Measuring deck height
 

Hi,

A quick quesiton to confirm - I need to measure the deck height of my 87 930 p&c. According to Wayne's book, since its a CIS cylinder, I can simply use a Vernier instead of the more "BA procedure". Is this correct?

And, how can I be 200% sure that the cylinder that I'm measuring is exactly at TDC; not that the pulley is off so there's no marking to tell?

Thanks.

For CIS pistons, there is a small flat spot (right on top of one of the wrist pin holes) that you can place the tip of a dial gauge or the end of a dial caliper on. I like using the dial gauge as you can mount it on one of the head studs and easily find TDC for that particular piston.

It's more accurate to use a dead stop device to locate TDC. The crank can travel a number of degrees while the piston is sitting still at TDC.

You can fabricate some flat stock that spans the top of the cylinder and held on by two opposing studs. Drill and tap the material in the middle so a stop bolt threads into it.

1. Position the piston some distance down the cylinder.
2. Adjust the depth of the stop bolt, then lock it in place with a nut.
3. Rotate the engine clockwise carefully until the piston stops against the stop bolt.
4. Mark the pully opposite the crankcase split line.
5. Repeat 3 in the opposite direction.
6. Repeat 4
7. Halfway between your marks is TDC for your engine. Notch the pulley with a file, then apply white paint to ID it.

I noticed Jeg's carries a "TDC Plate and Stop" (#128-40200) for about $12.00. They also carry a TDC Locator that screws into the spark plug opening that does the same thing. Make sure the rocker arms adj. are backed out so they don't interfere with the valves (I haven't tried the latter, so I can't vouch for adequate clearance using this method).

Sherwood

Hi Sherwood, thanks for the detailed procedure. In my case, the cam housing & cylinder heads are all removed so I won't have any valve clearance issue for now :)

But I guess there's no other way to guarantee TDC other than to pay for a tool :( ? Where's Jeg's BTW?

Jeg's is at www.jegs.com

Sherwood

Hmm, this seems like overkill to me. Just put it at what you think is TDC, and make a few measurements of deck height around those points (rotating the crank slightly in both directions). You can eyeball it pretty darn well.

Also, the Turbo pistons are different than the typical CIS pistons - they are flat to begin with...

-Wayne

I suppose eyeballing might be okay to determine deck height @ TDC. However, for cam timing, it's better to know where TDC is exactly.

Sherwood

Absolutely! Although by the time you get to cam timing, I would hope that the pulley was on again. Snowman's method for double-checking the pulley is also very good - do a search in the archives - it's good info.

-Wayne

Hi Wayne / Sherwood,

Thanks for the response & advice - will take your suggestion & eyeball it :)

Wayne, about your comments on turbo pistons -

It means that I can safely use the vernier caliper to measure the height then?

In my case, I found out that the PO had shaved the head. I'm also changing cams & I got a very good set of P&Cs which sadly also had been shaved a tiny bit, so I want to be doubly sure that I'm not bumping my compression beyond safe & there's no valve clearance issues.

thanks again.

Guys,

The critical issue in fitting non-standard configurations is the piston-to-head clearance (deck and squish) and the piston-to-valve clearance.

If you have an absolutely stock engine and need to replace the pistons and cylinders with stock OEM parts, that is a bolt-on operation. As soon as you stray from that, you need to start paying attention to the clearances. The higher compression and more radical engine, the more careful attention must be paid. This may (in extreme cases) include building the engine, running it, disassembly, measuring, adjusting the clearances, running again, etc.

Number one rule; the parts (piston-to-head & piston-to-valve) can never, ever, ever have contact. Close is OK but no touching is allowed.

