Piston-Valve clearance when miling.

[Fatman]

At what rate does piston to valve clearance close up when flat milling 23 degree heads?

Did a quick calculation and got that piston to valve would close up by .97 thou for each 1 thou that was flat milled. Is this right?

[MadBill]

OK, since no one else has stepped forward, I'll take a crack at boldly going where I haven't been for 40 years (The Dreaded Den of Trignometry): My dim recollection is that Cosine of an angle is the adjacent side of the triangle over the hypotenuse, so if the amonut milled is 'adjacent', and the 'hypotenuse' (valve stem) is angled off at 23 degrees, my scientific calculator says Cosine 23 = 0.920, so I believe a 0.010" cut would close up the clearance by 0.010/0.92 = 0.01086"

[Fatman]

Thanks for having a crack mad bill but i think that you piston to valve clearance would actually decrease by less than what you are milling, not more.

If my numbers are right, it's not really much of an issue. I was hoping that a .050" flat cut might see piston /valve clearance close up by .040" or so. something significant. Not .0485" which wont make much difference to most builders.

Anybody else? am i on track here or not even in the ballpark?

[MadBill]

Thanks for having a crack mad bill but i think that you piston to valve clearance would actually decrease by less than what you are milling, not more.

If my numbers are right, it's not really much of an issue. I was hoping that a .050" flat cut might see piston /valve clearance close up by .040" or so. something significant. Not .0485" which wont make much difference to most builders.

Anybody else? am i on track here or not even in the ballpark?

Well, other voices are more than welcome, as I've been wrong before at least twice , but my reality check goes like this: If the valve guide was at zero degrees (i.e. parallel to the bore) it would see a one for one reduction. If it was at 89 degrees, moving almost horizontally across the top of the piston, starting say 0.005" above the deck at zero lift and finally contacting it aftere 1.0" of lift, then milling the head 0.005" would reduce the valve clearance to zero. Therefore any angle between zero and 90 would cause a clearance loss of no less than 100% and up to infinity vs. the amount milled.
If the pistons are not above the deck (or if you set one to 0.000" deck clearance just for the test), you could always check the clearance 'as-is', then repeat with the gasket removed. More scientific: Set a valve in the head on its seat, with a dial indicator on the tip, and another, fitted with a wide contact foot, on the rim of the valve head with its travel perpendicular to the cylinder head surface. Raise the valve say 100 thous off its seat and check the resultant travel in the plane of the bore.
Do let us know your findings!

[jacksoni]

Once again probably opening my mouth with no knowledge but seems to me if you flat mill it the valve moves to the bore the amount you mill. the valve angle makes no difference. The distance to the piston parallel to the stem may not change the same amount but the edge of the valve to the piston in the bottome of the valve pocket will. If you angle mill it might make a difference but on the flat when the valve is open and has x clearance to the piston (at the edge of the valve), you move the head y closer to the deck the valve moves y closer to the piston. I think you are confusing trying to figure what cc the chamber changes when you flat vs angle mill the head.

JMHO

[SupStk]

Common sense would indicate the edge of the valve would get closer to the piston by the same amount as milled. I've seen pistons where there isn't enough room to change valve location on the radial axis by .050

Considering piston to valve clearance on the valve guide axis that number would be reduced less than a one to one basis.

[Wolfplace]

You all are makin my head hurt

I just use 1 to 1 & measure it when I am done,,

[jacksoni]

You all are makin my head hurt

,

[LilRacr]

Yep, you guys are thinking too much, I also just figure 1:1 ratio.

I agree if you are flat milling the head the valve height changes the amount milled from the head.

[Fatman]

Madbill

When i said piston to valve clearance, i meant between the flat valve face to the flat piston relief that you would find on a typical 23 deg dome piston. Not the clearance between the valve margin and the piston. Sorry bout that.

Using your method of thinking. If i'm correct, a 0 deg valve angle will have a 1:1 drop in pv clearance for the amount milled. A 90 deg valve angle (perpendicular) to the bore would see the valve face get no closer to the piston relief regardless of how much you milled.

If the difference between the amout milled and amount that the piston to valve has closed up is insignificant then it really aint a big deal. Were in a situation where we have .120" p/v clearance on the intake and have to flat mill for max comp. Happy to pull that p/v clearance down to .080". Was going to go .040 off but then thought that if you took valve angle into account then more could come off. If it's .041 as i calculated then who cares. But if it's .045" then its worth worrying about.

[MadBill]

I really think you should set up one of the tests I outlined. You could even just lay a head on the bench with a valve in it and a couple of thin spacers under it, check the valve travel to contact with the bench top, pull out the spacers and re-check. Then you won't need anyone's opinion or common sense assumptions, you'll have the real data. Remember, it's the available valve travel that matters, not the static vertical clearance.
However the difference is small at any real world Chev valve angles and 5 or even 10 thous variation doesn't mean that much when you miss a shift; it's still a crap shoot as to whether or not you get away with it...

[Fatman]

MadBill.

Wouldn't sitting it on a bench give a 1:1 ratio when the spacers are pulled out?

think the best would be on a short with a domed piston. Put some feeler guages under the head, measure, pull them out and measure again. That way you are measuring between the valve face and dome rather than the margin and deck.

[Rick360]

Cosine (Valve Angle) x Amount Milled = Amount valve is closer to piston valve relief.

This is just for the flat part of the piston valve relief to head of valve, not at the lower edge of the valve and valve pocket.

Example:

23* sbc milled .050"

Cosine (23*) x .050"

(0.9205 x .050") = 0.046" closer

Rick

[Fatman]

Thanks Rick

drew my diagram again and came up with

sine 67* x amount milled

same as your cosine 23* x amount milled.

Don't know how i got my numbers in my first post.

Worth worring about especially if you're going a big flat mill. save some time a least anyway.