compression test

I tested the compression after the top end rebuild, borrowed a friends compression gauge and it measured 150 and 155 lb right after the rebuild.

re bored 20 over, new pistons, rings, valves, valve guides and springs, had the valves seated.

after 800 miles I purchased my own compression gauge and now it measures 140 lb on each cylinder

not sure what could be going on. could it be a difference in gauges? I would think if it was rings or valves it would show as one cylinder being different. not both having the same compression.

What is a good compression number for an otherwise stock 71 commando? Do I need to think about taking it back down and looking for a problem? Just ride it and forget it?

Will probably try and re borrow the original gauge first

Dennis

We did my 1968 Commando at a NENO monthly meeting, it has 152 lbs on both cylinders. I am assuming that you had the throttle full open when you kicked it?
Also make sure you give it a lot of really hard kicks to build it up to maximum pressure. It makes a big difference.

MF

Are you checking it with the engine warmed up? That can easily add 10# over a cold engine. Could be your guage is a little off also. If it has a hose and the release valve is at the top it will be adding the volume of the hose and adapter and could lower the reading. Actually those don't sound like that bad of numbers and its really good they match side to side. If it was wear they still might match but damage probably wouldn't. If the plugs look clean and no oil smoke, I believe I would just ride it a little.

both tests were with the engine cold

The gauge has a hose and the pressure release is at the gauge

Throttle open and several kicks after the gauge stopped moving.

Dennis

Ta.

There are compression gauges and compression gauges. Some cannot possibly give an accurate reading and are only good for comparison purposes.

Usually if the check valve on the gauge is close to the spark plug thread, the reading is more accurate.

The guage with the hose in it has extra volume to be filled so will give a lower reading. If your bike goes well, then just enjoy it. No need to be concerned about compressions on a new engine unless something is wrong.
Dereck