Tool for Measuring Cylinder Bore Diameter

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#1
My service manual says to measure the bore diameter in several places around the cylinder from top to bottom.

My question is: (1) how important is it to do this and (2) where can I get the tool to do this measuring?

Thanks,
Bill
'99 YZ 125
 

WoodsRider

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#2
You'll need a bore gage. You can get one that reads in inches or millimeters. I buy lots of different gages mostly from my local Mitutoyo sales rep. You can find less expensive bore gages through Harbor Freight and other liquidators.

You may also check with any local machine shops that perform precision work. They might measure one up for you for a small charge.
 
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#3
Thanks WoodsRider. We have a Harbor Freight and Northern Tools near by. I'll check them out.:)
 

Jaybird

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#4
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#5
Is it really important to do this precise measurement?

I took a picture and description, of a dial bore gage that I was thinking of getting, to the mechanic that helped me with my top end. This guy has been riding and racing for 15 years. He owns his own shop that sells new bikes and repairs used ones. He told me not to even spend my money on something like that. Todays cylinders don't require it.

Here's the approach he uses. He takes a dial calliper and measures the top and the bottom of the bore.Then uses the same calliper to measure the top and botton ot the piston. And finally compares the measurements.

What do you guys think? Is this "close enough" approach a good way to go? Is using the dial bore gage a little bit of extra insurance?
 

Jaybird

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#6
Did you check out my link? A telescoping guage doesn't cost alot. But you will need something to measure the dimension. (micrometer, caliper, etc..)

Good bore guage may cost 4-500 dollars. Telescoping guage may cost 15-20 dollars.
 

WoodsRider

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#7
Bill - Calipers, depending on the manufacturer and the resolution, are only accurate within 0.002". Depending on the size of the engine, tolerances of the components, number of revolutions per minute, coefficient thermal expansion, etc., etc. that 0.002" can be the difference between success and a loud, expensive noise. :eek:

That being said, your mechanic may be very skilled and comfortable using a caliper. Myself, I prefer to use a dial bore gage graduated in either 0.001", 0.0005" or 0.0001". I've never had an engine fly apart <knocking on wooden desk>. Telescoping gages can do the job, but are definitely not as accurate as the bore gage.

You'll need to know the bore size in order to determine what size piston you'll need.
 
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#8
I found a dial bore gage for $80 at an industrial supply shop. The brand looks like Fowler. I haven't ordered it yet.

Specs

Graduation 0.0005"
Range 2 - 5"
Probe Depth 6"
No. of Anvils 11
No. of Spacers 2
AGD Group II dial indicator


For $80 I'm sure this isn't the best quality. Do you think it would work? I'm not sure what the anvils, spacers, and AGD Group II are for. Hopefully instructions would be included:)
 
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#9
Fowler makes outstanding tools and that bore guage is a very good choice for your purposes. I have the same one sitting here and it's excellent quality plus it even comes in a nice wooden box. Best bore gauge I've seen for under $100.
 
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#10
hey......
In reality you can get away with jsut using dial calipers to measure your bore diameter. But, the problem you have is when measuring from the top of the cylinder, you will only be able to measure the diameter of the cylinder ridge formed in the area the piston does not reach....when measuring from the bottom you only get the smallest measuremetn in that bottom quarter inch. because you cant reachinside the cylinder like you can with an inside micrometer, you cant check for cylender warpage, and uneven wear in the middle of the cylinder....both of these can obviously cause major problems. The micrometer would be your better bet.
Tim
 

WoodsRider

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#12
Bill - One more thing I forgot to mention. Make sure you have a known "master" for setting the bore gage close to the size being measured. At work we use ring gages or a cadillac (height transfer) gage. Both very expensive, but extremely accurate. You can get by using a caliper or micrometer if you're careful.
 
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