TAChad

I have my engine all tore apart, and im trying to figure out why my car smoked pretty bad and burnt oil. I was hoping to find something wrong with the rings or something, but everything looks great. the cyllinder walls look brand new besides a couple of minor scuffs, and the rings look great as well. i had the block bored .030" for the stroker, and i measured the cyllinder diameter and it was right at 4.030", so i measured piston (lunati) diameter at the piston skirt and got 4.020". Is there a certain way or area on the pistion to measure to get a correct reading? The piston skirt shows a little wear scuff perpendicular to the wrist pin, but it doesnt "look" bad. could this be my problem or am i measuring incorrectly? any other suggestions on the oil consumption problem? every exhaust port had quite a lot of black carbon build-up/residue from the burning oil, so it seems like it was a problem with each of the cyllinders. the rings' (total seal moly plasma) gap was facing opposite of eachother also, so i dont think that was the problem. thanks in advance for the help

Chad

jonaddis84

I dont like the sounds of this already

Sure you got the largest part of the piston? If you cant get a caliper to the largest part of the diameter, which not many arms on the calipers are long enough..I would somehow make it into a square so you could measure easier. The piston obviously wont be 4.03 or else it wouldnt fit right, it will probably be around 4.027 or 4.028 most of the time I believe, so make sure you are reading the caliper perfect, a digital would help if thats not what you are using.

TAChad

thanks for the info, my calipers may not have been large enough, ill try some larger ones at work tomorrow. Where is the best location on the piston to take a measurement? mid-way between the wrist pin and the botom of the skirt? when you say the largest part of the piston, what do you mean by that? is the pistom somewhat tapered from the top, down, or straight throughout?

Chad

jonaddis84

I would say the very top ring land would be the best place to measure. And when I say the largest part, I mean since it is a circle, duh, if you were to find the exact center and draw a perfectly straight line directly through that center, that would be the largest diameter of the circle (the actual diameter). Hard to explain what I mean, but basically if the arms of the caliper are not directly across from each other travelling through that center point, you are not measuring the actual diamter of the circle.

rskrause

Different pistons are supposed to be measured at different points to determine the correct bore (piston to wall) clearance. For example, I usually use JE pistons. They come with a spec sheet that specifies the correct measuring point. So do Ross, and other quality pistons. You can call the manufacturers tech line of you don't have this information.

Rich Krause

OldSStroker

Yep, what Rich said.

Most pistons are measured on the skirt perpendicular to the pin. Often pistons have oval skirts to allow them to expand along the pin axis. Try measuring in the area of the scuff marks.

You need to be able to measure closer than a caliper will. Use a 4-5 inch micrometer that reads in "tenths" (.0001) for the piston. For the bore the best thing is a dial bore gage. Set it with the same mics you used on the piston. If you have to use expanding gages (shaped like a "T"), take lots of measurements if you don't use them often.

Lots of burnt oil? You don't have a ring or two upside down do you? It can happen.

TAChad

Well I called lunati and they said to measure .600" down teh skirt, at a 90 deg angle to the wrist pin, and i used a large digital caliper that read to the .0001" and got 4.011" on one of the pistons. I dotn know what happened, but i guess they are shot. cyllinder walls look brand new, but i know looks can be deceiving so ill have it all checked anyway by a machine shop. I think i only had about 18,000 miles on this setup, and i didnt drive it THAT hard. I was hoping I had some rings upside down, or broke, or lined up wrong, but i didnt, and it seems that the oily residue and carbon build-up is constant on all of the plugs, piston tops, and valves, so i think that the problem occurred in all cyllinders. think i should go with JE pistons or stick with some new lunati's? Ive heard some great things about JE, any bad about lunati?

jonaddis84

Something doesnt seem right. I would take them to an engine shop and have them looked at before you go buying a new set of pistons, I could see if they measured like 4.023 or something, but somethins fishy, unless you were given the wrong pistons in the first place and the assembler didnt notice the nice gap.

Couldve gotten a set of .020 pistons on accident

TAChad

I was thinking the same....youd think if they were that far off they would be noticeable beaten up, but i can still see the machining marks and the detonation grooves that were machined in them(i think thats what they are called) i was hoping that i just got some .020 pistons, but lunati ran the numbers stamped and said that they were in fact .030 pistons. I may still be measuring wrong, but hell, i checked and checked. Ill jsut take them to the machine shop, cuz ill be going anyway for the block. any recs. on piston types? thanks

Chad

Mad Machinist

Can't rely on a calipers to measure accurately within .001 at best. I've never seen one read in tenths (.0001) nor would I trust it if it did.
When you say you used a "large" digital caliper, what is "large"? The reason I ask is because a standard 6" or 8" (range) caliper does not have adequate throat depth to measure a 4" piston. You would never get a measurement at the centerline of the piston and your measurement would always be on the small side.

As OldSStroker said, you need a 4-5 micrometer to do this job correctly. Now, before you run off to buy one. Your best bet is to take the piston to an engine rebuilder or machinist and have him or her check the piston. Also let them know what Lunati told ya cause that'll save them some legwork.

From there you can investigate things further.

Good day to ya.

rskrause

Something seems srong with your measurements. I agree with the suggestion to take the pistons to a good machine shop.

Rich Krause

TAChad

Thanks for the replies. when i wsa talking of a large caliper, i meant with a large throat...i made sure the caliper ends extended well past the centerline of the piston. these calipers definately measure to the .0001...i agree it should be measured with a micrometer, but even if the digital caliper was off a few thousandths, something still would be out of spec with the pistons beacuse say the piston is supposed to be 4.027" and im getting a reading of around 4.011 give or take a few thousanths if the calipers are not quite accurate, that would leave me with about .015" of piston slop, right?

Chad

Mtrhds94Z

Try a feeler guage with the piston in the bore[at the 'skirt contact point' partway down the bore..
That will give you a better idea of the clearance than what you are doing now I think...

TAChad

ok I'll try that, but with a gap that large, i dotn know if it will work. I remember when i was taking the pistons out of the bore, i could actually see how teh piston wsa laying on the one side of the bore, adn on the top of the piston (between the piston and the cyl. wall) I could see how much of a gap there was (this was looking at the top of the piston) i didnt measure it but it looked around 1/8 to 3/32 ofa gap....it just seems like the piston is too small, all around but lunati said they were .030 pistons. i dunno, somethins not right

jonaddis84

Sounds like you might be getting Lunata to pay for your troubles, sounds, right now at least, that they sent you a set of .010 or .020 pistons packaged and stamped in .030 badging