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#82857 - 11/02/14 10:39 AM Piston to valve clearance
70Nova Offline
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Finally a small step forward. Checked piston to valve clerance today with play-doh. I didn't bother measuring what seems to be about a 1/4". HUGE clearance.

I am happy about that, but I have to ask those with more experience, is this normal? This is the first Chevy inline I ever worked on so I'm still a newbie to these engines.

Zero deck, Comp Cams 268H grind with .499" lift. Timed straight up. Looks like there's plenty of room to experiment with cam timing if need be.





Edited by 70Nova (11/02/14 10:39 AM)

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#82863 - 11/02/14 03:21 PM Re: Piston to valve clearance [Re: 70Nova]
Twisted6 I.I #3220 Offline
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most want to see .100 piston to valve clearance. Some will run them tighter.
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#82864 - 11/02/14 03:31 PM Re: Piston to valve clearance [Re: Twisted6 I.I #3220]
CNC-Dude #5585 Offline
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I don't see where your valves even made an impression in the play-doh. Did you use a solid checking lifter? Like Larry said, anything down to .100" is safe.
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#82867 - 11/02/14 06:45 PM Re: Piston to valve clearance [Re: 70Nova]
70Nova Offline
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Valves never hit the dough. Used new lifters and did several rotations to get clearance to zero, adjusting more and more each time. Never came close. the dough is about 1/4 inch thick. You can see where the quench ledge squished part of it thin. The dough you see is how I patted it down before I bolted the head on. Wish I had a solid lifter to play with. I'd sleep better.
Just wondering if this much clearance is to be expected in this engine with this lift and zero deck. It would mean it should be easy to run much more lift than my .499" without clearance issues.

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#82868 - 11/02/14 07:13 PM Re: Piston to valve clearance [Re: 70Nova]
CNC-Dude #5585 Offline
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Yeah, you can't accurately check P to V with a hydraulic lifter because it collapses in on itself. Find an old used one and weld it solid or take it apart and put shims in it until its solid.
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#82869 - 11/02/14 07:44 PM Re: Piston to valve clearance [Re: 70Nova]
70Nova Offline
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Well I tried to have it collapse all the way until it had no more to go, very slowly. I could make it "flex" at first after zero lash, but I kept going until there was no more give at all when pushed from the rocker. Now I want to do what you said and take one apart and rig it solid. I got plenty laying around smile

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#82872 - 11/02/14 08:27 PM Re: Piston to valve clearance [Re: 70Nova]
CNC-Dude #5585 Offline
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You may be ok then since you realized the lifter kept bleeding down. But it might be good to double check since you have old one's to play with. These engines typically aren't interference type engines until you start getting really huge with the cams and very large valves, but it's always good to know what you have as a baseline.
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#82879 - 11/03/14 06:47 AM Re: Piston to valve clearance [Re: 70Nova]
THE FRENCHTOWN FLYER Online   content
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I have seen around .300" clearance with streetable camshafts on late Chevies. Big Fords too, for that matter.

Sleep easy.
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#82881 - 11/03/14 06:54 AM Re: Piston to valve clearance [Re: 70Nova]
THE FRENCHTOWN FLYER Online   content
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One more suggestion: set up your valvetrain - spring pressures, valve weights, etc. - so the intake valve will float a little BEFORE THE EXHAUST VALVE does. That gives you an extra margin of safety should the engine over rev as the intake will be tossed off the cam lobe when the piston is further down, resulting in backfires but no valve-to-piston contact, whereas if the EXHAUST valve hangs open when the piston passes thru TDC ugly things start to happen when pistons contact the exhaust valves.


Edited by THE FRENCHTOWN FLYER (11/03/14 06:56 AM)
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#82896 - 11/03/14 04:15 PM Re: Piston to valve clearance [Re: 70Nova]
70Nova Offline
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well an hour of tweaking two old lifters I now have two gutted lifters with a ball bearing and two steel washers inside, to create a solid lifter with identical dimensions to the original. Had nothing better to do tonight. I'll repeat my test later this week.

Interesting bit about the float. The intake valve is heavier by design and should float first anyway, shouldn't it?

After I sort this out I will figure out what length push rods i will need. Suggestions for source and material?

Jan

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#82900 - 11/03/14 06:31 PM Re: Piston to valve clearance [Re: 70Nova]
THE FRENCHTOWN FLYER Online   content
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Originally Posted By: 70Nova
Interesting bit about the float. The intake valve is heavier by design and should float first anyway, shouldn't it?



I can hear it now. "Titanium valves are junk. I got a good deal on some oversized titanium intake valves and when I tried to see how much higher I could rev it the engine blew up."

Usually what I will do is check spring pressures at installed height. The springs with the heaviest pressures I install on the exhaust side.

But to answer your question, yes, the heavier intake valve should float first, assuming the intake and exhaust valve springs are the same and the ramp rates on the cam lobes are the same.


Edited by THE FRENCHTOWN FLYER (11/04/14 05:34 AM)
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#82905 - 11/04/14 12:22 PM Re: Piston to valve clearance [Re: 70Nova]
70Nova Offline
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All springs (and valves) are new and shimmed to about 100lbs. All took about the same amount of shims. Almost everything I can think of on the head has been machined, lol. $$$.

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#82955 - 11/08/14 07:54 AM Re: Piston to valve clearance [Re: 70Nova]
70Nova Offline
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My little helper assured me that there is plenty of clearance. Second test with solid lifters at zero lash. Play doh must be 1/8" to 1/4" thick, no contact.




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