In a few weeks I'll be to the point where I need to measure my new motor for pushrods. I read up on the procedure and have an adjustable length pushrod already, but have probably a very stupid question about measuring pushrod length with hydraulic lifters. Since I'm running hydraulic lifters, won't the lifter compress when valve spring pressure gets on it and therefore make the lifter 'shorter' than it would be with the motor running -- and therefore make it appear that I need a longer pushrod than I really do?

I have to be missing something very basic here...

Thanks!

 

Mark:

I’m not quite sure I understand your question. When you measure push rod length you should be on the base circle of the cam for the valve you are measuring push rod length and the rocker should be off (or loose). Insert your adjustable push rod at this point. Reinstall your rocker and tighten until the rocker just takes up the slack. Check your rocker geometry and position of the rocker on the valve stem. Adjust your push rod length until you have the correct geometry and position. If you really want to get exact, add another .030 to .060” to the length for pre load. (Remember when you final install the push rods to add the pre load which is roughly ½ to 1 turn more on the rocker nut. Check you lifter instructions.)

What used to throw me for a loop is you have to be careful to not compress the lifter as you take this measurement. The lifter has a light spring that will keep it at its uncompressed height, but you can squeeze the oil out of the lifter and compress it. I’ve found it takes a delicate touch to make sure the lifter plunger is not depressed. No big deal if you go too far, back it off and wait a minute or so for the lifter to uncompress.

Here’s a good write up from Crane: http://www.cranecams.com/?show=hydraulicLifters

 

Mark,
Not exactly sure how you are doing this, but there are a couple of ways. I prefer to check pushrod length by using the soft springs that come with most timing degree wheel kits for the intake and exhaust valves. This reduces any negligable lifter pressure. Mark the complete valve tip with magic marker and carefully install the roller rockers and adjust them to suit 0 or the desired lash. You should get a good finger feel for the adjustment since normally the valve spring will start to compress before the lifter does but look at the lifter plunger in the process to make sure it isn't compressing too. I usual run about 1/2 turn lash on stock roller lifters in operation.(Your choice). I then crank the engine over about 5 or 10 seconds with no plugs in the engine to allow maximum freewheel of the crank (about 180 rpm's) and reduce battery load. This should give you enough cranking oil pressure to insure lifter is extended. Remove the rocker and inspect contact pattern for an acceptable narrow width and center location. Make pushrod length adjustments as necessary and repeat. Wipe off old marks and remark for next attempt. I normally do this on the engine stand using a spare starter and battery setup which makes it easier to do than in the car.Hope this helps.
Good luck.
Rick

 

Thanks guys -- this gives me a much better idea!