Ducati Monster Forum

Moto Board => Tech => Topic started by: monsta on September 28, 2015, 08:20:23 PM

Title: Valve overlap clearance ?
Post by: monsta on September 28, 2015, 08:20:23 PM
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?
Title: Re: Valve overlap clearance ?
Post by: Howie on September 28, 2015, 08:56:16 PM
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.
Title: Re: Valve overlap clearance ?
Post by: monsta on September 28, 2015, 11:48:52 PM
nah, what I mean is the distance between the valves when they are both momentarily open in the cycle.
Title: Re: Valve overlap clearance ?
Post by: koko64 on September 29, 2015, 03:04:59 AM
Ah, valve clip.
Title: Re: Valve overlap clearance ?
Post by: Nibor on September 29, 2015, 05:39:19 AM
Don't want clipping, Tony, want clearance!  :P
Title: Re: Valve overlap clearance ?
Post by: koko64 on September 29, 2015, 05:44:22 AM
Yep.
Title: Re: Valve overlap clearance ?
Post by: Howie on September 29, 2015, 05:47:49 AM
Clipping means you get to do it all again :P  Can't add much to overlap clearance on a Ducati, but as far as I know a good general spec is 1 mm.
Title: Re: Valve overlap clearance ?
Post by: Duck-Stew on September 29, 2015, 06:52:51 AM
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...
Title: Re: Valve overlap clearance ?
Post by: koko64 on September 29, 2015, 12:38:07 PM
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.
Title: Re: Valve overlap clearance ?
Post by: Speeddog on September 29, 2015, 01:40:29 PM
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.
Title: Re: Valve overlap clearance ?
Post by: koko64 on September 29, 2015, 01:58:02 PM
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.
Title: Re: Valve overlap clearance ?
Post by: Speeddog on October 01, 2015, 12:29:29 PM
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.
Title: Re: Valve overlap clearance ?
Post by: monsta on October 01, 2015, 06:56:55 PM
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!!  [laugh]

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

(http://i133.photobucket.com/albums/q41/monsta904/20151002_070922_zpseuwkrcin.jpg)
Title: Re: Valve overlap clearance ?
Post by: Speeddog on October 01, 2015, 07:35:27 PM
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?  [popcorn]

Title: Re: Valve overlap clearance ?
Post by: monsta on October 01, 2015, 09:20:17 PM
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..   :(
Title: Re: Valve overlap clearance ?
Post by: koko64 on October 04, 2015, 10:37:43 PM
Interested to see your solution. Good luck.
Title: Re: Valve overlap clearance ?
Post by: Speeddog on February 04, 2016, 09:49:14 AM
Monsta may have solved the issue by this time, but I was doing some heads and took advantage of the situation to swap parts and take a few measurements.

'95 900SS V2 heads, stock size valves with minimum cut on valve seats and valves to freshen them up.

With the carbie cams (OHT, VHT) valve overlap clearance was 0.042".

With ST2 cams (OHZ, VHZ) valve overlap clearance was 0.033".

Fitting larger valves reduces that clearance by 0.017" for every 1mm increase in valve diameter.

So...... if you go +1mm on both valves, with ST2 cams, they'd just touch.  :(


Going from carbie to ST2 cams, the shimming had to change, thinner closers and thicker openers:

                  CLOSER   OPENER
INTAKE         -0.18      0.41   
EXHAUST      -0.23      1.35

<Caveat: The above numbers are influenced by the manufacturing tolerances of all of the parts, and I didn't fiddle the shimming down to the last thou. YMMV.>
Title: Re: Valve overlap clearance ?
Post by: monsta on February 08, 2016, 02:26:15 AM
thanks for that SD..   [thumbsup]

I've done the heads, the valves got trimmed a bit more. I bought a decent magnifying glass and blued the seats so that I could see the sealing surface better then turned them down to that.
Ended up with about 0.8mm. I'm pretty sure that'll be ok.

the heads are still sitting on my bench!  [bang]
But I have a new job that'll give me a LOT more time at home.
Hope to get onto it this week or next if I dont let this  [drink]  doesn't get in the way!   ;)
Will be around 43 deg C (109 F) here for next few days! so wont be going out to the garage much.