Checking deck height/piston-cyl/piston-valve clearances

[85RedCarrera]

The Porsche 911 engine design certainly requires more man-hours, in regards to checking dimensions, than any other engine I have worked on, due to its separate cyl/head stacks combined with an overhead cam box.

Since my 3.2 to 3.4 build includes 98mm Max-Moritz style pistons and a 964 cams, I must check not only piston/deck height, but piston dome-to-head, and piston-to-valve clearances. Yes, the heads were chamfered for these pistons, but I still need to check everything.

I am wondering what the most efficient way to do this is, so that I don't have to repeatedly re-mock up the engine...

I plan to use clay to measure the piston dome to head clearances.
I plan to use solder to measure the deck height.

Can I use valve checking springs in the #1 and #4 cylinder heads, a crank degree wheel and a dial indicator, to measure piston-to-valve clearance, without installing the cam towers? Or- is it easier to just install everything, time the cams, and use clay or checking springs and a dial indicator?

I was hoping NOT to have to assemble everything twice AND have to time the cams, twice.... perhaps I only need to get the cams "roughly" timed for this check?

If doing everything twice while being precise (hey- that rhymes) is really the only way to do this right, then of course that is what I will do.

How are most of you handling this chore?

Thanks,

Dave

[stownsen914]

For mockup, you can assemble #1 and #4. For piston to valve, I can't think of a way that doesn't involve installing the head, cam tower, camshaft, and rockers. You can do only #1 and #4 of course. Personally I measured deck height with head off and a depth gauge on my digital caliper. Most say solder over clay if you separately need to check piston to head clearances due to clay's tendency to deform easily when measuring. For piston to valve, setup springs make it easier, and then use a degree wheel and dial indicator to manipulate the valves manually and check clearance. Make sure you have a good TDC measurement. Personally I used a dial indicator and the degree wheel to be as precise as possible.

[85RedCarrera]

Originally Posted by stownsen914

For mockup, you can assemble #1 and #4. For piston to valve, I can't think of a way that doesn't involve installing the head, cam tower, camshaft, and rockers. You can do only #1 and #4 of course. Personally I measured deck height with head off and a depth gauge on my digital caliper. Most say solder over clay if you separately need to check piston to head clearances due to clay's tendency to deform easily when measuring. For piston to valve, setup springs make it easier, and then use a degree wheel and dial indicator to manipulate the valves manually and check clearance. Make sure you have a good TDC measurement. Personally I used a dial indicator and the degree wheel to be as precise as possible.

I think the most challenging part (for me), if I use a degree wheel + checking springs + dial indicator (and no cam tower installed) is going to be determining WHERE in the crank rotation I should be measuring valve travel available, and how much valve travel I should have at that location....

If there is a spec for those dimensions where I can simply bring the piston to TDC and push the intake & exhaust valves down until they touch the piston, measure and call it good, that would be great- but I am guessing that data is not publicly available, so the process is going to remain more labor intensive?

I'm sure that engine builders have their own proprietary notes from prior builds (combinations of cams/deck heights/piston domes) that make this process more streamlined. I guess first time around, it's just going to have be a lot more work....

[targa72e]

For valve to piston check you can use the fact that the pitch on the rocker adjuster is 1.0MM. Assemble and time everything. Then set intake and exhaust at zero lash. rotate adjuster in the amount of clearance you desire. Rotate engine carefully by hand. If there is no binding there is no interference, and you are good to go. You can continue turning the adjuster in until it binds (valve hits piston) if you want to know the actual clearance.

john

[85RedCarrera]

Originally Posted by targa72e

For valve to piston check you can use the fact that the pitch on the rocker adjuster is 1.0MM. Assemble and time everything. Then set intake and exhaust at zero lash. rotate adjuster in the amount of clearance you desire. Rotate engine carefully by hand. If there is no binding there is no interference, and you are good to go. You can continue turning the adjuster in until it binds (valve hits piston) if you want to know the actual clearance.

john

Yes, thank you- I had read a while back that one rotation of the rocker adjuster equals 1mm- I had forgotten that! That actually makes things easier.

For the initial assembly, if I have to install the towers and time the cams, I am planning to install the pistons without rings (and only one circlip so that I can easily slide the pins out afterwards), so that I can rotate the engine with little effort and "feel" any interference when I get to that point.

[stownsen914]

If you are doing the valve adjusting screw method, another option is to turn the screw in and back out at different positions before and after TDC. I’ve heard recommendations for 5 and 10 degree increments. I checked every 5 degrees since I am using a high lift race cam and knew clearances would be close.

