Pushrod Lengths:

Pushrod Length

One thing that most people overlook is the pushrod length. Milling the heads, block, using a thicker or thinner gasket, new valves or even just a valve job can change the required length. Aftermarket heads with longer valves will also change the required length. Any time any part of the valvetrain is changed, the geometry should be checked. Changing the pushrod length is the easiest way to change the rocker to valve tip contact point, in most cases the pushrod length can be adjusted to compensate for best power and reliability. Is some rare cases, the rocker stud might have to be relocated, like when 1.7 big-block rockers are used on a small-block Chevy or when a longer valve is used in a stock head (this is usually only a problem when the valve is +.200" or more).

Checking Pushrod Length

A rockers pivot point is lower than the pushrod cup and the valve contact points for a good reason, so the wiping action across the valve tip is the greatest at the lowest spring pressures, this helps reduce guide wear. When higher than stock lifts or any of the modifications listed above are done, the pushrod might be too long. This would cause more side loads at peak lift, which is where spring forces are the greatest. Imagine a line from the center of the pivot point to the valve tip contact point. As that line passes 90 of the valve stem, the wiping action across the valve tip is minimal. So it makes sense to put that point where the spring forces are highest. For best valve guide life that point should be at about 2/3 of peak lift. If the rocker pivot point is in the correct place, the tip should be in the center of that valve at that point also. To figure the best length, you must use an adjustable pushrod and adjust it until you get what you need, you can then order a set of pushrods the same size as you adjustable one, you should do this with a solid lifter to avoid any leak down and remember add any hydraulic pre load to the length. Manley offers an inexpensive pushrod length checkers for most popular engines, these work fine if you know how to use them, and most people don't. For an example, the tool to check length on a small-block Chevy is set to check correct length with 0.600" lift at the valve. If your valve lift is not 0.600" you will need to do some math, or you can just use it as a starting point to adjust your adjustable pushrod. Manley will provide you with the information needed for your application. On a small-block Chevy, any lift more than 0.600" gets multiplied times 0.22 and that is subtracted from the pushrod length. So if the valve is lifted 0.650", you will need to multiply 0.22 with 0.050 and the pushrod will need to be 0.011" shorter than measured. If the valve is lifted less than 0.600", you take a different approach. Subtract your lift from 0.600 and divide it by 3. Then take a feeler gauge of that thickness and put on the valve tip when checking length. If your valve lift is 0.500" the difference would be 0.100", divide it by three to get 0.033", now insert a feeler gauge of that size between the checker and the valve, and the measured length will be correct. Once you adjust the pushrod to the right length, you can then measure it and order the correct length pushrods.

The pictures shows the Manley pushrod length checker for small-block Chevy engines. The engine here has a cam with 0.510" lift. In order to get the proper length, I need to subtract 0.510 from 0.600 to get 0.090". 0.090 divided by 3 is 0.030. One last thing to consider on a hydraulic cam is that when you pre load the lifters, the plunger will change the pushrod position. A 3/8" rocker stud has 24 threads per inch. You generally pre load the lifters about 1/2 turn, which will put the pushrod about 0.021" lower. This needs to be considered as well. So I need a 0.021" as well as a 0.030" shim on the valve tip to make my measurement. I simply put 0.051" feeler gauge on the tip of the valve between under the checker. Now I just need to adjust the checker pushrod to just touch the checker, lock it, and pull it out to measure it. In my case, with the hydraulic roller, the length cam out to 7.345" long. It so happens that 7.350" is a popular size, and that's pretty darn close, so that's what's going in. Pushrods generally come in 0.050" increments. It's better to go a bit long that short, so it's better to round up unless you are less than 0.015" short.

Geometry For Best Power

Setting the correct pushrod length will get the most mileage out of the valve guides, but when radical cams are used they might not be the best for power output. What many engine builders don't realize is that the rocker ratio is not constant. The rocker ratio changes through the lift cycle due to the geometry, this gives a chance to change the lift curve slightly. On most engines, the peak piston velocity will fall in between 73 and 78 after TDC, so the higher you can get the intake valve lifted at that point the easier it will be to fill the cylinder. Once you get the pushrod length correct, try adjusting the rod a little longer and shorter. See where the point is that you get maximum lift at 75 ATDC. On most engines this should be pretty close to the correct length. You will also find that when geometry is optimized for performance, the point of maximum rocker ratio will be close to TDC. After you find the best lift at 75 ATDC go back and recheck your rocker to valve contact point. The geometry for best power will usually be close to what was measured. Small details like this could mean 10 hp on a 500hp race engine, so it pays to check. I have heard a more than once that the proper length pushrod will give the highest peak lift, this is simply not true. You want as much lift as you can get at peak piston speed, which is the hight airflow demand. With todays technology, we just can't get the valves open fast enough. The point of peak airflow demand comes somewhere between 73 and 78 after TDC, but we can't get peak valve lift until more than 100 after TDC. What I'm getting at is that if the piston speed peaks at 75 ATDC and peak valve lift is at 108 ATDC, extra lift will help more at 75 ATDC than it will at 108 ATDC.

 

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