Hey guys,
First time I've posted here in power adder, most of the time I'm posting in TPI. Anyhow, I had a problem with my engine and I was hoping that you could help me. I just pulled my recently rebuilt engine because the 6th cylinder is dead and I brought it back to the machine shop. They took a look at it today and they said that the exhaust valve is hitting the piston.
I've got an SLP cam and 1.6 ratio rockers so I'm guessing this is where I *should* start. I'm thinking if I lower the ratio to 1.5 this would be a good solution.
Now I'm also thinking if I change the cam timing by a degree, I might be able to avoid the exhaust valve hitting the piston. My question is, what are the benefits and or losses that would come from advancing the timing by a degree? I guess I could be putting my intake valve in jeopardy if I were to do that? (my timing set has keyways for advancing and retarding the timing by a degree)

Thanks in advance,

-Bruce

 

Bruce, I see where you're going here. I have been in similar situations myself. Advance the cam and the exhaust valve gets closed sooner- less chance of slamming a piston into the exhaust valve near TDC on the exhaust stroke.

Don't do it. That is an absolute last resort. The slight advancing of the cam will not make much difference. Instead....

Are you sure you've got the right valve springs for the cam? Weak or imporperly matched springs can cause the valve to bounce as it hits it seat and slam right back into the piston.

1.5 rocker arms will help, if you're willing to downgrade to them but they will also make not usually make a ton of difference, like retarding the cam. But this depends somewhat on the actual specs of the cam- some might see more difference than others.

Did you degree the cam??? If it's actually installed several degrees retarded due to any number of factors you might have your answer right there. (Improperlerly located woodruff keyway in the crank, improperly indexed lower timing chain gear, improperly indexed upper timing chain gear, cam that's not ground to spec, etc.) There's a reason you want to properly degree a cam and make sure it's all exactly to spec versus true piston TDC. This is one reason why.

You should "mock up" the final engine assembly (using solid lifters on the cam and zero lash on the rockers temporarily) and put a blob of clay on top of the piston. Rotate the engine over several times and the valves will imprint on the clay showing you positively how much valve-piston clearance you have.

If you have checked all of that and can't get the clearance you need then your only recourse is to machine the valve reliefs deeper in the piston or choose a different cam. Ouch!