Hello folks, I have a stock 1968 Toronado 455, I changed the camshaft to a Comp Cam 260H grind number camshaft (part number #42-228-4). I am using new Comp Cam OEM style replacement lifters, OEM replacement rocker arms and brackets. I have an adjustable push rod to measure for the ideal length. I have measured for push rods before on small block Chevys but never on a big block Olds. Since the 455 I have is equipped with non-adjustable rockers the measuring method is different that what I have done in the past. Does anyone have a detailed list of steps on how to measure for push rods on a 455 Oldsmobile. Or if someone has used the same setup as what I am using and knows what length of push rods I need to use. Thanks for any help in advance. I attached the cam specs for both stock and aftermarket cams.

 

Page 6
http://www.compcams.com/Instructions/Files/145.pdf

 

Cam specs are irrelevant. The diameter of the base circle is what determines any potential changes.

 

I had seen this document but I was curious if there was another way of doing this. I know that there are always different was of approaching valve train geometry especially on these 455s, but this maybe the only way.

 

www.youtube.com/watch?v=o5is9BsH5OU

Push rod length best explanation

 

Olds with non-adjustable rockers is not a SBC with adjustable rockers.

With non-adjustable rockers, pushrod length does NOTHING except set lifter preload. If you've got "some" preload, but not "excessive" preload...you're pretty much done. Some lifters have more than 1/4" of plunger travel, most have not quite that much, and certain "high performance" hydraulic lifters have only a few thousandths. Typically, the plunger would be depressed 0.050--0.100. Anywhere in that range is fine--so long as the lifter has at least that much plunger travel.

 

Because Oldsmobile rockers are non-adjustable, the cam manufacturers typically adhere to the factory specifications for the camshaft base circle dimensions unless you are running an extreme effort solid roller or something with very high lift (not hydraulic cam ranges), or huge stroker crank that requires a reduced base circle cam.

The only variations you are likely to see here is if the heads were shaved or different thickness head gaskets were used. Even then, you should be in the range of hydraulic lifter tolerance.
The best way to measure to is (with intake off) measure the gap between the lifter snap ring & the plunger. A .040 depression is about ideal. If it is far off, you can buy shims that go under the rocker stands to lessen the preload.
I've read that .020-.070 is an acceptable range.

 

The length of the pushrod must be changed to match the base circle diameter. It doesn't matter if it's a Olds or a Chevy. Even with adjustable rockers, if the base circle is smaller the rocker contact on the valve will be wrong unless the pushrod is changed.

 

True for adjustable rockers. NOT true for non-adjustable rockers (shaft-mounted, or Olds pedestal style) because the rocker pivot is unchanging, so is the contact at the valve tip. Pushrod length in that case changes ONLY the lifter plunger preload.

 

BUT, if you,or in this case" I" am dealing with a cam with a "smaller Base circle" than stock ( Smaller by .150" diameter). Olds 350 Non adj valve train. So wouldn't it require .075" Longer pushrods to make up the difference because the lifter is .075" farther down in the lifter bore ??? thank you

 

You need whatever pushrod length gives you appropriate lifter preload. Pushrods are the LAST thing you buy to complete a long-block, because so many things influence the required pushrod length--base circle, decking the block, decking the heads, head gasket thickness, etc. It's a little easier with non-adjustable pushrods, because rocker-to-valve-tip geometry is "built into" the rocker and pedestal system. 'Course, you still need to have valve tip heights correct.

 

Is there some reason you chose that cam ? Are you towing a trailer ?