How do I measure a cylinder?

[700rpm]

I want to determine whether the cylinders in one of my extra blocks are worn, out-of-round, or tapered. How does an amateur garage mechanic do this (what is the correct tool), and what are the acceptable tolerances before a rebore or sleeves are needed?

[Tod]

Originally Posted by 700rpm

I want to determine whether the cylinders in one of my extra blocks are worn, out-of-round, or tapered. How does an amateur garage mechanic do this (what is the correct tool), and what are the acceptable tolerances before a rebore or sleeves are needed?

There are several ways to do it if you have the tools. If it is just a bare block maybe you could just take it to a local run-of-the-mill machine shop and they should be able to measure it with bore mics, tri-mics, telescoping gages, or even inside dividers. Probably do it in a couple of minutes for free.

Tod

[motordr]

Put in a piston ring and measure the end gap with feeler gauges near the top and bottom of the cylinders for taper.

[edmondclinton]

Originally Posted by 700rpm

I want to determine whether the cylinders in one of my extra blocks are worn, out-of-round, or tapered. How does an amateur garage mechanic do this (what is the correct tool), and what are the acceptable tolerances before a rebore or sleeves are needed?

The amateur garage mechanic needs a couple of tools just like the pro, i.e., either a dial-bore gauge or a telescope gauge/snap gauge, and then measure the gauge with a micrometer. The latter combination of instruments isn't usually as reliable as a dial-bore gauge in my opinion.

Clean the cylinder walls and then inspect them for pits, roughness or scoring. If evident, the cylinder must be bored or honed oversize to clean up or in other words eliminate these imperfections and be bored or honed to the correct specs for the smallest available oversize piston.

The bore should be measured at the top, center and bottom of piston travel and in two different directions horizontally at each level. To start, measure the bore perpendicular and then parallel to the engine centerline, at the top of the bore (below the ridge or below the area where the rings do not make contact). Lets call these two measurements X top and Y top, where X top is perpendicular or at a right angle to the centerline of the engine and Y top is the exact opposite or parallel to the engine centerline.

Make the same type measurements again at the center of the piston travel. Call these two measurements X center and Y center.

Make the same type measurements again at the bottom of the cylinder within say a 1/2 inch or so above where the top of the piston would be (or is) at its lowest travel or in other words, Bottom Dead Center. Call these two measurements X bottom and Y bottom.


To find the Taper:

Subtract or find the difference between X top and X bottom.


To find Out-of-Round:

Subtract or find the difference between X center and Y center.


To find the Bore Service Limit:

Equals the average of X center and Y center.


If all the measurements turn out to be within the spec limits of the standard bore size or within the size and specs limits of the pistons that came out of the block (or are still in there) or the ones you plan to put in of that same size, then new rings will give satisfactory results on a used cylinder. And if there are no remaining cross hatch finish pattern visible then honing to remove the glaze and restore the proper cross hatch pattern must still provide a bore size within the spec limits of the above.

Otherwise, the cylinders must be bored and finished honed to the specs for the next size available oversize piston. And of course if the bore is worn greater than the spec limits for a 4 inch piston including the proper cross hatch finish then it's time for a sleeve.

[700rpm]

Great suggestions! Thanks! Any more?

[tbirdtbird]

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Ray, there are always some Starrett brand inside mic sets on ebay under 100 bucks....several on there now...

http://www.ebay.com/sch/i.html?_from...rrett&_sacat=0

These sets have one micrometer barrel and several extensions depending on your bore diameter, which in the case of an A you want the 3-4" extension of course. You have to get the entire set, of course.

You'll have to play a bit to get the feel for them but it is not that hard. You keep one end anchored and move the other end up and down and back and forth until you have expanded it all you can. You should just barely be able to move the tool in the bore when it is fully expanded....kinda like feeling to get the correct feeler gauge gap when setting points....not called 'feeler' for nothing

Starrett is a good brand. For the longest time until the chinese invasion they and Brown and Sharpe had the machine tool market cornered

[Charlie Stephens]

Have you checked your local tool rental places for measuring tools?

Charlie Stephens

[Kurt in NJ]

Using a piston ring works quite well, just remember that the change in gap is a little over 3 times the actual taper ---I used to carry a new .030 ring in my Hershey backpack to use as a gauge

One of the recent engines I took apart had over .035 taper, and .010 out of round, just cleaned up at .040 ---amazing it could run with that much taper, must have had a new set of rings every year- it was used running a irrigation pump ---probably kept the mosquitoes at bay downwind

[700rpm]

Originally Posted by Charlie Stephens

Have you checked your local tool rental places for measuring tools?

Charlie Stephens

Good idea, Charlie. Thanks. Until this thread I had no idea even what tool to use.

[700rpm]

Tbird, thanks for the link. I really should get one of these sets!

[johnneilson]

Originally Posted by 700rpm

Tbird, thanks for the link. I really should get one of these sets!

Rather than getting a inside mic, which is very difficult to get repeatable measurements look for a used bore gauge set.

I use bore gauges, inside mics and snap gauges daily. All have a purpose and application. bores need to be checked for taper top to bottom and across two axis. The later measurements are to determine true wear in the cylinder. Typically, I measure axis inline with crank and at 90° to crank axis.

If, you have a block bored you will be able to double check the finished product. You may find some very good work, and then, you might not.

Enjoy, John