at risk.

There's been a number of deck height discussions over the years because hitting 1mm is hard. Especially when the heads have been lightly resurfaced. I recall when Grady Clay chimed in (RIP, great guy) on these discussions with his experiences, the discussion got very detailed. Here's a good one I found http://forums.pelicanparts.com/911-engine-rebuilding-forum/294214-9-8-10-1-a.html

What I got out of the above thread was, besides the fact that detonation involves a number of different circumstances, if you're pushing tight deck clearance, especially with piston domes that are prominent at the outer edge of the piston, it's wise to have a bevel at the edge of the combustion chamber. Additionally, this bevel aids in wedging or directing the air + fuel mixture in the squish zone up into the combustion chamber toward the spark plug

On the other hand, if you throw a 0.50mm base gasket in there and your deck ends up at 1.15mm, I don't think you're going to be highly prone to edge detonation. I had an engine that was running with a whopping 2.4mm of deck and it was showing some noticeable detonation on the pistons. I suspect that if the compression ratio was much higher from let's say a larger piston dome volume (it was a single plug Carrera 3.2L to 3.4L 98mm piston that was spec'd for 9.8:1 compression, misused with SC length rods) then the detonation would have been more severe. Look at this piston picture and you can see the pock marks around the periphery that show signs of detonation out there.



You shouldn't have any issues with piston-to-valve clearance since you're running the 964 cams. But worth checking anyway just so you know. It's a load of fun to check......... That process is annoyingly tedious

[kclem19]

There is one sentence in waynes book at the end of the piston to valve clearance section. Something like "instead of doing all this, just extend the intake or exahust valve by the respective cleanrance spec and rotate the crank slowly by 2 revolutions." I took this route on my 2.4 with 964 profile cams and 9.5:1 JE pistons. I didn't hear anything or feel any resistance so that made it easy and also ensured that I checked the clearance at all crank angles (ie not just at 5 degree increments). However, this probably isn't the best way if you know you're building to high compression with high lift/duration cam profile.

[Trackrash]

You need to be concerned with three things. However the piston to valve clearance probably won't be an issue for you.

The deck height needs to be measured, since if it is too little your piston could contact the head. This needs to be checked at the edge of the piston. One mm is usually considered to be minimum.

The compression ratio should also be measured to make sure you have what you want or need.