Piston to head clearance (again)

[David]

I've got a 79 930 with 98mm Mahle pistons with 0.5mm base gaskets. I checked the piston to head clearance with 0.050" solder. The very end of the solder (about 0.040") on both sides of the piston was crushed to about 0.035". Should I (A) not worry about it, (B) cut 0.010-0.020" off the top corner of the piston, (C) cut 0.010-0.020" off the squish area of the head, or (D) space up the cylinder more. I would prefer not to raise the cylinder so I don't lower the compression any more.

[Eagledriver]

According to Bruce Andersons 911 performance hand book the minimum piston the cylinder head clearance is .035 inches. You are at the minimum but if the minimum wasn't good enough it wouldn't be the minimum.

-Andy

[David]

I just got off the phone with EBS (who did my machine work) and they said 0.035" was OK. I will be checking all six pistons instead of just #2 and #5.

[Grady Clay]

David,

0.035” is just a dimension. A critical issue is where it is measured. Tomorrow, I’ll make some diagrams and analysis of why the dimensions are important and where they are measured. Bruce is absolutely correct but there is more to the story than he could put in his book.

Do some research on Pelican using "deck clearance" and "piston clearance" as key words. Use my name.

Best,
Grady

[David]

Grady, I read some of your posts and I still have some concerns with clearance. Here's what I got.

I was using pieces of solder and I was concerned they were moving so I used a single piece 98mm long across the full width of the piston. I got 0.025" on one side and 0.032" on the other in the squish area. I've never run tighter than 0.026" on my YZ125 so this has me concerned. My thought as this point is to machine 0.005" or 0.010" at a 5 degree angle off the top corner of piston. Thoughts?

[Grady Clay]

David,

First a couple of questions:
What kind of cylinders?
What is the piston-to-cylinder clearance?
Do you have all the rings on the pistons?
What is the wrist pin bearing clearance?
Did you torque the head in place?

I find it necessary to have solder at four places N-E-S-W and sometimes eight.
I use varying sizes of hollow (rosin core) solder so they are just barely squashed. Scotch tape works well to hold solder in place, just not where squashed.

You might find it necessary to torque three heads and cam housing in place even when only measuring only one piston

In the first round of measurements I use modeling clay. I take an X-acto knife and section it and measure with vernier calipers. Not as accurate but gives a good overall picture of the squish and deck areas. This also equally supports the piston around the perimeter. This is also good for valve-to-piston measurements.

I agree 0.025” is too close if that is an accurate measurement. The significant difference between the 930 and the YZ is a roller bearing crank and rod bearings. We have four CR80s that have around 0.012” squish and come “close” at 13,500 RPM.
I have three 91x66 mm race engines that I use clearances in the 0.025” range but they are disassembled for new bearings, rings, etc. every 25 hours. I also “crept-up” on these close dimensions during repeated rebuilds.


In addition to the clearance issue there is the shape of the squish area. The clearance should be least at the perimeter of the piston-head and gain clearance toward the center. Never have the clearance tighter as you go away from the perimeter. That can cause detonation above the ring land and disaster follows.
Notice how the dome of the YZ piston (I’m assuming it is similar to a CR) follows the contour of the combustion chamber for equal clearance and then angles away. This is much more difficult on a 4-stroke but your turbo has more latitude being lower CR.

To answer your question, I would choose to cut the head where the flat “deck” transitions into the combustion chamber. If you can follow the contour of the piston, so much the better. Remember the clearances should be opening up slightly. I would not cut the piston.
I think the company that makes the CR race cylinder base gaskets in 0.001” increments also makes custom 911 base gaskets. I don’t remember their name.
You must have all three cylinders on one bank the same height.


When you are rebuilding a running stock 911 (or 930) engine with stock OE parts, it is almost a “no brainer.” As soon as you start stirring the parts pot and machining pieces, the care and measurement that is necessary goes up exponentially (X^3, X^4?)


