Pelican Parts Forums - My 3.2L Deck Height, Piston to Head Clearance Measurements and Compression Ratio

Went ahead and did some final measurements to prepare for final installation of my pistons and cylinders.

I only took measurements from #1 & #4 pistons/cylinders for this step.

Deck Height Measurements

For both piston/cylinders #1 and #4 I used:
- A single 0.25mm cylinder base gasket
- Cylinder held down with 4 Stomski cylinder nuts, finger tight
- Vernier caliper placed against the top of the cylinder wall, extension protruding/resting against the flat outer edge of the JE piston

Piston #1 deck height measured 1.03mm
Piston #4 deck height measured 1.09mm

Satisfied with these #'s I figured I would move on and check the piston to head / combustion chamber clearance on #1.

Piston to Head / Combustion Chamber Clearance

I used the solder method described in Wayne's Engine Rebuild book to measure the clearance between the piston and the head's combustion chamber.

I had some 3mm solid core solder handy so I used it. Wayne's book recommends acid core. Other than the possibility that the acid core crushes a bit easier - I can't see how the solid core solder result in a significantly different measurement as compared to the acid core solder (after all - this process is really just a sanity check as opposed to looking for a precise measurement/specification - or at least as it relates to a non-race engine).

For piston #1, I came up with the following clearances (measurements of the crushed ends of the solder):

- 2.04mm and 2.05mm
(Two pieces of solder placed on the outer edge of the piston/against the cylinder wall and parallel to the piston pin).

Thoughts on the Solder Crush Measurements

While these measurements appear a bit large, I wasn't surprised considering my heads cc'd larger than expected:

http://forums.pelicanparts.com/911-engine-rebuilding-forum/792313-measuring-head-volume-ccs-compression-ratio.html

Either way - it is what it is.

Compression Ratio Calculation

With the deck height measurement, I can now run my numbers for my compression ratio (I've been using this calculator 404page.html

- 95mm bore (3.2L engine spec)
- 74.4mm piston stroke (3.2L engine spec)
- Leave calculator fields for head gasket bore & thickness blank (it is considered in the deck height measurement)
- 92.4cc combustion chamber volume (measured in above link)
- 35.1cc piston dome volume (from JE specs for 9.5:1 pistons).
- 1.03mm piston deck clearance

This produces a compression ratio of 9.16:1.

While I would have liked to have been closer to 9.5:1 compression ratio - the larger volume of my head combustion chamber accounted for the CR drop.

Good to go. I should have a solid, safe running engine that leaves plenty of room to rev and play with timing.

Fun stuff,

Gordo