During recent maintenance I found a broken valve spring on one of my cars. Not wanting to go to the trouble of removing the cylinder head, I was easily able to do the job with the head in situ. I filled the bore with rope, wound the cylinder up to TDC and the engine locked solid, so the valve was going nowhere. Cable tied the rocker arm out of the way. Replaced the spark plug and fitted some old coolant hose over it to protect the plug and so I could get a good grip with the valve spring compressor.
I am pleased to say the transformation was remarkable, and I am no longer suffering a 'kicking' from the engine when driving hard in 3rd gear, which I now know was attributable to poor valve action.
No harm seems to have been done but I will carry out a compression test anyway. I have some compression figures from years ago so I will compare the present readings with those.