Deck height

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1930

1930

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I am going to be working toward getting a true compression ratio on my parts truck by taking the necessary measurements.

I have read that.........the stock piston compression height is 1.765

If I am reading that correctly that comes out to 1 3/4 (approx) down in the bore measuring from the top of the deck is where the piston top sits at TDC. That does not seem right. How would the engine build any compression?

I turned the parts truck engine over so that the piston is at approx TDC and as you can see the piston is maybe 1/4 down from the deck so I am confused?

Can someone clarify for me which is correct? ( approx )

225 slant, dont know the year yet of the engine, havent gotten that far yet.
 

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Compression height is the measurement from the center of the piston pinhole to the top edge of the piston.
 
what your thinking is piston to deck height. On the 225's you can see anywhere from .140-.210 below deck.
 
RustyRatRod said:
Compression height is the measurement from the center of the piston pinhole to the top edge of the piston.
Click to expand...
This has me confused, would another way to put it be...........Compression height is the measurement from the center of the wrist pin to the top edge of the piston?
 
805moparkid said:
what your thinking is piston to deck height. On the 225's you can see anywhere from .140-.210 below deck.
Click to expand...
Thanks kid, isnt the typical way to find compression on an engine to measure deck height ( the measurement from the deck down to the top of the piston using a depth gauge ) and then CC the head chamber.

Can you remind me of the formula?
 
1930 said:
This has me confused, would another way to put it be...........Compression height is the measurement from the center of the wrist pin to the top edge of the piston?
Click to expand...

Since we're talking in thousandths of an inch, I prefer to use the pin hole as a reference. Less variation. Besides, that's where the measurement is taken from for the spec. The pin is irrelevant. besides that, the pins are not a press fit into the piston and have clearance between the pin and pin bore in the piston. That might be splittin hairs, but accurate is accurate.
 
RustyRatRod said:
Since we're talking in thousandths of an inch, I prefer to use the pin hole as a reference. Less variation. Besides, that's where the measurement is taken from for the spec. The pin is irrelevant. besides that, the pins are not a press fit into the piston and have clearance between the pin and pin bore in the piston. That might be splittin hairs, but accurate is accurate.
Click to expand...
I do understand and agree, laymans terms I have always heard wrist pin, never heard anyone describe it as pin hole, my first thought was ....what pin....

Now I am clear and thanks for the accuracy and clarification.

I will assume that since the pins are not press fit they are clipped. I believe I have seen this set-up before.
 
None are press fit into the piston. That's what I was referring to. Press fit pins press into the rod and are allowed to swing on the piston so the piston and rod are not locked together and can rotate through their travel with the crankshaft in the cylinder. Full floating pins float both in the piston and the rod and require some sort of retention such as spirolox or c clips or wire retainers.
 
to get a piston recession value that is meaningful for use in claculations for compression
1) you must have the piston in the bore that is being measured at top dead center. The way to do that is with a piston stop. Install the stop, install a degree wheel, rotate the engine clockwise so the piston hits the stop, observe / mark the reading on the degree wheel. Rotate the crank counter clockwise, so the piston comes to the stop. Observe / mark the reading. Remove the piston stop and move the crank to the degree rerading exactly half way beteen the two. The piston where the stop was will be at TDC
2) a method to measure accurately,, depth micrometer, or block of known height, feeler gauges and a straight edge..

and yes, the poster is confusing a piston measurement, center of pin bore to top of piston ( compression height) with piston recession.
 
my68barracuda said:
to get a piston recession value that is meaningful for use in claculations for compression
1) you must have the piston in the bore that is being measured at top dead center. The way to do that is with a piston stop. Install the stop, install a degree wheel, rotate the engine clockwise so the piston hits the stop, observe / mark the reading on the degree wheel. Rotate the crank counter clockwise, so the piston comes to the stop. Observe / mark the reading. Remove the piston stop and move the crank to the degree rerading exactly half way beteen the two. The piston where the stop was will be at TDC
2) a method to measure accurately,, depth micrometer, or block of known height, feeler gauges and a straight edge..

