Useing a Dial Bore gauge

[geoff5269]

When I am machining the cylinders for my engines I am beginning to realise the importance  of being able to measure the bore to check it is accurate size and is not tapered and so I bought this Dial bore gauge for approx. £40.00. 
After reading the instructions it seems you set the gauge with a know size and then zero the dial, so you are not really getting a measurement but looking for a deviation from your setting. what surprised me was that the dial is a standard item and seems to read the wrong way which is confusing,  the more you squeeze the pins closer the higher the reading, or perhaps I'm doing something wrong.
I wanted to use O-rings for the piston and wonder how tight should they be in the cylinder, any advice?
Geoff

[Don1966]

Geoff I don't own one but I believe that the gauge is suppose to read any offset from the set value. The reading will be greater if the hole is too small so you will need to cut more. If the value is less you have over shot the value you set the gauge at. I hope this helps.

Don

[petertha]

I'm no expert but I think you have the gist of it. I have seen different styled +/- deviation gauges like attached that make the 'direction' a bit more obvious. Depending on your tool model, you might be able to swap in a different gage, particularly finer resolution or digital for that matter. One issue is matching stem diameter & gage body dimensions. The other issue is whether the displacement action on the contacts properly translates to the gage reading displacement. For example if one were to swap in a 0-.5" gage with 0.0005" grads vs maybe what you have 0-1" with 0.001" grads. Will defer this issue to someone with more experience here.

The other (real important) thing is your measurement is always seeking a 'min' reading as the contact bars are arcing through the bore until the least measurable distance is realized. That translates into the dial also decreasing to some minimum position which is the bore 'reading'. ie. not just eyeballing the contacts across the bore as what you think as perpendicular. Same procedure goes for the initial dimension setup on a mic or reference bore. 

[derekwarner]

Geoff...if you are machine finishing a bore as opposed to reaming it, even in model applications [small bore] it is far more critical to get the bore without taper and of an acceptable surface finish for O-rings ...than it is to achieve a set predetermined toleranced diameter

[0.4 umRa {max}] is a value universally nominated by seal element manufacturer's for dynamic sealing....[this is actually a value of surface roughness]

From there, the piston O-ring seal cavities can be adjusted to provide the desired O-ring crush within the bore...in this one setup, the piston OD or clearance to the bore is also achieved

Remember the larger diameter section  O-ring chosen will always be easier to attain the set % crush, than a smaller section O-ring .... Derek