Piston Sizing

[Hedd_Jones]

LOL. Just looked at the 'overhauled' block. Vernier says 2.18. (it been linered to standard by the looks of it) 

Think I need that internal mic!

[Bob Culver]

Nails can be filed to a very exact length , used to gauge the bore, and transferred to external mike. Reduces cumulative errors. Despite the seeming accuracy of any readout, verniers do not match a good micrometer.
Bores are measured at botom where unworn. Max wear is usually at or just below top ring and takes soem hunting. Bores are quite tricky to measure with simple gauges. Have to sweep in little arcs seeking the minimum at the maximum diameter!

[Chris KC]

+1 for a proper bore micrometer.
FWIW those cheap Chinese telescopic gauges are more or less useless.

[Ivor Hawkins]

If you need to borrow a set of internal mics Hedd, give me a shout, I’ll send them to you.

[Hedd_Jones]

thanks for the offer Ivor

my friendly machinist is all kitted up, I'll wait till I can see him on Tuesday

[JonE]

could someone please perhaps post a typical link for something secondhand or new which is of the sort of spec that is reliable enough to be of use? I thought Chris Garner had posted a few months back something Chinese that he thought WAS up to the job, (which was reasonably priced) but I'm uncertain now.

[Chris KC]

https://www.ebay.co.uk/itm/124807386440?...SwGUxg4baf

The first one that came up - obviously choose sizing to suit what you plan to measure, this is a bit OTT for an A7.

n.b. with the caveat that these things tend to be sold on when either worn out or a bit has been lost... it's a good idea to make reference measurements on something of precise known size to check accuracy. In industry special calibration blocks are kept specifically for the purpose.

n.b. 2 even when you have the right kit there's a definite art to using them... Really nice micrometers have a 'clutch' which helps you judge the right level of tightness.

[Bob Culver]

Everyone should have a 1inch or metric micrometer. The conversion 25.4 is dead accurate so either does. For most purposes a quality vernier calliper is adequate. I find the 8"' easier to handle than 6"'. Direct reading OK with a magnifying glass; does not give a false sense of accuracy. Greatcare necessary no to burr the tips etc. A gap of .0001" is visible against strong light.I f filing nails is too tediuous can make up a threaded tube and long screw with locknut. Hold with a clothes peg or some such. Laregr micrometers really need tets pieces to check the zero.T Unless badly mistreated in the past,  the accuracy is usually more in the use than the instrument Any oil film can add .0005 or more.

[Chris Garner]

Personally I would use a Dial bore gauge every time over an internal micrometer. So easy to determine the wear over the length of a cylinder bore.
My imperial one came from Machine-Dro. co. uk, in a range from 2" to 6" and cost around £35 I think. It has been excellent.

[JonE]

thanks for the links, both.
Their dial bore gauge has gone up quite a lot...
https://www.ebay.co.uk/itm/254577852249?