Piston Sizing

[Michael D]

They shouldnt leak, your quite right, but they often do.

Remember that excessive oil above the piston rings usually means something is wrong. Well, on an IC engine it does.

Coupled with crusty and fouled plugs....it means.......

...... it means you're in for a full rebuild.. at least I was with my Rosengart engine when oil was gathering in the spark plug wells.....(and blowing heavily from the sump-vent...)

[JonE]

as I have a decent, vintage, 2-3" micrometer now with a 2" trueing block I decided to try a set of these - 13.50 delivered or so from the cheapest china ebay seller, and came within a few days. Sprung rams on each side and a lock function at the base. I realise they aren't going to last forever, but they feel useful, low tech, and good to experiment with?     

[GaryCullen]

JonE, I bought exactly the same set from aliExpress. For using on my 1933 Austin 10 checking the bore before deciding on a re-bore and new pistons or a de glaze and new rings. New rings it was and cured my low compression and oil on top of the pistons.
The ends of the "T" piece have hardened and slightly rounded tips so that once locked you can swing the device out of the bore easily. Since I was measuring 6 points in each bore, bottom to top and at 90 degrees horizontally you have to be careful locking the measurement. A couple of times the stem unlocked or moved slightly giving me an erroneous reading. Other wise for the price I think the set was a good investment.
Gary
Auckland

[Bob Culver]

You have to recheck with them locked. As above can acheive the same with filed nails held by a clothespeg. If file a thou short can stretch by hammering the middle. 
Care is necessary transferring to external mic as the ratchet stop reading is slightly different from static.
It is quite diffivult to find the exact max wear in old cyls and I often wonder just how much hunting for guidelines presume.
On old engines and esp Sevens piston clearnce including wear, and rock seem to lead to top ring vertical wear, much reduced in moderns