Bore and Stroke Explained

When the specs on an engine says it is a 5 x 7 inch bore and stroke does
this mean that the cylinder diameter is 5" and the stroke is 7"? if this is
and I was going to measure the taper in the cylinder would I need a bore
gage that would measure down to 7" in depth?>

I see that you've received several replies which partially address your
questions, but I'm going to try to pull them all together in a more detailed
response which hopefully gives you all you need.

Bore x Stroke is commonly quoted in that form - bore diameter first followed
by stroke length. In our older and slower engines the stroke is normally
longer than the bore. I have a few older type engines where the bore and
stroke are the same, and this is referred to as a "square" design. It is
only in modern high speed engines that we generally see "oversquare"
dimensions where the stroke is shorter than the bore diameter.
When measuring a cylinder bore for wear we are interested in three things -
bore diameter compared to standard, taper and ovality. All you need to
measure all these is an inside micrometer, or a telescopic gauge with an
appropriate tool to read the dimension such as an outside micrometer or a
good vernier caliper. When measuring, remember that the rings don't go all
the way to the top of the bore, so at the very top there will be no
appreciable wear. Likewise at the very bottom of the bore. Use these to give
you the standard bore size if you don't have the specs.
Measure the bore diameter just below the uppermost extent of ring travel
(there is usually a 'lip' on a worn bore) and measure in two places 90
degrees apart. Make one measurement across the bore parallel with the wrist
pin and the other at right angles to the wrist pin. This last one is
measuring the 'thrust side' of the bore, and will probably be slightly
larger than the other measurement. The larger figure will give you the
amount the bore is worn from standard, and the difference between the two
readings is the ovality of the cylinder. Now repeat the measurements at the
bottom of the piston ring travel. Once again, the difference between the two
cross measurements gives you ovality, and now you can compare the readings
with those from the upper end. The difference between these figures gives
you the lengthwise taper in the cylinder. Unless the cylinder is very worn,
taper and ovality will probably not be an issue, but they are handy to know
if a rebore is being considered.
I hope I haven't oversimplified this for you Paul in trying to make it as
clear as I can.
Regards
Brad

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This article is part of the Stationary Internal Combustion Engine Frequently Asked Questions (FAQ). This series is a combination of my views and knowledge and the views and knowledge of other people - most of them members of the Stationary Engine Mailing List (More info on ATIS). Those articles which were written by others are © Copyright to the author. Those articles written by myself are © Copyright to Paul Pavlinovich.

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