Race Engine Forum UK

Hi Guy

Which would be the best way to cut the valve clearance (de-shroud) after fitting large valves, is it the normal to hand grind or mill? I have gone from 1.55” to 1.75” inlet and 1.4” to 1.6” exhaust valves.
I could just clock the combustion chamber on each valve and use a 60 deg cutter and cut the clearance to de-shroud the valves, the existing angles are 60 deg so grinding a cutter and using a micro boring tool would work.

Alan

Photo please Alan!

GC

Hi Guy

The clearance from the edge of the valve to the chamber wall

Poor Alan!

You must think I'm 'getting at you!'

I'm not, but I need something this big to figure out a useful answer..

GC

Hi Guy
Will try again, hope this is ok,

Alan

OK, thanks,

For starters there seems to be a fair bit of room from valve to head gasket, which gives you freedom to work on the valve region. The depth of the inlet valve seems to be say, 1mm below the head face. This can impact on the low lift flow. I say 'can' because on some heads I have removed that step and found no difference; but if in doubt take it away because it doesn't help anything and FWIW (I'm sure you know this) certainly on a cast iron head like this (Ford V6?) it presents a sharp edge that could, just, set off detonation.

60 deg? No. On the inlet side I would relieve that region around the valve seat with a wide cut or grind at an angle that matches the valve head if possible. I don't know the valve shape in this case but many valves have an angle on the head 9on the non-firing side), varying from very shallow 10 deg or even none at all (penny on a stick type) to quite a steep angle - 20deg or more. I often put a 30 deg back grind on TC valves and others that, as standard, have a head angle of 15 deg (measured relative to the firing side of the valve), and a quite wide 45 deg contact face. This has two benefits. It reduces the width of the valve contact face (it should be the same width as the face in the head or fractionally smaller) and, importantly - if combined with a 30 deg top grind around the seat it enhances the flow regime thru the valve curtain area at lower lifts. Moreover - of course - it will deshroud the area you refer to, and at the same time, if grind right onto the contact face it will assure that the top edge is actually concentric with the guide and perpendicular to the valve axis, because without doing a top grind (or cut) you can end up with a contact face between valve and seat that varies in width all the way round. Mostly my top grinds (except on some multivalve heads where the valves are quite deeply recessed below the chamber) end up 15, 20 or 30 deg. Of course doing a top grind can put the valve deeper in the head so be careful your tip heights don't totally wreck the rocker ratio setup.

On the ex side I would do more or less the same; although the top grind angle is less critical a good aerodynamic lead into the ex throat region helps the ex flow and - importantly - the 'crossflow' on the overlap (in valve to ex valve cylinder purging). That makes a big difference to throttle response. Naturally one must be careful to ensure that the top cut doesn't undercut the gasket fire ring.

I have found that on the inlet side on many heads that the inlet flow is best when the angles are left sharp, not radiused. And yes, a micro boring tool is quicker than grinding and would be ideal.

I hope this gives some ideas to think about, feel free to come back to me.

GC

Hi Guy

WOW, I only hope I can put all that into my heads, fantastic reply.

ALan