Britbike forum Forums Thread Like Summary - Need a bore/cylinder gauge

Hi All -

Time to add a new tool to my garage. Anyone have any suggestions? I'm sure there are good ones and poor ones. I'm rebuilding an A75 Rocket 3 after I get the Royal on the road, so I'll have a chance to use it again...

Cheers,

Steve

"Back in the Day" when I was early into motorcycles (and Adam was a lad) we had no access to internal bore gages of any description.
The best we could do was an external micrometer.
In order to judge whether a rebore was needed we would put the piston (no rings) into the bore and insert feeler gages between piston OD and cylinder bore.
Measure the piston OD with a micrometer and you have a bore diameter.
Repeat at different positions around the circumference and up and down the bore and you get a pretty good idea of the state of the bore.
In particular comparing the measurements at the bottom of the bore where it was unworn to the swept part of the bore gave some pretty good indications of bore wear.
I must admit I get very wary of anybody pretending that they measure to one or two tenths of a thou unless they are measuring in a temperature controlled environment using equipment recently calibrated to known standards and traceable back to international standards.
Just my two cents worth of course.

Hate to pile on but I’m one more vote for an actual bore gauge and micrometer as being the accurate and repeatable way to measure a cylinder. Buy the bore gauges and micrometers in sets.

But why do you need that level of accuracy? What you have, calipers and a telescoping gauge, will tell you useful stuff like what oversize of piston you have, or roughly how much taper you’ve got, then the actual machinist can take it from there and bore it precisely. Assuming you aren’t going to get a boring bar and try to bore the cylinder yourself.

No, you rock it til it’s at dead opposite points, so you’re measuring the full diameter.

The diagram shows how all 2-point instruments work.

If the handle attached to the probe isn't parallel to the axis of the cylinder the probe will register a value that's too large. As the handle moves from tilting one way, through parallel, to tilting the other way, the length of the probe will go through a minimum. The minimum length of the probe is the diameter of the cylinder, so the problem becomes one of finding that minimum..

Telescoping gauges, inside micrometers, and 2-point bore gauges with dial indicators all use the same principle, although this list is in order of increasing accuracy and sensitivity.

Attached Images

I guess in a sense we are saying the same things, MMan.
Measuring a dimension is a crap shoot----measuring differences in dimensions is much more real world.
So in the "good old days" we measured the difference (with feeler gages) using a piston in an area of unswept bore and in a swept area.
The difference gave the bore wear.
And assuming that the bore was correct when new this gave one the wherewithal to judge whether a rebore was necessary.
Much more real world and reliable IMHO to trying to measure the bore size in absolute terms and comparing that with the book new bore size to judge bore wear.
Just my old mind wandering around these matters.

"Escalated"? Don't you mean "descended"?

Scottish tape measure.

Attached Images

Only by steps of approximately .003"

Found a set of telescoping gauges (Mitutoyo). Thanks everyone.

As for learning how to use it, let the lessons begin! So far, it seems the best way to use these is to have the bore on a horizontal plane. This allows the tool to "fall" instead of having bore on vertical and "pulling" the tool.

[video:youtube][/video]



Steve

Hi, With bore gauge you watch indicator needle. As you rock handle the needle will stop for instant & go the other way. You work it until the needle is at that stopping point. Pin of tool is at the straight across position.

How should I describe this? I don’t know. I know this it takes my apprentice about 20 seconds to get the feel of this in real life.

Learning how to read micrometer & setting gauge to zero & understanding what deviation from zero means takes a little more time.

I have Starrett inside micrometer. It’s pretty accurate, but it takes a very deft touch to get repeatable results. Measuring my bores you can feel difference of .0002”. But not that easy.

The dial bore gauge is costly, but works really good.

When/if you get into fitting main bearings into case & on crank, press fits & C clearance of bearing becomes critical. Does .0001” make a difference? Trust me it does. Splitting hairs is needed. Just went through retro fitting timing side roller on my ‘73 Tiger.

Engineering tables on bearing website gives all info you need. My problem was I couldn’t measure case bore accurately enough. Was an interesting learn experience. One thing I learned is I need a high precision bore gauge.

You take a shaft what will just slide through a hole. Increase diameter of shaft by .0001”. You just learned how tight a .0001” press fit is. Just the heat of holding the part or micrometer in your hand will skew results when we get to precise measuring.

Suppose you fit piston just a wee bit too tight. Gets pretty warm up that hill on 100f day. Piston expands another .0001 or .0002”.
Remember that .0001” press fit we were talking about? Now the piston is thinking about it & how not scuff. This stuff matters!
Don

I use a old oversize hone, I believe it's 3.25 inch. It leaves a nice hone pattern...When in doubt, ask the manufacturer.