When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
Tech / General EngineIs your car making a strange sound or won't start? Thinking of adding power with a new combination? Need other technical information or engine specific advice? Don't see another board for your problem? Post it here!
Ignoring the fact that the intake valve seems to have a slightly different pattern than the rest (got me?), it appears every other valve looks like the exhaust valve on this picture.
Looks like the pushrod is too short, as the pattern is wide and sweeps towards the intake.
This is a 7.266" pushrod. What would you increase it to ?
I don't have an adjustable pushrod and don't want to wait a week for one to show up, and then wait another week for more pushrods.
I do have a proform pushrod length tool, the silly plastic thing that slides over the stud and touches the top of the valve, and you measure between the pushrod and the back of the tool. But I can't help but feel like pushrod length is dependent on the rocker used, and it's ratio?
When using the tool, I can fit about a .041" feeler between the ball and the tool. So that would be about 7.307" pushrod. The closest they sell is a 7.300".
Pushrod length should always be chosen to give the smallest pattern on the tip of the valve. The smallest pattern is not always on the center of the valve tip. Your exhaust pattern looks very wide so it definitely needs a different length pushrod.
Your measure method with the mockup rocker tool is being done correctly. It may not be 100% exact like using an adjustable pushrod but it should get you close enough. You'll never get a pushrod the exact length so rounding to the closest available pushrod is acceptable.
7.266" seems like a strange length for a pushrod. OEM?
Pushrod length should always be chosen to give the smallest pattern on the tip of the valve. The smallest pattern is not always on the center of the valve tip. Your exhaust pattern looks very wide so it definitely needs a different length pushrod.
Your measure method with the mockup rocker tool is being done correctly. It may not be 100% exact like using an adjustable pushrod but it should get you close enough. You'll never get a pushrod the exact length so rounding to the closest available pushrod is acceptable.
7.266" seems like a strange length for a pushrod. OEM?
It was the recommended starting point for retrofit hydraulic rollers according to Howards. (running #91364-16).
Well, I ordered some 7.300", since that's sorta what the proform tool is looking for. I'll give that a whirl.
Yea they only sell p-rods from on the shelf in 0.050" increments unless you want special order p-rods. My word is close is good enough and the for modest lift cams that plastic measuring tool does the job.
I'm saying you did the right thing here. Please give us some feedback on what you find though.
BTW the adjustable p-rod method is torture to do with the engine in the car.
If you take a 7.300" nominal push rod and just measure it with a mic, it very well might come out to that; since the hole in the end makes it slightly shorter. GM specs theirs that way. Aftermarket mfrs however, recognizing that the hole might be different sizes in different mfrs or models or end styles, usually use the "theoretical" overall length that it would be if there were no hole there at all. So a stock spec 7.266" might well be a 7.300" theoretical with .017" off of each end due to the hole, which seems entirely reasonable.
Personally I find the plastic gauge thing to be UTTERLY USELESS. Mostly because, if the relative locations of its fulcrum (theoretical since it doesn't pivot), its push rod seat, and its tip at the valve, don't EXACTLY match the corresponding dimensions of whatever rockers you're using it with, it will be WRONG; and because, it doesn't take into account the variation in total lift (it "assumes" ... and we all know where THAT leads ... that it's at half-lift, which would be fine if true, but ONE tool can't possibly be exactly half-lift for .450", .500", .600", etc. valve lifts); and because, it doesn't take into account the possibility of variation in the angle of the valves or the studs; and because, it doesn't take into account the ACTUAL ratio of the rockers you are using.
The ONLY truly accurate and reliable method is an adjustable push rod and some light "checking" springs, using the EXACT surrounding hardware that will be installed: the cam, lifters, valves, studs, & rockers.
Alky is exactly right that the correct indicator of correct geometry is the NARROWEST POSSIBLE sweep, since that means that the sweep is exactly centered at half lift (equal distribution of rocker tip arc length on either side of this point). The location of the pattern on the valve stem is TOTALLY irrelevant, within reason. This results in maximum transfer of lobe lift to valve lift, and minimum "scrubbing" of the rocker tip across the valve stem, and therefore minimum side loading and therefore minimum guide wear. The plastic toy thing will NEVER be able to approach the same level of accuracy except by pure dumb luck. Yes it might be "close enough" in some or even "most" situations; but you have no way of knowing that, for YOUR specific hardware combination. You'd be proceeding on the basis of blind faith, not anything resembling "measurement".
Last edited by sofakingdom; 06-30-2018 at 12:29 PM.
06-28-2018, 08:33 PM
