Valve overlap clearance ?

[monsta]

So.. I'm finally getting some time to put the heads back together.
Some may know I have had J Precision do the CNC porting and they went for the 44.5 mm  inlet valve.
I am fitting IE cams..

problem is, I'm only getting about .6 mm clearance on the valve overlap.
from experience, (not a hell of a lot) it should be about 1 mm?
the old VeeTwo specs were minimum 30 thou but we'd go for 40 thou (1 mm)

I would have thought J Precision would have similar specs...

I'd ring em, but I have to get up early because of time difference..

what clearance do you guru's use?

[Howie]

I'm not sure what you mean by overlap clearance, but if it is what we, on this side of the pond, call seat width the seat contact area after lapping that sound really small.  1mm should, IMO be a minimum.  Either way, do contact them, they might have their reasons.

[monsta]

nah, what I mean is the distance between the valves when they are both momentarily open in the cycle.

[koko64]

Ah, valve clip.

[Nibor]

Don't want clipping, Tony, want clearance! 

[koko64]

Yep.

[Howie]

Clipping means you get to do it all again  Can't add much to overlap clearance on a Ducati, but as far as I know a good general spec is 1 mm.

[Duck-Stew]

That value (I would suspect) would change if you were to allow the closing shims to get looser in tolerance.  So, if you're going to run those...  MIND THE CLOSER SHIMS.  Also, I have no idea if 0.6mm is sufficient or not.  Seems too tight for me, but hey...

[koko64]

The inlet valves are 44.5mm not the usual 44 for an oversize valve with stock valve angle.
I'll remember that shim advice, thanks.

[Speeddog]

IIRC, in one of Brad's reports, he mentions an acceptable valve-to-valve clearance on overlap.

I don't recall the clearance, nor which report it was in.

[koko64]

Iirc, he mentioned 44mm inlet valves being being as large as you could go without changing the valve angle. I dont remember the clearance spec.

Had a look and couldnt find it. Monsta could call Brad.

[Speeddog]

Back in the day, I ran into the same sort of thing with big hairy cams in my Yamaha XV920R racer.
The OEM Yamaha valves were a bit chunky, so I put a bit of a chamfer on the offending edges of the valves.
I tortured that engine mightily, and had no issues with the valves.

An option that I would recommend running past the valve supplier first.....YMMV.

[monsta]

yeah, Speeddog, I have already put a 30 deg chamfer on the exhaust because it's pretty meaty.
I left the thickness at the edge of the valve at about 1.2 mm which is about what we used to do back when...

I also turned down the diameter of the inlet a fairy's fart, just to bring it to the edge of the sealing surface..

This has given me the 1 mm clearance if I put the feelers in at an angle, but about .6 if I go straight in.
Thats what i'm worried about...   

I may have to just reduce the diameter of the inlet to 44 mm and re-cut the seat?
I have emailed Jprecision, but they are not that good to repsonding to emails..
I ahve also waded through some of Brads note, but haven't found anything... there's a LOT of info there!! 

I'm in no hurry!    I'll just wait and get the info and make sure it's right...

[Speeddog]

You could cut the seats a bit deeper.

Then you'd have thicker closer shims and thinner opener shims, that may or may not be a problem, depending on where you're at now in the range.

That costs you a smidge of compression ratio, and shrouds the valves a tiny bit.

If you cut each seat ~0.25mm deeper, then that'd gain you roughly 0.25mm more of clearance.

Have you checked valve to piston clearance? 

[monsta]

shim range is pretty good, I'm not that concerned about that if I go deeper.

hmmm  maybe I should check the valve to piston now so I don't get any more surprises....

everything has already been ceramic coated, I hope its ok..