Thread: Piston deck height/ Quench question

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I yet have another question haha. Our rotating assembly calls for 12:1 CR with a 64cc cylinder head. (With a stock deck height/ 9.025) Problem here is after using UEM calculators I have found that we are -.020 down in the hole.

3.480/2= 1.74+6+1.265= 9.005 - Half the stroke+Rod Length+ PCH

So instead of being at 12:1 static compression ratio using a 0.041 gasket... we are at 11.3:1

If we used a cometic .027 thick head gasket it would bring the CR back up to 11.7 at .047 quench
Question is can we get away without being at zero deck running a thinner head gasket. Im scratching my head though why its advertised at 12:1 CR with stock deck height. We ordered the rotating assembly through competition products. I checked speedway motors rotating assemblies and it was the same scenario 12:1 advertised but when you add all the numbers up your -.020 in the hole. Am i calculating something wrong here or is this in fact true. Sorry if some of these things come off sounding funny I'm still learning and this will be my first high compression motor. Race fuel Motor lol
Thanks y'all

I preach putting all the components on paper and figuring it all out before you ever buy one part. If you had done that and run the math, you would have realized that you needed to perform some other operations on the block and make some different parts choices before you bolted the whole mess together.

You didn't bless us with the dome volume of the pistons, but I'll assume Icon IC6732-030 @ 6.3cc's.
4.03" bore x 3.48" stroke, 64cc chambers, 9.005" stack, 9.005" block deck height (zero deck), Cometic C5245 head gasket, 4.060" x 0.036" would have given you a 12.13:1 static compression ratio with a 0.036" squish/quench. The motor would be ready for aluminum heads and a 248/252 solid roller cam.

The block deck height is not sacred, you have to be prepared to alter it to suit your needs.

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Yes it's icon -6.3cc's. The block is currently at the machine shop.
Calling up there this morning and get him to check the deck height and if it measures out to be 9.025, I'll get him to take -0.020 off the deck. Appreciate the reply

Probably wouldn't be a bad idea to get the block back first and mock up the motor without the rings though just incase it may be a few thousands of one cylinder to the other. Then I could tell them exact measurements to be at true zero deck.

Originally Posted by Dirtracer5

Probably wouldn't be a bad idea to get the block back first and mock up the motor without the rings though just incase it may be a few thousands of one cylinder to the other. Then I could tell them exact measurements to be at true zero deck.

What? Any variation cylinder to cylinder is going to be in either the rod dimensions, hole to hole; or the piston wrist pin to top dimension, hole to hole, right? Either is reason to return the parts for replacement, unless you want to take cuts off of the piston tops, then re-balance the whole shebang? Your "zero deck" is to first ensure that the surface is dead flat relative to the crank bore, while also providing the desired deck height relative to the piston top, unless I totally mis-understand.

If there is a difference in cylinder to cylinder you can off set bush the small end of the rod without any further balance issues unless its an aluminum rod and then there will be a small correction needed


and its pretty complicated to MEASURE a block for deck height--you have to establish the main bore center line ---precesion bushings plus alignment bar ( usually a 2 inch steel perfectly straigt bar) and some tools toestablish a plane square to the bores then measure each hole to the lowest point( that is minimum of how far you'd have to machine it to get flat square to crank bore surface)


Its easier to determine the deck height you want and have it decked square to that deminsion OR have it decked minimum to square and choose head gasket to establish squish you want.


Now if you don't have pistons with any flat surface you are in a whole different pile of horsefecus to determine squish clearance between ubeven piston crown and combustion chamber of the head--


its about as complicated as having having married your best friends girl while he was gone to war????

Originally Posted by jerry clayton

If there is a difference in cylinder to cylinder you can off set bush the small end of the rod without any further balance issues unless its an aluminum rod and then there will be a small correction needed


and its pretty complicated to MEASURE a block for deck height--you have to establish the main bore center line ---precesion bushings plus alignment bar ( usually a 2 inch steel perfectly straigt bar) and some tools toestablish a plane square to the bores then measure each hole to the lowest point( that is minimum of how far you'd have to machine it to get flat square to crank bore surface

Its easier to determine the deck height you want and have it decked square to that deminsion OR have it decked minimum to square and choose head gasket to establish squish you want.


Now if you don't have pistons with any flat surface you are in a whole different pile of horsefecus to determine squish clearance between ubeven piston crown and combustion chamber of the head--


its about as complicated as having having married your best friends girl while he was gone to war????

It's a dome piston we are working with. Which assuming you take a dial indicator with a magnetic bridge I guess you could call it. Bringing the motor to TDC and then taking a caliper gauge and see how far it down The piston is down in the cylinder. I Have watched this be performed before at a engine builders shop about 5 years ago. Saw him rock the piston back and forth and then on the back side of the caliper he took his measurement on the flat spot of the piston. Like I said before I'm learning while building this engine for a friend I'm still trying to understand this process. So at this point in time I may sound like a idiot but thank yall for all the help.

It your scenario you basicly have to assemble the engine to take measurements to the oiston but basicly you don't have a deminsion to which to deck the block and you still don't know what the deck height is.

Originally Posted by jerry clayton

and its pretty complicated to MEASURE a block for deck height--you have to establish the main bore center line ---precesion bushings plus alignment bar ( usually a 2 inch steel perfectly straigt bar) and some tools toestablish a plane square to the bores then measure each hole to the lowest point( that is minimum of how far you'd have to machine it to get flat square to crank bore surface)

Jerry, all I've ever needed was a 12" dial caliper, pencil and piece of paper. Measure from the main bearing bore to the deck, moving the jaws of the caliper around some to get the lowest reading, write it down on the piece of paper and add the radius of the main bearing bore. Voila, block deck height. You can do all 4 corners of the block in your home garage in less than 10 minutes.

http://www.jamisonequipment.com/picPage/BHM-24-1.gif

Here's a Chinesium 12" dial caliper for 50 bucks. If it's accurate to within 2 thousandths, it's good enough for what we need.
12" inch Dial Caliper Imperial Vernier Workshop White Face Accurate Machine DRO | eBay

Avoid purchasing a digital tool of any kind. Every time I used to reach for my digital calipers, the batteries were dead. Dial calipers and manual micrometers are bulletproof.

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Originally Posted by techinspector1

Avoid purchasing a digital tool of any kind. Every time I used to reach for my digital calipers, the batteries were dead. Dial calipers and manual micrometers are bulletproof.

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I have several Starrett calipers and micrometers that were my fathers - still my favorites. And be sure to have your glasses on when you read the calipers... I keep a set of 1.25 magnifiers in the tool box!!

I agree that that will give you an accurate measurement , however its only accurate at the center line of the cylinder-if the block has ANY tip/angle to it at all from the 90 degree------ like for instance a 348/409 chev or the 383/410/430/462 Lincoln, Mercury/Edsel ?????????


On about every chev engine I have done, it is pretty common that they are tipped aprox .010 and up tp .020 front to back. They broach the block to surface them and I think it is because of the offset in the cylinders from side to side that as the block is forced thru the machine it cocks it a little bit------