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Any machinists out there?

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MikeH(Tx)

05-18-1999 19:11:29




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I have my JD H torn down and am assessing whether I need a rebore. I don't have all the tools I need, so I went down to Harbour Freight today and bought a set of cheapo male/female calipers to measure inside diameter of cylinder walls. I set them in the middle, keeping as level as I could, and then used a decent pair of measurement calipers to measure the distance between points.

OK, good in theory. However, I can't measure repeatably any closer than about .020.

I measured five independent readings and averaged them. Info says I need a rebore, and that the JD H has already been bored .045 over. OK, the cheap tools have done their job and now the machine shop can take over. However I want to know the right procedure for taking these readings.

How do you measure the inside diameter of a cylinder wall?

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Christopher Tate

02-03-2001 05:36:19




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 Re: Any machinists out there? in reply to MikeH(Tx), 05-18-1999 19:11:29  
You need something that will accurately transfer the actual diameter to a good measuring instrument
One person suggested what are commonly referred to as telescopic bore gages if you do not want to purchase these you can make your own with some items from the hardware store. If you will email me I will send you a drawing and instructions. You can obtain good measuring tools for a small investment from pawn shops and the newspaper classifieds.

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MikeH(Tx) with an update

05-24-1999 17:02:59




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 Re: Any machinists out there? in reply to MikeH(Tx), 05-18-1999 19:11:29  
Hi all. Been working on the JDH. The cylinder block is out and measured. Looks like I don't have to have a rebore, but it is a judgement call (.008 step). Since this is going to be just a toy, I am going to hone it, put new rings in it, paint it and go. The paint ought to fix what the rings don't.

A friend suggested using a piston ring and measuring the gap while it is in the cylinder. Since I can accurately measure the top of the cylinder, I can get the diameter there. Changes in the gap (divided by pi) will give me changes in diameter all the way down the cylinder.

Not only does this give an easy way to measure changes in diameter, but it allows 3x the accuracy since you are measuring circumference instead of diameter. Measuring the gap with a feeler gauge seems to be accurate to about .002, so my diameter measurement should be accurate to .0007 or so. Yes?

Of course all this assumes a round cylinder.

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Pancor

05-19-1999 05:05:06




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 Re: Any machinists out there? in reply to MikeH(Tx), 05-18-1999 19:11:29  
I don't know if you have one or not, but it is fairly easy using a vernier. Hold one of the outside measuring jaws against the cylinder wall, and open the vernier so that it touches the wall on the other side. slide the free jaw along the cyclinder wall, all the while looking at the reading. The peak reading will be the true diameter of your cylinder wall. This would be much easier with a dial caliper or an electronic vernier, but they can run pretty expensive. good luck. If you need more help, post another message.

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jim

05-19-1999 00:03:25




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 Re: Any machinists out there? in reply to MikeH(Tx), 05-18-1999 19:11:29  
easy method for novice is too sand top of bore where top ring stops travel.you will want to primarily look on thrust side,becdause this is where most of your wear is going to be.keep in mind that what you see is only approximatly half the wear. Also if ridge needed reamed to remove piston you can be sure than rebore is needed.



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Charlie Okla

05-18-1999 22:54:41




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 Re: Any machinists out there? in reply to MikeH(Tx), 05-18-1999 19:11:29  
Go get a set of ID Mics. Cheap ones will do. Set it so it is just snug when you find you are straight across the bore. Too tight, you can't move it through, too loose, it falls through. Measure over the mic with a OD mic for a true reading. ID mics are VERY subject to temperature changes and are never correct for long. Doing it this way you can get readings to within .0002.
You can use telescope gauges, but they also require getting the right feel. (and getting your hand to the bottom of the #&$^$^!! bore)

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Bill

05-18-1999 19:43:32




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 Re: Any machinists out there? in reply to MikeH(Tx), 05-18-1999 19:11:29  
The problem your having is that you are measuring
with non-presice instruments, any error is amplified when you take a reading from the calipers with a micrometer or vernier. The best
way to measure is with a set of inside micrometers. I've got a set of Mitutoyo's that
covers 2 to eight inches. It also comes with an
extension for reaching into holes, i.e. cylinder
bores.The set cost me $165 Canadian and has been
invaluable. I hope this helps, maybe someone else
has some suggestions.

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jeff w

05-26-1999 07:11:02




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 Re: Re: Any machinists out there? in reply to Bill, 05-18-1999 19:43:32  
This is a real good,cost effecient way to measure
but not the best. bore gauges & air gauges are what they use in factory to measure. Problem with inside mikes is you have to get your hand down inside the bore to turn thimble on the mike. I actually prefer a snap gauge & outside mike for this application.



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Mike

05-20-1999 16:07:43




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 Re: Re: Any machinists out there? in reply to Bill, 05-18-1999 19:43:32  
That's pretty much the way I've done it on the gas compressors at work, but like you indicate, temperature does make a difference. When we check the bore, we also use claibration strandards to check the measuring devices before and after the measurements. When you're throwing twenty inch diameter pistons around, you don't want the wrong clearances!

Another way to get an idea is to take a known good piston ring of a certain size and place it in the bore, and measure the gap between the ring ends.

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MikeH(Tx)

05-20-1999 19:19:43




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 Re: Re: Re: Any machinists out there? in reply to Mike, 05-20-1999 16:07:43  
Mike, a friend suggested the piston ring method to me offline. Sounds like a very good way to measure diameter, if you can measure the gap accurately, then reproduce the gap with the ring outside the cylinder. I haven't quite figured out the entire procedure, but it sounds like a great possibility.

Again, thanks to all for the suggestions.



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