If you screw in and out at those positions, you’ll know actual clearance. A little gentler on the parts too in case a valve does touch a piston.

[PeteKz]

Ditto on using the adjuster screws to determine valve to piston clearance, and at 5 and 10 degrees before, and after the recommended checking point.

You will need to set the timing pretty close to specs to prevent the valves from interfering with the pistons during your mockup, so don't be alarmed if the pistons interfere with the valves as you crank it through manually. Just stop, recheck the cam timing, and try again.

[JoeMag]

I would put rings on. Put piston in cyl without rings and see how much you can rock back and forth. Min piston to valve will occur 8-10 btdc exh and 8-10 atdc intake unless you have something out of the ordinary going on.

Measure deck height above the wrist pin (minimize piston rocking effect).

Regarding timing cam twice you can mark parts and disassemble, and put back together aligning marks and be in time.

Do not assume anything was made correctly… check everything.

[85RedCarrera]

Originally Posted by JoeMag

I would put rings on. Put piston in cyl without rings and see how much you can rock back and forth. Min piston to valve will occur 8-10 btdc exh and 8-10 atdc intake unless you have something out of the ordinary going on.

Measure deck height above the wrist pin (minimize piston rocking effect).

Regarding timing cam twice you can mark parts and disassemble, and put back together aligning marks and be in time.

Do not assume anything was made correctly… check everything.

All very good points- thank you. And GREAT pictures too!

I was not going to install the rings for the first assembly, since I fully expect that the .25mm base gaskets will need to be increased. I was also trying to avoid re-loading the pistons in the cylinders more than once (to reduce the probability to breaking a ring on assembly)...but performing a good measurement will always be more important.

You are right- any spec given by Mahle for piston/head clearance will almost certainly be predicated upon having the piston rings installed and having the piston fairly well centered in the bore while measuring.

If I have to change to thicker base gaskets, my cam sprocket location/timing will be affected, but using the markings from the first assembly will put me so close, that I will surely save time when I have to do the final cam timing.

And a big thank you to everyone else- there are a lot of things to consider during an engine build, that you won't find in the books....

[stownsen914]

You can install the rings and just leave the ringed piston in the cylinder if you need to disassemble after you do your mockup checking. Just expose enough of the piston to pop the pin out.

[85RedCarrera]

Originally Posted by stownsen914

You can install the rings and just leave the ringed piston in the cylinder if you need to disassemble after you do your mockup checking. Just expose enough of the piston to pop the pin out.

Agreed- and that will be my plan.

[PeteKz]

Measure piston to head clearance at the sides of the wrist pin--it doesn't rock in that direction.

[85RedCarrera]

Originally Posted by PeteKz

Measure piston to head clearance at the sides of the wrist pin--it doesn't rock in that direction.

Yes, thanks Pete!

[Neil Harvey]

If I can add something here that may help.

Measure the Piston deck height as has been explained. Check across the pin axis but measure both sides as there could be a difference. Use the minimum number. Adjust with base gaskets. Remember this will change the CR number. Its an easy input into your equation as the Piston dome and Chamber volumes do not change.

This is how I suggest you measure P/V clearances. Saves time up front and if you ever have to change the cams or timing you have all info you need. What you do need to do is to ask your Cam supplier the valve lifts at the same cranks angle's I suggest below. If they do not offer this service, we do for a small fee. . We need just one cam to measure.

Fit a degree wheel and TDC the wheel. Fit 1 head with the valves loose. No springs. You can fit soft checking springs. If you leave loose, make sure you hold one valve up against its seat when measuring the other valve drops. A rubber band works well.

Position the crankshaft 20° BTDC and with a dial indicator touching the tip of the valve stem, hold the valve up against its seat and measure the drop distance until it touches the Piston. Repeat this every 5° or 10° to TDC and ATDC to 20°. Do the exact same thing with the other valve.

These drop measurements are the maximum the valve will move to touch the piston. This dimension will never change unless you add or remove base gaskets, cut the seat deeper or machine the Head thinner.

Now the cam supplier will supply you the valve lifts at these same crank positions after you tell them the cam lobes centers or lift @ TDC you wish to use. Then you subtract the valve lift numbers they give you from the numbers you measured. This gives you the clearance.

Why is this a good way to do this? You could have interference, but its only on paper. Then with the amount of interference plus the clearance you want, you can add base gaskets, or cut the pockets in the valve deeper. You do not have to assemble the engine and time the cams often multiple times, either.

