Articles, Blog

Measuring cylinder bore, taper, and out-of-round

September 13, 2019

We’re going to demonstrate how to measure a cylinder of an engine. And, when we measure it, we’re measuring it for three things: for its bore size, for its taper (so, whether it’s wider at the bottom or the top) and for its out of round (whether it was perfectly round or whether it’s oval-shaped. And, we’re going to do that with a dial bore gauge. Of course, a dial bore gauge can’t measure anything by itself. And so, we’re going to use a micrometer just to set this at zero. So, we found a specification, right here, the standard diameter for the cylinder bore is 3.7795 inches. It gives us a range, but we’re going to set it at the lower number. So, I’m going to set the micrometer at 3.7795. We’ve got a set all ready and locked. Then, we’ll take this to a vise. I put a rag in the vise and I set this inside of here, just lightly, and I, and then I, very lightly, tighten that down. Don’t tighten it very much or you’ll distort this. Then, we find the appropriate attachments for this dial bore gauge. We’ve already done that. And, we put it in here, in between the micrometer. Then, I’m going to rock this back and forth. And, when we see that, we see that dial, that needle change directions. And, this is probably the trickiest part of this whole operation is getting this set to zero. It’s close already. I’m holding the back and rocking it. You’ll notice that needle’s turning around. Let’s see, where is it at? Really close to zero. It’s, turn it just a little bit more. Okay, I think we’re there. The needle’s turning around right at zero. I’ll carefully pull it out and we’ll come over here to the engine block. We’ll start with cylinder number one. And, what I’m going to do is make myself a chart. So, we’re going to measure each cylinder in six different places: three at the top and three at the bottom. So, I’ll make a chart like this– pen, work! So, under number one: top, bottom. And then, I’m going to say, “Position One, Position Two, Position Three.” Now, I’m going to take my measurements. So, I’m always going to use Position One, as it’s perpendicular to the engine block– this is the direction that the piston would rock back and forth. And so, this is where the most wear would occur if there is wear in the cylinder. So, I’ll call this Position One. And, I’ll go down, just below the ring ridge here, so, just down about a half an inch or 3/4 an inch into the cylinder. I’m going to rock this and watch that needle go around and around. And, it stops right there. And, on this dial, each little tick mark is a ten thousandth of an inch and then the large numbers represent a thousandth. So, this is actually just slightly more than two thousandths of an inch larger than the specification. that we set the micrometer to. In fact, to be specific, it’s 2… 2.4 thousands of an inch. So, I can write, at Position One: 0.0024. Now, Position Two. I’m going to do here and Position Three like this. So, we’re just slicing the cylinder into equal slices here. So, Position Two. There, it’s about exactly a thousandth of an inch. And, Position Three will be like this. And, right there, it changes directions. It looks like it’s right on the one thousandth of an inch mark again. So, what these represent is how far, how much larger than the specification the cylinder is here. So, I’ll do the same thing at the bottom of the cylinder. Position One at the bottom: looks like it’s about 18 ten thousandths or 1.8 thousandths. Try it one more time. Or, 19, depending on where I move it to, there’s a little bit of variation. So, we’ll say 18. Okay, now once we have these numbers, here’s where we can calculate our taper and out of round. So, if I look at Position One, top and the bottom, I’m going to come over here and see what was the difference between those two numbers. And, it’s .0006 right? Six ten thousandth of an inch. I’ll do that all the way through here. In this case, there was .0001 and here is .0008. The largest of these three numbers is this one. So, I’m going to call that my taper. The most taper that I found here was eight ten thousands of an inch difference between the top of the bottom of the cylinder. Now, to find my out of round, I’m going to compare these to each other. What’s the difference between the largest and the smallest number? It looks like it’s .0014. And here? .0007. It’s the difference between the largest and smallest measurements we took. The larger of these two numbers, obviously, is this one, so that’s my out of round. Now, you’ll say, “What,what was your exact bore measurement?” So, I’ll come back to my specification here. 3.7795 is my specification. And, I find the largest number that I measured, which was this one. And, I add .0024 to that. So my bore measurement–the largest, the largest measurement in that bore was 3.7819 inches. And now, I’ve taken all of the measurements. And I, I can compare those against my specifications and find out if I need to do anything to my cylinder: if I need to have it bored out or it’s okay to use, as long as everything falls within that specification. And, I’ve also got specifications for out of round and for taper. As long as these numbers fall within those specifications, I can reuse the cylinders like they are: just hone them and reuse them. Otherwise, they have to be bored out. So, so one thing to be aware of is as you take this measurement, if it’s to the right of the zero, It’s actually smaller than the specification. So, in this case, it’s to the right of the zero, it’s smaller than the specification that I set the micrometer to. That doesn’t happen often. Cylinders don’t get smaller, But, for some reason, either our specification is wrong or the cylinder is distorted, that this is showing us It’s three thousandths smaller. If, on the other hand, it stopped over here on the left of the zero, somewhere on this side, that tells us that it’s larger than the specification. That’s most typical.


