Cylinder head TDC

[D.Martijn]

Hey guys,

I just assembled my cylinder head but noticed something..

Some info; I got a brand new Kolbenschmidt camshaft, new IE HD rockers, new BMW rocker shafts, new BMW eccentrics, valves and valve seats got a regrind.
Cylinder head got surfaced but still has the little dimples.

When I set the head at TDC, both valves of cylinder No1 are closed.
However the valves on cylinder No6 are slightly open.

My concern is that both pistons 1 & 6 are on TDC wont the valve of cylinder hit the piston this way?
Is it normal that the valves on cylinder No6 are slightly open?





Exhaust No6


Intake No6


Thanks,
Martijn.

[zaq123]

4 stroke engine you have.

1.Power stroke. The piston is at top dead center, intake and exhaust valves are both closed and the spark plug has just fired. The expansion of the ignited fuel/air mixture forces the piston down. Before the piston reaches bottom dead center, the exhaust valve starts to open.
2.Exhaust stroke. The piston is at bottom dead center and starts to come back up. The exhaust valve opens fully and starts to go closed. Before the piston reaches TDC, the intake valve starts to open and the exhaust valve is still partially open.
3.Intake stroke. The piston is now at TDC, both the intake and exhaust valves are partially open. As the piston travels back down the cylinder, the exhaust valve goes fully shut and the intake valve goes fully open and starts to shut.
4.Compression stroke. The piston is at BDC and starts to travel up the cylinder. The exhaust valve is still shut and the intake valve goes fully shut.
You’ll notice that during the piston travel, there is a time when both valves are open. This seems counter productive, but it is necessary for optimum performance. This is a term called "valve overlap"

[D.Martijn]

4 stroke engine you have.

1.Power stroke. The piston is at top dead center, intake and exhaust valves are both closed and the spark plug has just fired. The expansion of the ignited fuel/air mixture forces the piston down. Before the piston reaches bottom dead center, the exhaust valve starts to open.
2.Exhaust stroke. The piston is at bottom dead center and starts to come back up. The exhaust valve opens fully and starts to go closed. Before the piston reaches TDC, the intake valve starts to open and the exhaust valve is still partially open.
3.Intake stroke. The piston is now at TDC, both the intake and exhaust valves are partially open. As the piston travels back down the cylinder, the exhaust valve goes fully shut and the intake valve goes fully open and starts to shut.
4.Compression stroke. The piston is at BDC and starts to travel up the cylinder. The exhaust valve is still shut and the intake valve goes fully shut.
You’ll notice that during the piston travel, there is a time when both valves are open. This seems counter productive, but it is necessary for optimum performance. This is a term called "valve overlap"

I did some more googling and also found out this is called valve overlap.
Thanks for clearing that up.

So, I can just go ahead an start getting the head on the block then. Or should I still test it with some playdough, just to be sure they don't hit the piston?

[Lugnuts]

I'm not an experienced engine builder by any stretch (only the M20 in my E30), but personally, I would be checking piston to valve and piston to cylinder head (quench) clearance. Plasticine for one, 2mm solder for the other. Saying that, you might be able to just do P2V and call it good. The need for close attention and measuring (imho), depends on how aggressive the cam is (you're using a stock one?), how much material was removed from the head/block, if the pistons/rods/crank are exotic/modified for more stroke/compression etc...

Just my $0.02.

[zaq123]

I did some more googling and also found out this is called valve overlap.
Thanks for clearing that up.

So, I can just go ahead an start getting the head on the block then. Or should I still test it with some playdough, just to be sure they don't hit the piston?

it's technically overlap on the intake stroke (highlighted in red in my post above). TDC cyl1 = intake stroke Cyl.6. Both pistons of cyl 1 and 6 at TDC; cyl 1 valves are closed; cyl 6 valves are both open and EX valve is about to get closed when IN valve is opening.

This is why one needs to check P to V clearance. Piston is closest to the EX valve right before TDC and closest to IN valve right after TDC