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Right way to measure Piston to Wall clearance?

Posted by bluegs03, Dec 5, 2008
Newbie Thread Graveyard - Repetitive questions already answered in the FAQ Forum, Tech Articles, other New Member required reading, VFAQ.com, etc. This is a read-only forum. If your thread ends up here, you didn't do the required reading.

  1. bluegs03

    bluegs03 Proven Member

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    Hey guys ive been taught to do piston to wall clearances before but i wanna clarify something.

    I have .020 wiseco pistons, and i am sticking the feeler gauge into the cylinder 90 degrees from the wrist pin.

    My question is this: Do you take your measurement at the top of the piston or do you try and stick your feeler gauge down and reach the skirt? Basically if you can get a feeler gauge in between the top of the piston and the wall but only a few centimeters further is this the piston to wall clearance? Or is it which feeler gauge can fit all the way down the piston?

    Here's another way of putting it in pictures!!!

    Top of piston
    _________
    ------------| P to W clearance by which gauge you can stick in here
    ------------|
    ------------|
    ------------| or Here?
    Middle of piston

    The reason i ask is because i can fit a .010 feeler gauge at the top of the piston but it only goes a few centimeters down before it cant go farther, If i use the middle of the piston i get a measurement of .004-.003ish Which is BARELY in spec but i wouldn't have to have the block honed out again.

    Thank you all!!!
     
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  2. treebonker

    treebonker Proven Member

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    Wiseco are measured 1.300" from bottom of oil ring groove, 90 degrees from wrist pin hole. A feeler gauge is about worthless though. Use a micrometer/bore gauge!!
     
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  3. bluegs03

    bluegs03 Proven Member

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    Yes but i dont have access to a micrometer and a bore gauge. I was taught that you just stick a feeler gauge inbetween the two and the largest one that fits is the size. Is the micrometer / bore gauge the only way to determine p to w clearance?
     
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  4. assault187

    assault187 Proven Member

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    Stick a feeler gauge strip between the piston and the cylinder wall, You want to use the largest you can slide in there tightly, then divide that value by 2 and you'll have your PTW clearance




    I.E. If your value is .008 divide by 2 for a .004 PTW clearance. Not the best way, but it works
     
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  5. absolute_DSM

    absolute_DSM Proven Member

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    Mic and bore gauge.

    How can you stick a straight object between 2 round objects?
     
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  6. bluegs03

    bluegs03 Proven Member

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    wow ok thank so much guys i had completely forgotten about the dividing by 2 thing! Just to be sure i will measure the width of the piston and the cylinder with a outside/ inside diameter micrometer. I will most likely have to take it too a shop.

    Thank you again!
     
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  7. BoredDSM

    BoredDSM Proven Member

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    Using a feeler gauge is absolutely the worst way to measure ptw clearance. The reason you need a micrometer and bore gauge is because the piston is different in size at different points of the piston like treebonker stated where to measure for the wiseco's. Every piston manufacturer gives you there ptw clearance to where they measure the size of a piston. If you cannot get a bore gauge and micrometer have a shop do it. It would not make sense to let a block and piston possibly get destroyed when you can pay a shop 50 bucks to check it for you, most of the time its even free when they bore and hone the block for you.
     
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  8. tkelly27

    tkelly27 Proven Member

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    +1 for the mic and bore gauge. A guy at a shop I go to told me a trick, you can use a cheap bore gauge with a nice dial indicator and calibrate it with a good mic and still get good readings. Didn't you have your pistons at the shop while they were boring/honing it?
     
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  9. assault187

    assault187 Proven Member

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    While were on the subject, Who knows where to get a good quality machinest mic set? I cant find a decent set to replace the set I have now...
     
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  10. SlowSpyder

    SlowSpyder Proven Member

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    starret
     
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  11. johnnytsi

    johnnytsi Proven Member

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    These guys have any engine rebuild tool you could possibly need:

    Goodson Tools and Supplies for Engine Builders

    John-
     
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  12. assault187

    assault187 Proven Member

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    Great, Thanks. The starlett site you have to call for all pricing :notgood:
     
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  13. bluegs03

    bluegs03 Proven Member

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    Ok guys i got a shop to do the measurements.

    Pistons are 3.359 (or 3.36) it was close
    The walls were 3.45-46 varied a slight amount per cylinder

    So this gives me a P to W clearance of .005-.006 non divided by 2. Im liking the .003 p to w clearance better obviously.

    Are you guys 100% sure you divide by 2 because the machine shop was telling me that these are too loose, then they preformed the slip fit test where they use the long feeler gauge and it passed that with .003-.004 clearance. Basically they hadnt ever heard of dividing by 2.

    What would you guys do? Put it together and run it? or start over again
     
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  14. delta448

    delta448 DSM Wiseman

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    Don't divide by 2. It's supposed to be a clearance value, it's not dependant on whether the piston is sitting dead center in the bore, because it obviously won't when the engine is running. The best way to measure PTW is with a single correct range outside mike and dial bore gauge as treebonker stated. The ribbon feeler gauge method is less accurate and you should completely forget about using a set of standard non-ribbon feelers.
     
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  15. bluegs03

    bluegs03 Proven Member

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    Ok so according to the non divided by 2 measurement there is way to much P to W clearance for me to even think about running this motor, thank you for comfirming my worst fears im gonna go yell at my machinest now.
     