and yes, the poster is confusing a piston measurement, center of pin bore to top of piston ( compression height) with piston recession.
Click to expand...
You must have read my mind for my next question
 
you can make a piston stop from a strip of steel, long enough to bridge across one bore.

drill holes in the strip that align to two head bolt holes, get shorter bolts to bolt the strip across #1 bore. It needs to be flush down to the block deck.

drill a hole in the center of the strap that will accept a bolt and nut. Position the bolt and lock with a nut so the stop protrudes maybe 1/2 inch down in the bore. The depth into the bore needs to exceed the piston recession being measured, but where to set it is not critical, put it where you get a workable range on the degree wheel.

the strap needs to be rigid enough so it does not deflect when stopping the piston.. I use a piece of boxed channel.

you can print a degree wheel off the internet,, or buy one at Summitt, Jeggs, AZ,, bigger diameter has better resolution, less error.

remember, when you get #1 at TDC,, after measuring it you can go right to #6 and measure it as 1 & 6 are both at TDC at the same time, that will get you machining run front to rear on the block. Piston crowns can have a thou or three run across the top, so to get repeatability measure approximately the same spots on the pistons..I measure about 1/2 inch off piston center.
 
Ok so can someone walk me thru the steps to finding where my compression is going to be at with this engine.

I can get the required measurements just need to know specifically what they all are thats needed.

I like the write-up above posted by Crazycuda but since my engine is still in the vehicle and is slanted I dont see how I am going to be able to get an accurate measurement of fluid.

I measured the bore and it is stock, as far as what I have read stock bore is 3.40. For some reason this one comes under that at 3.95 or so.

I can get the piston deck height in the A.M and I am confident I can CC the chambers on the head.

As far as I know that is all that is needed. Once I have these figures what can I do with them?

I understand there are on-line calculators but Id like to figure it the old fashioned way and understand my work.

I have been told with the way fuel is going that I will not want to shave the head any more than is necessary to attain a 9.1 ratio.

Now that I think about it if I decide to use the larger valves I have purchased isnt that going to change the volume of the chamber in the head.

Do I need to do this head work before I can decide how much to take off the head to increase compression?

I dont just want to remove a 100thou and hope for the best. Id like to know what and why things are being done.
 
you need the CC of the cylinder hear
bore and compressed thickness of the head gasket (Felpro is .037 thick for both 3.4 and 3.510 bore gaskets)(stock steel "shim" gasket is .021 thick for both 3.4 and 3.510 bore)
Cylinder bore
Piston to deck height: you can do the stop method but its alot of work... just use a dial indicator and turn it till it stops coming up and measure it...

then use this calculator
http://www.wallaceracing.com/cr_test2.php
 
1930 said:
Ok so can someone walk me thru the steps to finding where my compression is going to be at with this engine.

I can get the required measurements just need to know specifically what they all are thats needed.

I like the write-up above posted by Crazycuda but since my engine is still in the vehicle and is slanted I dont see how I am going to be able to get an accurate measurement of fluid.

I measured the bore and it is stock, as far as what I have read stock bore is 3.40. For some reason this one comes under that at 3.95 or so.

I can get the piston deck height in the A.M and I am confident I can CC the chambers on the head.

As far as I know that is all that is needed. Once I have these figures what can I do with them?

I understand there are on-line calculators but Id like to figure it the old fashioned way and understand my work.

I have been told with the way fuel is going that I will not want to shave the head any more than is necessary to attain a 9.1 ratio.

Now that I think about it if I decide to use the larger valves I have purchased isnt that going to change the volume of the chamber in the head.

Do I need to do this head work before I can decide how much to take off the head to increase compression?

I dont just want to remove a 100thou and hope for the best. Id like to know what and why things are being done.
Click to expand...