Best,
Grady

[David]

Grady, thanks for the additional info. I'm using 98mm Mahle pistons and cylinders with 0.002" clearance. I did not have rings on the pistons when I was checking the clearance, but I did torque the heads.

I can see now that machining the pistons would not be a good idea since I could end up with a wider opening at the edge of the piston.

Since the heads were machined 0.008", I'm planning to stack a 0.5mm (the previous size) and a 0.25mm base gasket and recheck the piston to head clearance. I was really trying to avoid stacking gaskets.

[Grady Clay]

David,

Installing the rings will help square-up the piston in the cylinder. Daubs of putty between the solder will help also. If you use your new rings, be sure to grease everything for testing.

I will speculate you will get dramatically different clearance numbers with the rings installed and eight measuring pieces.

In your first image, is that tool chatter on the angle cut? If so, I recommend getting it smooth and slightly radius the corners. How well does the head transition from flat to angle and fit the piston? Exactly where is the closest clearance? This is a critical issue.
While you can run the flats of the piston and head somewhat close, it is the clearance at the angle cut on the head that needs particular attention. The clearance measurement at the angle is perpendicular to the surfaces, not the axis of the cylinder. That clearance should be slightly more than the flat “deck” clearance around the perimeter of the piston.

You can see how robust the piston is above the top compression ring. This is an indication of how concerned the engineers are about collapsing the piston and grabbing the ring.

I don’t have a problem with multiple copper cylinder base gaskets. I’ll try and find the custom gasket supplier.

When you think everything is done, I would cc the combustion chambers to verify all are equal and as expected. The twin plugs will help a lot with many combustion issues. I assume you know you don’t need to have as much spark advance as with single plugs. Make sure the sparkplug pockets are not-too-smooth transition and all are equal. Indexing the sparkplugs may help depending on type.

I would recommend some data acquisition; particularly boost, head temperature, and knock sensing. This will let you get the most from your 930 without damage. If you track your car in hot weather, I recommend the “Rubbermaid Solution” times two.


Best,
Grady

[David]

There's not an angle cut on the head, it's just a radius from the sealing surface to the combustion chamber. I suspect that the radius started at the same OD as the piston before the machine shop faced the head 0.008". They did not re-radius or angle cut for a squish, so there is about 0.020" of flat area on the head before the radius starts. Do you think I should go back and machine a slight angle or radius there?

I was not planning to have a knock sensor, head temp (unless I use it for my coolant temp), or boost data (except for the gauge on the dash). I was planning to have a wide band lamda readout in the car and outputing to the ECU for logging.

I'm somewhat aware of the different requirements for twin spark timing. My plan is to be very conservative through the tuning process.

I haven't CC'd the heads yet, but I plan to.

Thanks!

[Carrera3.5L]

Originally posted by Grady Clay
[B]I don’t have a problem with multiple copper cylinder base gaskets. I’ll try and find the custom gasket supplier.

/B]

For specialized gaskets, shims, etc, try Charlie Spira at Wrightwood Racing in Oxnard, CA. He probably has what you are looking for as this is his specialty. Phone number is (805) 385-7191.

Ralph

[snowman]

On my race engine, I have runnig clearences as close as 0.005". Thats Close, Very very very close. And thats at the real mininum clearence, determined by many to many measurements. But that what makes it a RACE engine, no bucks spared. On a street engine, I would only accept anything above 0.035" min. For many reasons. First is the compression is just to high for ANY street engine, next a street engine has to have slack, slack for whatever you may run across on the street, ie bad gas, dirt, whatever. A street engine has to be some kind of echonimacal (sp) meaning bucks are a factor. In my race engine, if you were to install the specified spark plugs, you will have holes in the pistons, just cranking it over.

As to a typical saturday weekend "race" engine I say stay with the 0.035" number, its orders of magnitude and I mean orders of magnitude cheeper than something less.