TOPIC:

wrote:

Isn't it true that the piston crown height is lower on an aftermarket piston to allow for planing the head?

NO! 
And unless otherwise stated all quoted compression figures are based on a zero deck height which only takes into account stock head gasket thickness,stock combustion chamber volume and piston crown volume.
For every 0.5mm of negative deck height at tdc you will have an approximate rounded average compression drop of 1 point ..

Hi thanks for the comments.
the debris around the top of the piston is just pieces of the old gasket I scraped off which I haven't cleaned up yet.
When I carried out the compression test the engine was cold and throttle was closed.
I've included a picture of the 2 compression gauges which I used and base gasket and the new copper head gaskets which I'm going to useI remove the barrels the checked rings pistons and bores all good
 

A cold engine with closed throttle will give low readings. A long hose with the check valve on the gauge will add a bit to the lowness.

To get the most accurate readings, the engine needs to be at operating temperature, the throttle must be held wide open or the carburetors removed, and the check valve should be as close to the combustion chamber as possible. A reliable gauge is also a plus.

If that green base gasket is from Athena, I would donate it to a local land fill and buy a good one.

With that copper head gasket.
If you don't already know..
From new they are work hardened from the stamping process.
Heat it cherry to anneal and quench in water to flash off the oxides.
Dry off and hang on a wire from one corner .
Then spray several light coats of Loctite 3020 leaving time to dry slightly between coats until you get an even solid pink colour.
Let it tack off overnight before fitting..
Done this way you have a vastly reduced chance of oil weepage etc that's normally associated with copper gaskets.
 

Of those compression tester you've shown I would use the rubber push-on type if the Schrader valve is up at the gauge.
But as another poster has mentioned, your problem may be elsewhere. I would definitely go over all my steps before I opened the engine up again.