Ok, here's what you need to know to figure out your compression:

Volume of combustion chamber in head.

Measure compressed thickness and diameter of "fire ring" on head gasket.

Volume above the piston to the top of the block when the piston is at TDC. If you have a flat top piston, you can just measure how far down the piston is from the head deck of the block, and then get the bore diameter. You can just use the stock stroke if you don't want to measure it, but measuring it will be fine.


Then I can walk you through the calculation or do it for you.


Also as far as measuring the volume of the cylinder at TDC while the engine is in the block, you can make a cover plate as I described in the link to my thread on how to check compression. Just put the fill hole to where it will be at the top of the block - even if it is not flat. A little "slant" is good also as it makes it easier to tell when the cylinder is full.
 
krazykuda said:
Ok, here's what you need to know to figure out your compression:

Volume of combustion chamber in head.

Measure compressed thickness and diameter of "fire ring" on head gasket.

Volume above the piston to the top of the block when the piston is at TDC. If you have a flat top piston, you can just measure how far down the piston is from the head deck of the block, and then get the bore diameter. You can just use the stock stroke if you don't want to measure it, but measuring it will be fine.


Then I can walk you through the calculation or do it for you.


Also as far as measuring the volume of the cylinder at TDC while the engine is in the block, you can make a cover plate as I described in the link to my thread on how to check compression. Just put the fill hole to where it will be at the top of the block - even if it is not flat. A little "slant" is good also as it makes it easier to tell when the cylinder is full.
Click to expand...

you dont need to use the swept volume method unless you have anything other than flat tops or dont know the piston specs.
 
805moparkid said:
you dont need to use the swept volume method unless you have anything other than flat tops or dont know the piston specs.
Click to expand...

You need to know your swept volume to calculate compression.

You can just use the book values, or measure it. Measuring it will be most accurate. Book values will be close enough for swept volume calculations, but you need to measure the information to figure out clearance volume.
 
805moparkid said:
you dont need to use the swept volume method unless you have anything other than flat tops or dont know the piston specs.
Click to expand...

If you mean measuring the volume above the piston, yes, I recommend using the method that I have in my how to link. You don't know where it was from the factory, or if anybody has machined it since. The only way to know what you have is to measure it.
 
krazykuda said:
You need to know your swept volume to calculate compression.

You can just use the book values, or measure it. Measuring it will be most accurate. Book values will be close enough for swept volume calculations, but you need to measure the information to figure out clearance volume.
Click to expand...

krazykuda said:
If you mean measuring the volume above the piston, yes, I recommend using the method that I have in my how to link. You don't know where it was from the factory, or if anybody has machined it since. The only way to know what you have is to measure it.
Click to expand...

you can either use water, or just measure the bore and piston to deck height and move on.
 
805moparkid said:
you can either use water, or just measure the bore and piston to deck height and move on.
Click to expand...


That's exactly what I stated above. If he has a flat top piston, just measure how far it is below the head deck at TDC, and the bore, then calculate.

If it's not flat top, then you have to use water to measure it to be accurate.
 
805moparkid said:
you need the CC of the cylinder hear
bore and compressed thickness of the head gasket (Felpro is .037 thick for both 3.4 and 3.510 bore gaskets)(stock steel "shim" gasket is .021 thick for both 3.4 and 3.510 bore)
Cylinder bore
Piston to deck height: you can do the stop method but its alot of work... just use a dial indicator and turn it till it stops coming up and measure it...

then use this calculator
http://www.wallaceracing.com/cr_test2.php
Click to expand...
1-Quote: you need the CC of the cylinder head..............Ok I can get this I believe with no problem.
2-bore,for both 3.4 and 3.510 bore gaskets...........It sounds like there is more than one size bore gasket. Can you elaborate on this, which should I be asking for and why the different sizes?
3-Piston to deck height, you can do the stop method but its alot of work..........what is the stop method? Do you mean the piston stop method? My head is already off on this particular head so using a dial indicator is not an issue for me. I do not mind the lot of work part if it means more accuracy.
 
krazykuda said:
Ok, here's what you need to know to figure out your compression:

Volume of combustion chamber in head.

