PTV is over 0.200" ?
Safety Car
Joined: May 2006
Posts: 3,696
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From: Potomac MD
Well if you have say 7.35 to get zero lash run that in your piston to valve test using that pushrod. My guess is you will pick up .085 in PTV clearance - as compared to 7.4. .050 x 1.7 = .085. I think if you were with no clearance with the no head gasket and the 7.4s you will be like .125 with the 7.35s and head gasket. Remember the hydraulic roller will find a zero lash equilibrium once you start the engine so the plunger will collapse to account for this.
Thread Starter
Instructor
Joined: Jun 2019
Posts: 189
Likes: 22
So I did that. Solid lifter, 7.35 pr, and 0.050 head gasket and re-measured. The minimum clearance I had was 0.087 +- 0.001. so when I go to a thinner head gasket my minimum will be right around 0.77. Thats a couple thousandths shy of what is safe minimum for intake. I did measure the valve when putting the rocker arm on to make sure there was no negative lash. Your math above makes total sense, now I'm curious why I'm right at (under) the limit for safe intake PTV. I haven't rechecked the exhaust.
Safety Car
Joined: May 2006
Posts: 3,696
Likes: 493
From: Potomac MD
I would go with the thicker gasket the minor compression increase is not much but the .010 clearance is more helpful in your zone. Check the exhaust. I assume you checked the clearance a few degrees before or after TDC as the piston kind of hangs up there but the cam does not it keeps increasing. In general I do .100 and .120 I/E at TDC as this reflects this situation I just mentioned by adding .020 to account for that. Someone else can chime in here that may be more knowledgable.
Thread Starter
Instructor
Joined: Jun 2019
Posts: 189
Likes: 22
Yes I checked it a lot. Admittedly, I don't have a degree wheel. I checked the dial indicator as the valve began to open, and checked it every 0.005" of lift on the valve until I got to under 0.100" of PTV clearance. Then I checked it every 0.001" of lift on the valve until the clearance began to increase again.
I don't really want to use a thicker head gasket if I don't have to because 1) I already bought the Cometic 2) I had planned this build with the increase in compression in mind and have a target DCR I'm trying to achieve.
That being said, I don't want the disaster of my pistons and valves meeting either haha. I would think that 0.76-0.77 would be okay, but sometimes those extra couple thousandths are the difference between catastrophe and no catastrophe. I've seen where some have run as little as 0.065". I'm using .660 BTR Springs so I don't think valve float would be an issue, but I'm only so knowledgeable. Hence me being here and asking everything haha.
I don't really want to use a thicker head gasket if I don't have to because 1) I already bought the Cometic 2) I had planned this build with the increase in compression in mind and have a target DCR I'm trying to achieve.
That being said, I don't want the disaster of my pistons and valves meeting either haha. I would think that 0.76-0.77 would be okay, but sometimes those extra couple thousandths are the difference between catastrophe and no catastrophe. I've seen where some have run as little as 0.065". I'm using .660 BTR Springs so I don't think valve float would be an issue, but I'm only so knowledgeable. Hence me being here and asking everything haha.
