Topic: Piston to valve clearance at TDC?

[CrazyRocketMan]

Hey guys,

So I'm trying to calculate how much piston to valve clearance I will need for intake and exhaust with a 125-75 cam and also a 125-70 cam (.400"in .375"ex for both). I do not have either one on hand yet so..

I guess I'm having trouble figuring the whole timing and lift thing and I'm just not sure how to figure out how much intake/exhaust will be open at TDC and then how much clearance is fair in addition.

Unless someone knows lobe lift @ TDC for various centerlines for these cams. (Megacycle doesn't publish this info = weird)

Can anyone help shed some light for me?

Thanks in advance guys,

 - George

[Powderman]

None of your questions can be answered without knowing your cam specs.You need about .060 clearance regardless of specs.

[Sam Green Racing]

At TDC, both valves should be closed but if you are reffering to the valves in the other cylinder, here is a post from Big J that might help.

On rockerarm motors it is not necessary to use clay when checking piston to valve. You can just put the cylinder in the overlap phase and put an indicator on the retainer.  Using a makeshift tool you just pull the rocker arm down until the valve touches the piston while taking a direct reading on the indicator.

Sam.

[CrazyRocketMan]

Thanks Sam,

I thought they would be fully closed at TDC. At least this is what I remember from classes I took 25 years ago. 

However, after trying to refresh my memory by reading up again the following description (exhaust stroke portion) has me slightly confused.

Is this bad info?

1    Power stroke. The piston is at top dead center, intake and exhaust valves are both closed and the spark plug has just fired. The expansion of the ignited fuel/air mixture forces the piston down. Before the piston reaches bottom dead center, the exhaust valve starts to open.

2.    Exhaust stroke. The piston is at bottom dead center and starts to come back up. The exhaust valve opens fully and starts to go closed. Before the piston reaches TDC, the intake valve starts to open and the exhaust valve is still partially open.

3.    Intake stroke. The piston is now at TDC, both the intake and exhaust valves are partially open. As the piston travels back down the cylinder, the exhaust valve goes fully shut and the intake valve goes fully open and starts to shut.

4.    Compression stroke. The piston is at BDC and starts to travel up the cylinder. The exhaust valve is still shut and the intake valve goes fully shut.

 - George

[Sam Green Racing]

That's about it George but unless you have lumpy pistons or have had the head skimmed to raise the commpression, you shoulden't have any problems with the 70/75 cams.
I have had 60/65 and 70/75 cams and they have been used with after market 10.5 to 1 pistons with no trouble. I cant remember the lash settings but I am sure Megacycle suplied them.
Ask Mike Rieck or Billy Benton, I'm sure they will put you on the right track.

Sam.

[CrazyRocketMan]

Sam you are awesome. That info helps ease my mind a bit.

I called megacycle prior to your response. They are going to work out valve lift @ TDC for me. Since we are working with SOHC's there is not much leeway as far as centerlines and overlap. That will make things easier.

As soon as I have the numbers I will post for others to use in their builds. Should help those interested in the 125-75 (125-70 hardweld) looking for really tight quench in their motors.

Thanks again!

 - George