The picture below shows a Tom C 105mm piston on top and a Mahle 104.5mm piston on the bottom. I was trying to measure the piston to cylinder wall clearance for the Tom C pistons and discovered that the pistons are not round, as shown below. I thought that was a defect, so I measured a Mahle piston (also new) and found that it was also not round. Both are wider on the diameter parallel to the piston pin, and narrower across the pin.

1. I assume this is done on purpose? Why?

2. Assuming this is by design and normal, which diameter do I use to measure piston to wall clearances?

3. My bore measures 4.157 inches (105.588mm). So, if I use the 4.155 piston measurement below, I get a difference of .002 inches. Splitting that difference between both sides of the piston, I think that gives me a .001 inch clearance (as specified by Tom C). Right?

 

1 Different expansion rates that even up when at running temperature
2 The faces perpendicular to the pin ie the 4.155 and4.135
3 Need to check manufacturer's clearance specifications as they may be different.

 

The area around the piston pin has more material, thus it expands more when heated. When the engine is warm, the piston will become round. As Dan said, you take the measurement perpedicular to the piston pin (largest one).

Not sure about the clearance numbers, I had a machine shop do that for me. Hopefully somebody will chime in.

 

Well the factory manual and tech spec book confirm to check the clearance perpendicular to the pin. (The tech sheet that came with the pistons just say to measure at the bottom.) The piston supplier listed .001 (.025mm) clearance, so it looks ok. The cylinders were bored for these pistons, so they should be right, but better safe than sorry I figure.

 

"Splitting that difference between both sides of the piston, I think that gives me a .001 inch clearance (as specified by Tom C). Right?
"
no, you dont split the clearance- the clearance is .002.

How was the bore measurement arrived at, Tom?

 

Thanks. I've been reading up, and you're right for sure re the clearance. I used a telescoping inside-bore measuring thing (spring loaded T-shaped measuring rod) and then used a big micrometer to measure the measuring rod. I measured the bore near the top. I measured the pistons directly with a big micrometer, near the bottom of the piston (as specified with the tech sheet the pistons came with). I've had these (cheap) measuring tools for a while, but my confidence level is low -- particularly with the bore measurements (too easy to measure unsquare). Tips/reference material welcome. I've been reading up, and also realize I should have torqued on the lower crank girdle and measured lower in the bore.

 

Can you point to the reference that says you don't split the clearance dif? I don't see in the Porsche manual, perhaps i missed it.

Tom you are correct about measuring the bores, the T-bar is going to cut it. I had to use a Cylinder Bore gage.. you can pick one up at Enco. As you measure 0.0005" resolution you notice changes throughout the depth of the bore.

 

I will confirm with Tom C tomorrow, since the spec I am shooting for came from him. However, I've looked at a few generic engine building resources, and they do not divide by 2. Also, if you look at the tech spec book for the 951, it says the piston wall clearance should be .008 to .032. Then if you look at the tolerance group measurements for pistons and bores, you will notice that the difference between the piston and bore diameters ranges from .008 to .032 within each tolerance group -- which seems to confirm that that is how the clearance is measured. I knew that, but was having a brain freeze at 2am trying to get to the right answer. I'll get a better bore gauge and torque on the girdle and try again.

I am still curious how they measure pistons for advertising purposes. I note that neither my Mahle 104.5mm pistons or the Tom C 105mm pistons actually measure out to those diameters, assuming I calibrated by mic properly.

 

Tom

Are you sure you are not misreading those tollerences? I remember them as .0008" - .0032". I may be mistaken, but I dont have any 944 manuals at my work, oddly enough. .008" is sloppy for sure, even by Detriot standards. Even budget forged piston (lots of expansion) companies spec .005" as their "high" in terms of piston/cyl. tollerence for blown applications.

As far as buying a gauge, one option is to drop off the case @ a good machinist (emphisis on "good") and have them handle that step for < $100. Not a bad idea to have machine work verified by a third party. They have the expensive measurement devices (bore gauges that have .0001" accuracy), know how to get accurate readings from them and real-world advice on things like piston clearance and how to arrive at accurate measurement information. They might not have specific knowledge of Porsche things, but I'd trust them before I trusted advice I got on this board. I think you may find the .001" can be easily accounted for in measurement innacuracy, at least a portion of it.

In regards to "where do I have specific info. . . not splitting the dimension", I hold this truth to be self-evident, but you are a good man for testing the source.

I just asked The Racers Groups' cheif engineer Mike Fox and he concurred - the difference between the piston and the hole IS the tolerence. He is arguably the best GT3 mechanic in the country and pretty trustworthy as a technical source, IMHO.

 

I was quoting the metric numbers from the tech spec book .008mm to .032mm. Sorry to mix measurements on you.

My machine shop bored for these pistons, and told me they had a .001" or .025mm clearance. I was just trying to confirm that, as is my way. If I can't get confidence on the numbers myself, I just might get them confirmed profesionally. I have no specific reason to doubt my shop, which has a great reputation, but my confidence in them was shaken a bit by their antics. I find rennlist comments helpful usually, but never use them as a substitute for my own homework.

I wasn't the one questioning that. I agree. Once you said it, I thumped the heal of my hand on my forehead. It was late, and I was in the habit of dividing by two after spending time calculating the reduction in cylinder wall thickness for different bores.

 

Thanks for the clarification on clearance.

Tom, I went through a similiar exercise to accurately meaure bores and pistons.. drove me nuts. On the Mahle, at least, the shape profile is round on top and ovalizes as you travel down to the bottom of the piston. The round part was like 0.51mm smaller than the bottom (where you're suppose to measure). The bore was also not perfectly round (larger dia parallel to the pin and smalled dia perpendicular to the pin).. opposite of the piston. When looking for a clearance between 0.0003" to 0.0012" (0.008mm to 0.032mm), it felt like splitting atoms.. my readings jumped around.. then you factor in the Piston the coating which isn't perfectly even at 1 ten-thousands of an inch.

 

Just an update for those on the edge of your seats. I got a dial bore gauge from Summit, and re-calibrated my micrometer. Also bolted the girlde on and measured on the pistons and the bore where specified in the manual. The result is 4.132 bore on all cylinders (round) and 4.131 on the piston skirts (measured at the bottom across the pin) -- my earlier measurements were off becasue (a) the snap gauges are nearly impossible to center in a bore like a dial bore gauge, and (b) the anvil in my mic was not seated all the way. So, I'm content now that my clearances are set as specified by the piston supplier at .001" or .025mm. Stay tuned for bearing clearances.