The reasons are:
There must be a “squish” area around the perimeter of the piston-to-head that is very close. This forces the combustion gasses into the actual highest compression area of the combustion chamber at TDC compression. The ideal would be to have zero clearance in the squish area but mechanical realities prevail. There is the mechanical stretching of components (crank, rods, piston, cylinder, head, case), differential thermal issues (the hotter parts expand more), the transient issues (some parts heat up sooner than others), and wear. This is absolutely a NO CONTACT sport.
Some say to lower compression ratio, just add spacers under the cylinder. NO, never ever reduce deck/squish. This provides an area (used to be proper squish) where detonation can occur, and just in the wrong place. At TDC compression the unburned mixture in that area won’t burn or will be extinguished, it’s too little mixture and too much relatively cool aluminum surface. It will be prone to detonation! Of course this is the area just above the top ring land. Guess what, this is like a hammer hitting the top of the piston and can collapse the crown of the piston around the top (compression) ring. As soon as the ring isn’t free to move, compression is lost and much worse things follow. Most have seen pictures of detonation holes in the center of pistons. That is from extreme detonation. The loss of squish allows insidious detonation that is usually not audible. The squish (deck) around the perimeter should be the closest clearance, clearance should be the same or slightly increase toward the center. Never should there be “pockets” of increased squish between the perimeter and center.

So, how do you measure these clearances?
To start with, I like putty – plain old modeling clay. Add a few extra shims under the cylinder, and carefully put putty on either the head or the piston for deck/squish, your choice. Have the surface exceedingly clean so the putty will stick. Give the opposing surface a light coat of Pam or silicone spray. Don’t put much more putty than the clearance you are expecting. Too little, you can repeat the measurement – too much and it won’t turn over. Put it in at least four (preferably more) locations around the perimeter of the piston. Use some old rings on the piston (piston must have rings for this). Torque the head(s) in place (you don’t need a head gasket), and carefully turn the engine over just past TDC – just once, not stirred.
I use an X-acto knife to cut the putty and measure the thickness.
When you know the approximant clearance and know there is appropriate clearance (after machining?), then remove the additional cylinder shims. Repeat the process. At this stage, use rosin core solder of the appropriate diameter. Remember the symmetrical spacing around the piston because the piston will rock in the bore.

Only after you have the deck/squish exactly where you want it do you light into the piston-to-valve clearance. You will need to assemble each bank with heads torqued in place and the cam housing. You can use just the outer valve springs, but you must time the cams. Do the cam timing very carefully so you don’t contact the putty while the cams are not in perfect time. I like using a solid tensioner for this. Just use putty as solder can bend a valve. On most engines (particularly radical cams) the valve dynamically chases the piston or visa versa. Pistons that have full valve relief are safer for the valve than pistons with only a slight relief cut-out. That is so when there is valve-to-piston contact, the piston closes the valve and doesn’t bend it.

OK, back to the overall picture. Having close clearances (deck and squish) are not necessarily only for high compression, it is for proper combustion. This is as true on street cars (maybe more) as it is on the best race cars.

Above all: NO CONTACT, just close.

Best,
Grady

You must have 1.5mm clearance on the intake and 2.0mm clearance on the exhaust. The measurement check at assembly is done when the engine is cold - everything expands when hot. If you do not adhere to these specs, then you will have contact at high RPM when the engine is hot.

-Wayne

Wayne,
Right on, those are good, appropriate values for piston-to-valve clearances.

I didn’t give specific numbers because they depend on application and the degree of effort one is willing to go to in order to get “close.” There is lots of argument about how close to run these parts. Every builder seems to have their own numbers. There is no “pat” answer, only “safe” (too much clearance) answers. Too close is when there are signs of impending contact. Not close enough leaves the squish area looking the same carbon pattern as the combustion chamber and can leave room for detonation. Someplace just shy of ever any contact is the right clearance.

Look at:
http://forums.pelicanparts.com/porsche-911-technical-forum/148258-twin-plugging-question.html
emcom5’s stock SC piston shows proper squish on the left, rdane’s piston shows reasonable squish around the perimeter and the “shadow” of the intake valve. To the contrary is Bill Verberg’s RSR piston that shows consistent “gun metal gray” coloration over the entire crown. I interpret this image that it had no proper squish (too much clearance) and was detonating. On the image “98mm Mahle & JE” note the nice clean ring around the perimeter of the used piston. That is good squish.

