Deck height measurement with chamfered heads
 

Doing a top-end rebuild on a modified turbo engine here and realized that the heads have been chamfered (pic attached). The depth of the chamfer is 40 thou/1mm. Now I am curious about how this may change how I approach deck height. According to the good book, I can measure deck from the top of the cylinder down to the edge of the piston crown. However, given the chamfer, I could theoretically have the edge of the piston crown flush with the cylinder top ("zero deck") and still have 1mm clearance between the crown and the head. So is it correct to add the chamfer depth to the deck measurement to calculate total deck height in this instance?

Any insight is appreciated.

http://asbury.smugmug.com/By-Land/My...DSC5727-XL.jpg

Jacob,

No pro here but.... If this turbo has flat top pistons and the cut in the head is the first thing the piston can contact then yes it will add to the deck clearance. This could have been done to work on compression too but that is not what you asked.

Pistons are not flat topped, but have a small dome to achieve 8/1 compression ratio. The domed portion begins a few millimeter from the edge of the piston, so there is a flat area around the edge. For sure the cut in the head is the first thing the piston can contact. I noted when we tore the engine down that no cylinder base shims were used so it does appear that the original builder was trying to do something with the comp ratio.

Logic certainly suggests that the chamfer should be added to the deck, but I wanted to confirm. I appreciate your response.

The piston is only going to come closer to the valve if the recessed part of the head is what makes contact with the cylinder. If the cylinder is going to contact the head on the wider outer part, I don't see how the piston will come any closer.

Wouldn't the chamfer reduce the compression ratio?

That does not look chamfered. It looks like clearance to run the piston at zero deck to help increase compression.

Maybe you can check your deck height from the top of the cylinder to the edge of the piston and add the 1mm clearance that is machined into the head.

Just a thought but maybe somebody machined the head instead of getting the correct thickness gaskets.

There is an instruction in the Wayne's overhaul manual on using clay to check deck height. I suggest you do this test for all cylinders. Up side is you know. Down side is what the hell do you do with all the clay after.
:)

Bingo! This is what my thoughts were about the purpose of the head machining. I guess chamfer is the wrong term?

is the recommendation then that we size the base shims to achieve. Zero deck? This would leave 1mm clearance to the head and by my calcs get us to 8/1 comp ratio. Naturally we will verify valve piston clearances.

When you have a flat ‘squish area’ and a dome, there are some unique issues to consider.
Here are a few threads:
Pelican Parts Technical BBS - Search Results

Best,
Grady

Grady,

Not much dome on a turbo piston. The outside diameter is darn near flat on the turbo pistons so I do not think it will have much of an affect.

Aaron,

I am not sure how this zero deck setup would give any more compression than setting a 1 mm deck with a space under the cylinder. If the bore of the step in the head matches the cylinder bore anyway.

The combustion chamber volume has been reduced by (pi r squared) x 1mm. It is not the same as running a 1mm deck. The piston is closer to the chamber by 1mm which is a significant change.

Aaron,

Not wanting to sound ignorant here but if metal was removed from the combustion chamber the chamber volume is larger, larger chamber same deck same bore same stroke is lower compression. If the cut stopped at the same diameter as the cylinder how do you figure compression is increased? If the deck height is set to 1 mm via cylinder shims and the chamber is smaller (no cut) the stroke does not change my mind sees a higher compression with cylinder shims. Both assume the cylinder sealing surface is not touched.

The amount of material that was removed is very small but the area that the piston now displaces is appreciable. Keep in mind, the heads are now 1mm closer to the piston than it would be with a 1mm shim. The amount of material that was removed is approx .597cc's but the overall chamber volume is reduced approx 6.795cc's bumping compression.

Aaron,

Thanks that works for me now. Just could not quite get there. The info you provided did the trick.

Thanks,

No worries,

That one is hard to visualize.