  • Reply DaffyJeffy July 11, 2015 at 9:29 pm

    Can't thank you enough for this.  I'm just learning so I practiced on a 2-cylinder 1959 Johnson 5.5hp outboard motor.  Tiny little cylinders!  🙂

  • Reply Denny Mcfastlane September 17, 2015 at 10:45 pm

    Great information Justin! Here's a question for your expertise. I have an old motorcycle Cylinder that was obviously dropped(not me). At the bottom base, but not the base itself, the cylinder bore is dented into the chamber. Is this at all possible to repair? The jug is from a 1919 Harley Davidson, so it would be a shame to offer it to the 'Gods of Speed"! Thank You. *P.s. The dent area is around the size of penny and in about 1/8'' at the crest. C'ya.

  • Reply Naseem Sheikh November 28, 2015 at 7:58 pm

    thank you so much ive been looking for this calculation thing thank you 😃

  • Reply purple guy. January 10, 2016 at 5:17 pm

    thanks dude helpful

  • Reply Mickey Flowers March 5, 2016 at 11:36 pm

    Where can we get a similar printout for the cylinder bore specifications of our engines?

  • Reply T DeAngelis March 23, 2016 at 9:54 pm

    nice explanation….thank you

  • Reply Mohamed Usama May 13, 2016 at 4:09 pm

    my inquiry when we make oversize for piston what's the right machining for bore with out of round or should make it with refrance specific value

  • Reply Carel Meester August 12, 2016 at 12:53 pm

    Random question. What pen are you using?

  • Reply Antegas Hindoyan September 11, 2016 at 1:38 am

    Like!!!1 Justin thank very much I enjoyed explanation a lot

  • Reply Vlad T. October 19, 2016 at 6:26 pm

    very informative thanks

  • Reply aziz sarker December 13, 2016 at 6:44 pm

    what will show the indicator,when i measuring a stander linear?

  • Reply Stephen John June 26, 2017 at 7:16 pm

    How much to you rebore it by? By about 5 thousandths of an inch and then use bigger piston rings?

  • Reply heavy_ham July 31, 2017 at 2:43 am

    Very informative thank you for this!

  • Reply Anthony Wendt August 25, 2017 at 7:29 am

    This is good and straight to the point. Thanks.

  • Reply albert ngowi December 14, 2017 at 11:37 am

    Yes we learn about to use bore gauge so how about inside screw gauge?

  • Reply Mark January 8, 2018 at 1:09 am

    how come you didn't do the middle of the cylinder like i see most people doing ?

  • Reply Mark Lowe February 28, 2018 at 11:11 pm

    Super good video. I'm not a pro engine builder but all that info made sense. Very well explained. Thanks.
    Must admit freaked a little when you walked towards the vice with a micrometer…….

  • Reply e.g. john April 18, 2018 at 2:51 pm

    So on any dial bore guage like this is the numbers left Of the zero bigger and to the right is smaller ? Also my guage has 0-25 to the left and 0-25 increments to the right

  • Reply samarth balwanti April 22, 2018 at 7:47 am

    hey such a wonderful video this helps me in exams.

  • Reply Angel Curiel September 12, 2018 at 6:05 am

    Dont you use a piston to get size of bore

  • Reply Ashroyer86 December 1, 2018 at 12:47 pm

    You sound like a math teacher.

  • Reply boostedsil40 December 20, 2018 at 5:47 pm

    can I use venier calipers to read these for engine bore?Or is a micrometer a must?I need to check piston to wall clearence so if I buy telescoping guages and a micrometer "Those C clamp looking ones" this should work good enough right because I cant afford those bore guages and found T guages for $12 and micrometer for under $20,If I can use venier calipers to measure the T then I wont need the mic,Please respond anyone and thanks?

  • Reply scooter13 December 23, 2018 at 12:35 pm

    thanks great video learned a lot have you any videos on the electronic bore gauge I have one but really don't know how to use it right, There's no real videos that show how to really set it up easly

  • Reply Jayee December 26, 2018 at 10:14 pm

    I don't get the zeroing out step using the micrometer. You're zeroing it on the center rear of the cylinder gauge body and the rollers on the side of the tool looks like it extends out further than that. Wouldn't this throw off your zero?

  • Reply Through the Apex February 8, 2019 at 9:50 pm


  • Reply Vijay Vjn April 20, 2019 at 9:16 am

    thank you for your time in making this video have a good day

  • Reply Jonathan-Emily Powell June 30, 2019 at 1:57 am


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