I ordered no notched forged pistons and cam from TA while my '73 455 block was at the machine shop. TA said I wouldn't need notched pistons with the 288-94H cam, Stg 2 SE heads, & +.040 23cc dished forged pistons. The machine shop ended up zero decking my block to get the desired comp ratio of close to 10.5:1 with these low comp pistons. My question is will I need to worry about piston to valve interference since I had the block zero decked? I believe they machined off somewhere around .040" to get zero. Thanks Cam specs: Valve lift; .488 IN & EXH Duration; .230 IN, .240 EXH
Never assume anything when it comes to piston to valve clearance! Maybe the the dish in the piston is set up to act as valve notches l can't say , but you need to mock up the motor with very light checking springs , with out a head gasket in place , some clay in the chamber and check , check it twice in fact! The most critical clearance is the Intake valve as the valve is opening when the piston is rushing up, with the Exh valve the piston and valve are in the process of seperating when the closest point of possible contact comes into play!
Thanks guys. I guess I was hoping that someone would say that with this combo, I would have plenty of clearance.
Call ta, ask how deep the valve reliefs are compared tho the flat tops.......if close to the same depth, many ppl run over .600 lift with flats at or near 0 deck with no issues of piston to valve The only 100% way to know is measure. We just put my 464 together with a set of protrude flats, having 8 3 cc in the valve reliefs. Cam with 1.65 rocker is going near .620/.635 lift .040 gasket. 010 below 0 deck, no issues on mine
I believe that valve lift has little to nothing to do with valve to piston contact. My take on it here at the shop is the actual valve timing events that take place. Most contact would take place at about 10* ATDC on the intake stroke. At that point the Intake valve is chasing the piston (the piston is on it's way down, ... Not Up). I would check the exhaust at about 108* BTDC on the exhaust stroke. Of course if some folks are new to it then it would be advisable to simply rotate the crankshaft slowly and keep checking all throughout the piston travel (making sure to Stop if there is any contact!). Using light springs, degree wheel and indicator is a good way to check and the clay method is good for checking radial contact. Larry
Your right, my brain wasn't working at all this morning after being up all night with sick kid, the duration and opening and closing with events would be were the issue is
A flat tappet Hydro Cam with 175 duration @ .050" lift will have far more pTv clearance then a average stout race Roller with 195 to 200 duration @ .050"!
Clay on the top of the piston, not in the chamber. Either use 2 solid lifters OR take apart 2 hyd. lifters leave the spring out, find something that will fit inside them so that the cup can rest on top solidly(I have used aluminum foil before folded to fit in the lifter so the cup can rest on top). Setup the cylinder to be checked with adjustable pushrods(you'll need 2) so that with the cam on its base circle that all the slack is taken out of the pushrods. And as mentioned, replace the springs on the cylinder you're checking with light checking springs so you can feel the difference of the clay compressing and the valve hitting the piston. After the head is bolted on you'll want to spin the crank for 2 slow revolutions feeling for the valve compressing the clay or feeling for it stopping on the piston. If you feel that it has stopped, you'll need to remove the heads for inspection, with your setup it probably won't though. After 2 revolutions, remove heads and inspect clay, carefully remove it so you can measure the thickness of it where the valves compressed it. You're looking for .100" for ex. valve clearance and .080" for the intake. You'll have a better feel if there is only the one piston in the block when you do this check. GL
Thanks guys. After I check rod & main bearing clearances I will throw one piston in and check valve clearances. I just talked with Tim @ TA and he said that I have the option to send my pistons back and have them notched if I need to. The Stg 2 SE heads I just ordered are on back order right now so it will be a bit before I get them.
You have that backwards, the intake valve gets closer as the piston goes down after TDC and the piston gets closer to the exhaust valve on the exhaust stroke as the piston is going back to TDC.
my take on this, the intake valve chases the piston when it is most likely to separate from the lobe(float) the piston is chasing the exhaust valve when it is most likely to separate from the lobe( float) to me making exh clearance most critical/least forgiving. Most valves get tagged at some uncontrolled rpm like burn out or insufficient spring pressure.
