Valve to Piston clash - or not?

[david.g.chapman]

I have a question for twin cams with full width heads (or lightly skimmed width) and pistons with large cut outs in the crowns.

I have not measured this (wish I had now), but when I had the head off last time it looked as if the valve projection from the head surface at full opening was less than the depth of the cut outs in the pistons. So no valve clash at all.

Were my eyes deceiving me or can this be the case with engines with mild CPL2 cams, full width heads and large piston cut outs?

Dave Chapman.

[steve.thomas]

Dave
I snapped a camshaft a while back and feared the worst, but after removing the head there was no apparent damage to valves or pistons. I was surprised by this but concluded that the valves did not protrude enough to contact the piston at any stage. The engine is a standard big valve from a 72 s130 as far as I know.
Regards
Steve

[david.g.chapman]

I phoned QED about this some time ago and they seemed to agree that it was possible.

So the next time my head is off I will do a measurement of the valve protrusion and the cut out depth. If the cut out depth is slightly more, then it might be worth doing a clay test (see other thread) with the pistons set at tdc and the camshafts carefully rotated. You might have to remove a valve spring so you can feel if there is a clash.

In that way the slow running clearances are checked.

Then if the test passes (10 thou of clay left?) you can bask in the warm glow of not caring so much if your timing chain snaps

Cheers,

Dave Chapman,

[rgh0]

Not only do the valves hit the pistons but they also can hit each other so don't plan on snapping you timing chain to soon.

I guess its possible for the valves to avoid the pistons in certain circumstances but you have to the right set of build dimensions ( e.g. big cut out in the pistons, big piston deck to top of block height, standard cam lift, valves recessed well into the head, head not shaved to much etc) or be very lucky just when in the cycle the cam stopped. The original standard build dimensions certainly meant you had a clash between pistons at TDC and valves at maximum lift but there are not many absolutely standard original engines anymore.

cheers
Rohan

[david.g.chapman]

Thanks, Rohan.

As usual, you have the complete picture and not just part of it!

I think it is safe to assume there will be a clash and work with that in mind!

Cheers,

Dave Chapman.