Also, if you ever change the cams, or the timing, all you need are the numbers from the cam supplier at the new position, to see if you can move or what changes you have to do to the Pistons.

Hope this helps and understood.

[Neil Harvey]

Was asked today to show what a typical Cam file looks like. Question I was asked today was, if I send you my cams to map, what info will I get back. Here are some images of the info we can supply. Unfortunately without the software the files cannot be read in their normal format. Any Cam supplier will be able to do this for you.



Image 2584 and 2586 are the graphs of the two lobes with the Valve and cam lifts shown. I can move the lobe centers quickly and get the lift numbers at those centerlines. 2585 shows the actual lift numbers which change as I move the curser along the graph. These may come out grainy as they are screen photos.

As I said in the earlier post, the distance the valves travel from the seat to the piston never changes at each crank angle other than what I suggested earlier. So once you have the numbers off the engine, you can change the cam or move the centerlines and establish the clearance you have. Best of all, if you need to cut the pockets you know exactly what needs to be cut and how much and do it one time. No back and forth, no assembling the engine to check.

[Neil Harvey]

More typical info. We typically give this for every lobe so the customer has an idea that the cams and grinding was done correctly. Be surprised how much difference we have seen in the past.

[stownsen914]

I’ve seen several degrees difference between nominal and measured cam specs. Possibly measurement error on my part, of course.

[85RedCarrera]

Originally Posted by Neil Harvey

Best of all, if you need to cut the pockets you know exactly what needs to be cut and how much and do it one time. No back and forth, no assembling the engine to check.

That is an awesome tool to have…hours (days?) of labor saved…

Thanks for sharing that!

[85RedCarrera]

Originally Posted by Neil Harvey

If I can add something here that may help.

Measure the Piston deck height as has been explained. Check across the pin axis but measure both sides as there could be a difference. Use the minimum number. Adjust with base gaskets. Remember this will change the CR number. Its an easy input into your equation as the Piston dome and Chamber volumes do not change.

This is how I suggest you measure P/V clearances. Saves time up front and if you ever have to change the cams or timing you have all info you need. What you do need to do is to ask your Cam supplier the valve lifts at the same cranks angle's I suggest below. If they do not offer this service, we do for a small fee. . We need just one cam to measure.

Fit a degree wheel and TDC the wheel. Fit 1 head with the valves loose. No springs. You can fit soft checking springs. If you leave loose, make sure you hold one valve up against its seat when measuring the other valve drops. A rubber band works well.

Position the crankshaft 20° BTDC and with a dial indicator touching the tip of the valve stem, hold the valve up against its seat and measure the drop distance until it touches the Piston. Repeat this every 5° or 10° to TDC and ATDC to 20°. Do the exact same thing with the other valve.

These drop measurements are the maximum the valve will move to touch the piston. This dimension will never change unless you add or remove base gaskets, cut the seat deeper or machine the Head thinner.

Now the cam supplier will supply you the valve lifts at these same crank positions after you tell them the cam lobes centers or lift @ TDC you wish to use. Then you subtract the valve lift numbers they give you from the numbers you measured. This gives you the clearance.

Why is this a good way to do this? You could have interference, but its only on paper. Then with the amount of interference plus the clearance you want, you can add base gaskets, or cut the pockets in the valve deeper. You do not have to assemble the engine and time the cams often multiple times, either.

Also, if you ever change the cams, or the timing, all you need are the numbers from the cam supplier at the new position, to see if you can move or what changes you have to do to the Pistons.

Hope this helps and understood.

Thanks for sharing this process. I am going to contact my cam guy. As long as you have the data to work with, this is a very logical way to go about it…

[85RedCarrera]

[Edit] Measuring my deck height & piston-to-head clearances:

I placed solder on both sides of the piston above the pin (several times), torqued the head down, then brought the piston up. These 10.3:1 Max Moritz style pistons make it difficult to measure deck height on both sides of the piston, as the side with the steep dome slope terminates so close to the piston edge that it doesn’t allow enough room for solder. I was able to get a good, clean crush on the other side of the piston, where the dome slope is more gradual where it meets the piston edge… I can also get a caliper slide to seat cleanly on the edge of that side of the piston. Caliper says 0.43mm. Not enough deck height.

Piston-head clearance was smallest where the steep slope of the dome meets the piston edge, right at the head chamfer. I guess this is my quench area…I will measure all of this again after a 1mm base gasket goes in.

This was all done with a .25mm base gasket, so a 1mm gasket should get me close to 1.17- 1.2mm deck height.

Waiting on new gaskets.