I have seen many (too many) pistons embossed with the contour of the valves – not good for the machinery. That should only happen above 9000 RPM or jumped cam timing. Many (most?) times when you see broken rings and major failure around the perimeter of the piston, it is from detonation in the squish area. Overheated and seized pistons are the other concerns. This may be a “chicken or egg” situation.

Above all: NO CONTACT, just close.

Best,
Grady

Grady,

Can you please explain the difference between the deck height and squish area measurement.....if there is one?

My interpretation of these are:

Deck height - Distance from piston deck to top of cylinder at TDC

Squish area - Closest distance of the piston dome to the head at TDC, usually measured at the outer perimeter.

Are the squish and deck height measurements usually equal (as i hear them used interchangeably)?

I would think that squish can vary from one piston type to another (if keeping deck height constant) due to the different piston dome profiles/shapes, etc.

So, is the solder method for measuring 'deck height' really measuring squish area?

If so, you can see how the 1.25mm - 1.50mm 'rule of thumb' for measuring deck height using the solder method is a bit misleading.


Signed,

-Confused

Charlie,

Squish and deck are often used interchangeably.

Deck is usually reserved for the clearance of a flat piston top to the flat portion of the head. In our 911 case I use deck to refer to the clearance between the flat outer perimeter of the piston and the corresponding flat area of the head.

Squish is properly that part of the piston-to-head clearance that is at the limit of minimum clearance and includes both the deck area and portions of the “pop-up” part of the piston. Notice in the images referenced above how clean or lighter shade of carbon some of the “pop-up” parts of the piston are. These are where there is “squish” minimum clearance. The other areas of the top of the piston are where the combustion volume in minimum at TDC compression.

You are correct about the profile of the piston dome. High quality pistons are exceedingly close to each other, other (usually cheaper) pistons are not so good. The same is true of the profile of the head. Great heads (911) are very consistent, others usually are lacking.

The techniques using putty and solder are just indirect methods for measuring dimensions in a location where a conventional measuring device won’t fit.

Did this help? I haven’t figured out how to post images on this board but I can e-mail you, if you want.

Best,
Grady

Grady,

Thanks for the reply, but still looking for a little more clarification.

Here is a picture of a JE piston (high dome) I used in my rebuild.

http://www.dorkiphus.com/porsche/mod...istons_001.jpg

I used the outer shelf to measure deck height with a dial gauge.

Should my deck height measurements using the dial gauge be the same as the 'squish' area using the solder method?

I found my deck heights ~1.15mm (using the dial gauge method) and my squish area to be about .50mm greater (using the solder method). I think other's have noticed this phenomenon also.

So, when building our engines and using the 1.25-1.5mm 'rule of thumb', are we talking deck height or squish area? (maybe this is a question for Wayne, Walt, etc).

Charlie,

Yes, you are correct in measuring the deck (at the outer shelf as you referred.)
The deck should be less that the squish or at least the same. If the deck has greater clearance than the squish, that tends to create a “pocket” for detonation to occur.
The “rule of thumb” 1.25-1.50 mm refers to both. Bruce has good safe numbers there.
I measure the squish perpendicular to the surfaces.

Best,
Grady

Grady,

Understood. Perfect.

Thank you.

-Charlie

If too much of the head has been flycut I'm guessing?

.....I can't let it go

If the engine uses a head gasket this will add to the squish. Don't forget to add this thickness to the other measurements.

Sherwood

Sherwood,
Yes, you are right. That is true for the OE 2.0s.
In ’70 Porsche went to the C-E head gasket which is captive in a groove in the top of the cylinder. It is only slightly compressed and not squashed. The great part of this feature is the dimensions are all determined by metal-to-metal contact.

Best,
Grady