Measure compressed thickness and diameter of "fire ring" on head gasket.

Volume above the piston to the top of the block when the piston is at TDC. If you have a flat top piston, you can just measure how far down the piston is from the head deck of the block, and then get the bore diameter. You can just use the stock stroke if you don't want to measure it, but measuring it will be fine.


Then I can walk you through the calculation or do it for you.


Also as far as measuring the volume of the cylinder at TDC while the engine is in the block, you can make a cover plate as I described in the link to my thread on how to check compression. Just put the fill hole to where it will be at the top of the block - even if it is not flat. A little "slant" is good also as it makes it easier to tell when the cylinder is full.
Click to expand...
1- Volume of combustion chamber in head.........No problem, I will get this or work on this over the weekend.

2- Measure compressed thickness and diameter of "fire ring" on head gasket...........how do I get the compressed thickness if its not compressed? Do I have to torque down a new head gasket and run one just to get these measurements. Looks like these numbers are already know ( see post above ) but is that too inaccurate?

3-Volume above the piston to the top of the block when the piston is at TDC.........I can get this no problem assuming you mean measure the deck height or the height from the deck down to the top of the piston when the piston is at TDC, I do have a flat top piston.

If you mean the liquid than of course that is going to take a bit more.

4- then get the bore diameter...........I have this already as 3.95, I thought bore diam for slant was 3.40, why is mine a bit smaller?

5- You can just use the stock stroke if you don't want to measure it, but measuring it will be fine..........From what I have read I should not just use the stock stroke because of factory variances, I want to do everything I do correctly and most accurately. ( within reason, its not a high dollar racing engine that is going to see anything near 5 or 6 thousand RPM at any time ) I do not want to leave anything to chance, ( assuming that what I am leaving to chance will greatly affect the outcome I am trying to achieve) at the same time I want to know and understand why I am doing the things I am doing and that is why all the questions.

6- I am going to guess the reason you suggest using the liquid method is because ( in laymans terms ) I am measuring piston bore but that doesent account for cylinder wear below the top ridge in the cylinder. Is that correct?

How much would that wear change the end calculation, in other words if I were to just use the piston bore/compressed fire ring value and chamber CC # and not take into account the cylinder wear would it/could it throw off the end number a great deal?

Also as I mentioned above I guess I am going to have to decide what I am going to do with my head before I can CC it. Bigger valves are going to change that number I would think greatly.
 
Ok, let me see if I can answer your questions.

2. Yes, to measure compressed thickness of your head gasket, the most accurate way is to get one like you plan to use and torque it down on the engine, then take it off and measure the thickness and diameter of the fire ring. (or save a used one if you have one just like that has already been compressed).

3. For volume above the piston at TDC; If you have flat tops, you can just measure how far down the piston is at TDC, and the diameter of the cylinder bore and calculate from there.

You can also use the water method with the cover plate as described in the compression check thread.

You could also try to do both to check for accuracy, but they should come out the same if done correctly.

4. I don't get how your bore is 3.95 when is should be 3.40, and you say that it's smaller. Isn't 3.95" bigger than 3.40"?

5. I usually just use the stock stroke numbers from the books to calculate swept volume, but if you don't think that your engine is "to the book" then find BDC and measure the depth of the piston in the bore at TDC & BDC, then take the difference.

6. I recommend the liquid method if you can't measure it easily. (It's much easier if it makes a perfect cylinder and no other "variations" to calculate with bore and depth). The liquid method accounts for everything in your engine, but is not practical to measure at BDC (the water tends to leak when you are trying to fill this much volume). You can just use the bore diameter that you measure at the top, I don't think a little wear will affect the results that much.
 
He either means 3.395 or his block is bored .050 over with .005 attributed to wear.
 
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