01-18-2011 05:38 PM #1
SBC 350 pushrod length
I put a Comp Cams 12-250-3 Hyd. cam in my 350 where a 12-242-2 was. I decided to check pushrod lengths by taking off the #1 exhaust roller rocker and marking the top of the valve with a sharpie. Put it on TDC and re-installed the rocker arm and turned the engine over with the key a couple of rotations. Checked the mark on the top of the valve and it was from the center of the valve stem to slightly towards the exhaust. Everything I have read says that means the pushrods are too long. I bought the Comp Cams pushrod length checker that goes from 6.8" to 7.8". I adjusted it to the same length as my stock pushrods and they showed to be slightly less than to 7.8".
Then I rotated it -.100"= 7.700. It put the mark in the center of the valve. Better, but -.150 seems to put the make from the center of the valve towards the intake. My confusion is there doesn't seem to be a large variety of SHORTER pushrods. There seems to be tons longer than the 7.8" stock length. What is making my setup need shorter pushrods? I don't know if the heads where milled before I got them. They are basically stock 882 1.94 heads. I had hardened seats, new valves, valve springs, and keepers installed, but no milling. Any ideas? Maybe I'm wrong about something. Any help would be great.
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roller rockers----whose and what ratio??
Sorry, I should have been a little more descriptive on my setup. '71 2 bolt 350, 882 heads, 1.94 valves, Comp Cams springs, keepers. Crane roller rockers 1.5 ratio. No screw in studs or guideplates.
If you need shorter pushrods----stack up of tolerances from new seats maybe not being installed deep enough, valves to seat not being proper depth, and the crane rockers not the exact configuration could cause you to have a significant differance in geometry. did you check the valve stem height against specs??
Valve stem height
No, I leave that to the machine shop. It's a reputable shop though. The engine runs fine. I broke in the cam per intructions, Has great power with no valve train noise. I wanted to check the pushrod length for good measure because I had a little valve train noise with the other cam. There are shorter pushrods for purchase, just not the length my test is showing I need. Maybe I will do a couple different valves to make sure it is all of them and not just the one.
Go with it as it is. The roller stayed on the tip of the valve. It will roll across to somewhere on the valve tip, sometimes it goes out, sometimes back sometimes it'll go back and forth - as lomg as it stays on the valve you are there. Since you didn't check the previous cam and its' pushrod geometry who is to say that the new setup isn't a vast improvement.
I agree 100% with Jerry, very likely the machine shop had to sink the valve a bit further into the seat so taht it can seat - we don't like to see that but the other option is new seats/replacement heads (they might make a thicker 'problem solver' valve for all i know) but if the rocker stays on the valve then it is doing its' job.
Okay, I guess I will leave it. That engine sure sounds good with that cam!
at mild lift the rocker needs to be in the center of valve stem how are you working this thru ? you need very lite set up springs on the heads or you just push the push rod cup down in the lifter . when you get it all work out for the right push rod length your checking at O lash so you need to add to this if you want them right as the hyd needs preload off the lifter clip by 030 so they would be +.030 from O lash if you want to go at it the right way but some only 050 longer increments . roller rockers geometry can not be the same form crane to crower to harland sharp and steel stamp with stock length push rods. and you have stack up like jerry said .you can get in to longer valves .smaller basecircle cam. mill heads. deck block .head gaskets .the all adds up. valves up in the sunk in thats why you get the right push rod for the job every job i do this on less bone stock and takes me two hours of screwing around
This is the mark of where the rocker is contacting the valve
yep that not close ......well not for me.. lay out need to be in the center. how are you checking this?? you need a way to set up lite valve springs and test at mild lift .how are you checking this ? with stock spring s and test pushrods ? and hand cranking it thru??
Last edited by pat mccarthy; 01-19-2011 at 07:54 PM.
Pulled off the rocker arm, cooated the top of the valve with a sharpie. Put that cylinder on TDC, put the rocker back on and adjusted it to 1/2 turn after the pushrods stops spinning with my fingers, then turned the engine over a couple of turns by hand. That is the mark it leaves. This is what I read:
well your using a hyd lifter in the block how do you know your not pushing the in the lifter valving and taking up lash ? hard to do with your stock springs on it there is valving in the lifter any were from 060 to 090 then like i said need to be check at mid lift with a set of test springs should be in center of valve stem at mild lift .i do then all the time thats the rigth way. so thats is the only way i do them