Originally Posted by
studabear
I've seen a tool in a magazine which involves keeping the cylinders pressurized with compressed air to allow you to do the job, looked fiddly and iirc the tool was a good few quid too. Laser tools springs to mind.
You are closer than you think to being right
.
Basically, you pressurise the cylinder at TDC using a leakdown tester. With the cylinder pressurised you can then remove the cams (preferably remove them before pressurisation) valve spring retainer and spring assembly before changing the valve stem seal. You have to be careful with this method that you don't drop the collets down the oilways etc ; covering the engine with a rag helps.
In theory you could pressurise the cylinder somehow via the spark plug aperture, retain that pressure and achieve the same result.