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  16. BoredDSM

    BoredDSM Proven Member

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    You can still run the motor just get an overbore and run over sized pistons. Have it bored to 85.5mm and use 0.020 over sized pistons.
     
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  17. delta448

    delta448 DSM Wiseman

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    I still don't approve of the method by which you got those measurements, but which Wiseco pistons are you using? Standard or heavy duty? Different alloys need different clearances. If you're using the higher silicon alloy (4032) pistons you need much less than if you're using the heavier duty low silicon alloy(2618).





    1000th post...
     
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  18. bluegs03

    bluegs03 Proven Member

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    Read post 13, after recieving comments about how using a feeler gauge is wrong i correctly measured the piston to wall clearance with a bore gauge and a micometer. According to that i would have .008-.006 of a P to wall clearance which is far too much. Plus looking at wiseco's site which you linked they said that, a big block (which mine isnt) would need a maximum of a .005 BEFORE adding in the addional .001 to the measure i made for when the coating wears off. There is no way i can knurl these pistons to make them fit (i refuse to do that) So there is really no excuse as too why in the world my machinest would bore this block .028 over when i asked for a .018 over and hone the rest out to a .020 over


     
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  19. delta448

    delta448 DSM Wiseman

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    I meant no offense -nor to insult your intelligence. I honestly somehow overlooked post 13.

    I agree with you. The machinist f'-ed up. If you'll allow me to... I'll bow out now, you've obviously studied what you're attempting.
     
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  20. treebonker

    treebonker Proven Member

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    What was your wall size again? 3.45 can't be right...
     
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  21. bluegs03

    bluegs03 Proven Member

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    The problem with this is i have NO money left, i am a hard working young adult, who is paying his way through college himself. I have already called extreme psi and becuase i have broken the seal on the box they cant accept the pistons back.

    Plus if you read my original post, this block was meant to be bored .020 over but due to machinest error it was bored out to .029-.028 over instead, i think what you ment to say was bore it .040 over and use bigger pistons. The problem with that is what i said above $$

    Delta- i know you didnt and i sincerely thank you for trying to help me i do appreciate it..

    Tree- The wall diameter isnt actually 3.45 but i used the 3 inch extension and zero'd out the gauge before sticking it into the cylinder bore. The resulting measurement was .45 on the gauge on a scale of 0-100. Thats how i got the 3.45 I then measured the piston skirt 90 degrees from the wrist pin and got a measurement with a digital micrometer of 3.36
    I then subtracted the measurements to get my Piston to Wall. This is how the machine shop instructed me to do to and they watched me preform the tests and told me that there was a huge amount of piston to wall clearance. I mean if you want further proof if you insert the piston and rod onto the crankshaft and turn it to Top dead center, you can physically "jiggle" the piston inside of the cylinder, no way is that right.
     
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  22. treebonker

    treebonker Proven Member

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    You're forgetting pistons have the clearance built in, usually .004. If your pistons measure 3.359X, then even if the machinist bored it exactly 20 over you would still be left with .007 PTW clearance. That's why you need to measure the pistons prior to boring the block to account for manufacturing tolerance.

    I can't say much w/o kowing the exact numbers but if you gave the pistons along with the block to be machined, then the machinist should be shot and hanged. BTW, it's normal for the pistons to jiggle as the crown is smaller than the skirt area.
     
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  23. bluegs03

    bluegs03 Proven Member

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    ok now you really have me confused. Why would measuring the pistons before you bore the block help things? The pistons will be the same size no matter when you measured them they dont change? Im also not understanding what you mean when you say they have a clearance built in? I have never heard of such a thing and am curious. Is what your saying that the pistons can vary as much as .004 in there size from the manufacturer?

    If so why wouldnt my machinest know about this and requested the pistons before hand? Because he didnt have the pistons when he bored the block over he never asked he just bored the block .020 over.
     
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  24. BoredDSM

    BoredDSM Proven Member

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    You measure the cylinder wall before you before and then you add whatever amount onto that to get 0.020 over. You don't just take away .020 amount of material off whatever you have. You match the bore to the pistons, you do not match the pistons to the bore if you get what I'm saying.
     
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  25. bluegs03

    bluegs03 Proven Member

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    ooooh yea i see what your saying now i think tree just put it in a confusing way. My machinest has done alot of work before so if he made a elementary mistake like that i would be appalled but its sounding like thats what happened... also you have to account for the fact that the cylinder walls could of had material scrapped off in its 120000 miles of useage and know what the block size was to begin with, then you can tell what it is now and subtract the 2 to get the measurement you need to bore it .020 over. I imagine what happened is the cylinder walls had .005ish of material scrapped off of them from usuage, my machinest just stuck the bore in there and took an additional .020 over out and then honed it out another .003

    I thought you were saying that you had to bore the cylinder out depending on the size of the pistons and how much they varied from the manufacturer, which is wrong of course.

    Even so its not my resonsibility to tell the machinest to do that, i mean if every person who had machine work done knew every single detail of every step that he needed to take then they would just rent out machines and do it themselves thats why we take it too a expert machinest because they have these steps memorized and we wont mess up. Im still convinced it is the machinists fault
     
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