You do the math (Deck Height)

[Bobboloo]

Just thought I'd ask you guys to double check my math for me. As the years go by the knife just isn't as sharp as it used to be.

Building yet another 2.7RS motor. FYI this for the street. The case halves have been shaved and the case brought back to standard. Spigot decks cut and I'm in the negative deck height area. The pistons stick out above the cylinder deck .032" or .8mm if you will, with no copper base shims.

So I'm thinking a 1mm copper cylinder base shim/gasket will get me to .2mm deck height and if I add another .75mm cylinder base shim/gasket that will put me at .95mm deck height.

I want to keep the deck height as low as possible since the pistons are stock Mahle RS P+C's which have a published CR of 8.5:1. The heads are being cut as well so hopefully I can raise the final CR. I will be checking valve clearance to make sure I have at least 1.5mm for intake and 2mm for exhaust.

So is my math correct?

-.8mm deck
+1.75mm copper shims
=.95mm deck height

Do you think a .95mm deck height is a good?

[BURN-BROS]

Bobby, your at the bottom end of allowable deck height. You can run it , but check for any variances in your rods,pistons, etc that may push that # smaller than you would like.

That said, I would run it.

[Bobboloo]

Good advice. I'll check the deck on all six just to be safe.

[Steve@Rennsport]

As long as the pistons won't touch the heads when everything is hot, I'd run it.

[Bobboloo]

Thanks for the reply Steve. Your advice is as good as it gets in my book.

[ed martin]

What about checking the finished product using the lead solder technique just to be sure?

[Bobboloo]

I recently rebuilt a 2.4L with 2.2S pistons. Solder was the only way I could really check deck height for those pistons. With 2.7RS pistons it's quite easy to check deck height without solder. The caliper has a nice flat land to rest on because of the pistons design. I don't think using solder would yield a more accurate result specially considering the same caliper would be measuring the solder. Measuring solder with a micrometer might though.

Regardless the initial post was really about my math and whether running a deck height of .95mm for a street motor was wise a choice.

[ed martin]

Just thinking that the solder approach would be proof positive, and eliminate any math or calculation oversights.

[whackit]

doing the solder crush will also factor in your head machining.

[whackit]

Actually I just re-read your math and you are not taking into account your head machining:

-.8mm deck
+1.75mm copper shims
=.95mm deck height

-.8mm deck
+1.75mm copper shims
-.25mm head cutting (or whatever value you had done)
=.7mm deck height

I think that's the case. From P. 148 of Wayne's book:

... if your heads were fly cut 0.25mm then you will need to start with two copper base gaskets instead of one.

That was the case with my motor, Measuring the deck height with caliper on same setup as yours was 1.5mm with heads that were fly cut it was 1.25mm via solder crush test.

[Bobboloo]

Originally Posted by ed martin

Just thinking that the solder approach would be proof positive, and eliminate any math or calculation oversights.

You still need to measure the solder with the caliper and then do the math to determine what size shim(s) you need.

Don't get me wrong, I've used solder in the measuring process and it's the only way with a high dome piston that doesn't have a land but in this situation I feel more confident putting the caliper right on the piston and measuring. Solder can move and sometimes it's hard to actually determine where the deck is when you examine the solder.

Originally Posted by whackit

doing the solder crush will also factor in your head machining.

Deck height is the difference between the top edge of the piston and the top of the cylinder. Machining the heads has no effect on that correlation and doesn't need to be factored in to deck height.

Machining the heads does change the shape and size of the combustion chamber so I'll be checking valve clearance as well.