Measuring Piston To Valve Clearance on assembled motor

how easy is to bend a valve?

can i set the piston on TDC and then move the cam/cams (with the slightest force) until i feel the contact?

I don't think that will provide any useful info. Can you turn the crank 2 revolutions on the assembled engine (with your desired valve timing) without valve interference? Then you have some clearance.

If your engine has adjustable rocker arms, then you can try adjusting one valve until you find interference. Do some measurement / math to compare to the normal valve/rocker position and determine the piston to valve clearance. Or perhaps you could hold a shim of a known thickness at the cam or valve and see if that interferes.

Depending on the type of valvetrain you have, you could install a set of tester springs.

Different engines may have different ways of figuring this out - what is the one you're using?

Fiat 128 SOHC and VW 1.8 16v (kr)

I want to find how much I can retard/advance the cam in dyno testing.

both engines have aftermarket pistons and cam/cams

In theory you can work it out, to a close approximation, from the known clearance* at the spec' timing, the camshaft lift profile, and the piston/connecting rod/crankshaft geometry, but it is tricky. It's a timely reminder of why it's a good idea to check the clearances with the camshaft(s) advanced and retarded from the nominal timing spec', so one has a good idea of what one has to work with.

There is an exception to all this, though, and that's if the engine is a NON-interference design, where it's impossible to have V2P interference. I'm NOT sure, but I seem to recall reading something about one of the 16 valve VAG heads being non-interference on either the intake or exhaust, but that something you MUST check for yourself.

Even if you don't know all that, you can advance the exhaust, so it closes earlier, and retard the intake, so it opens later, without any interference concerns as it will increase the respective V2P clearances. DO NOT do the opposite, if you're in doubt, as that will reduce the clearance.

*You DID check, didn't you?

I don’t have such info available.

Thats why I want to try and measure it this way.

If you don't have the clearances at the spec' timing, it may be a good idea to bit the bullet and pull the heads to do a full check.

However, as others have mentioned, if you can fit test springs (just the inners with multi-coils work well) you can put a dial gauge on the valve retainer and by gently depressing the valve from~20 degrees BTDC on over lap, to 20 after (important as often the valves will be closest a bit before/after TDC), in 5 degree steps, with the camshaft(s) fitted and clearanced, you can check how much additional clearance you have. Then advance the camshafts in 2 degree steps to +10 degrees (should be more than enough, but make it more if you feel it's needed) while checking, then retarded. It's a lot of messing around but cheaper than a broken engine.

OH, just had a thought while typing that out - If you just want to check you have clearance, and not concerned exactly how much, pick up a borescope that can be used with your 'phone, they're quite cheap. You can put it through the 'plug hole and actually see how close the valves are coming to the pistons.