Piston to valve clearance

Slysc

Registered User
Does anyone know if I will need to get larger reliefs cut into my pistons for 1.6" exhaust valves and a larger cam?

I bent some pushrods after putting in the 1.6" exhaust valves and going with a .520" lift cam. I haven't been able to figure out what bent the pushrods and someone suggested that my larger valves combined with the bigger cam has got my valves hitting my pistons.

Has anyone done this valve/cam combo before with stock pistons?
 
Hmm?

Thanks for the info. I wonder what bent my pushrods then?

I've got Chevy valves that are about .100" longer than stock,
and 7.25" trickflow .080 wall stock pushrods,
a regrind cam that takes the cam surface down about .020", resurfaced heads about .010",
rocker shims about .110",
Comp cams 942 valve springs.

My car ran real bad when I started it up. Not much power, idled real high, it would overheat and then it wouldn't start at all untill it cooled off. My plugs are black and it smelled real rich when it was running. The boost was real high so I took my valve covers off to see if I had overshimmed my rockers and I noticed the #1 and #5 exhaust pushrods were bent.

I'm stumped.
 
I know from my Chevy days, that when I had a big lift cam (.600+ lift) I used guide plates in the heads for the pushrods. When I rebuilt my SC motor, I noticed there was nothing like this for the pushrod to align on, except where it goes through the head. Just venturing a guess here, and I may be way off my rocker (no pun intended). I know that there are guys out there that have bigger lift than that on their SC motors. Hopefully they respond. Good luck.
 
I run a very high lift cam and I haven't bent any pushrods. I used a one peice manley push rod and haven't had a lick of trouble. Either your rockers are too tight (Could be with those longer valves) or your push rods are too weak. YOu should adjust your valves so that there is about .030 - .050 of pre-load. I like to go a little looser, so I usuallu hover around .045" of pre-load. I was always taught a loose valve is a happy valve (Not out of spec loose but on the edge). The best rule of theumb is tighten the rocker down just until the play in the push rod disappears, then go another half turn. That should be pretty good. Any more you will be too tight, any less and you will be loose. On my re-grind I noticed that the only vlaves I had to shim were the exhaust valves, but I did end up using push rods that were .125" longer than stock. I hope that helps.
 
It depends on high tight the rockers are. If they were so tight that when the cam actuated the lifter and the lifter pushed up on the push rod and then maybe your valve spring started to bind.........something has to give and it is usually the pushrod.
 
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