run a 258 with a 4.0 cam with the 258 head?

Canoe · September 10, 2012, 04:32:45 PM

If getting a 258 block rebuilt, anticipating a 4.0 head added later, can I get it built with the 4.0 cam and run it with the stock 258 head for a while?

captspillane · September 10, 2012, 06:28:33 PM

Yes. The only potential issue is the length of the lifters and pushrods. A 4.0 lifter and pushrod together looked exactly the same as a 258 lifter and pushrod (individually they are about a 1/16th of an inch different, but both together are the same distance cam lobe to rocker arm). The rocker arms are also slightly different but exactly the same in the critical dimensions.

I know of one exception. There are two different lengths of pushrods for 258's. I think a casting in 1981 was slightly off and AMC compensated by having a slightly longer or shorter pushrod. I would stick with a later 258 head 1982 and up. Does anyone else here remember the specifics to that early oddball head? It's prone to having the pushrods pop out of place.

amcinstaller · September 10, 2012, 08:47:47 PM

also 4.0 cam has no fuel pump lobe

carnuck · September 13, 2012, 09:43:13 PM

You have to pull the head to swap the cam

Unless you:
pull the valvecover and oilpan,
flip the motor upside down,
undo the rockers,
remove the cam,
then roll the motor upright again to let the lifters fall out.
Install the new lifters from the bottom and put the pushrods in to hold it in place until the cam is installed,
then roll it over one more time, make sure the lifters are in place (I used a magnet and long screwdriver to align them to the holes when a pushrod/rocker broker and the lifter popped out of the bore) and finish assembling.
You could do it on it's side (dist side up) to avoid having to turn it.

amcinstaller · September 14, 2012, 07:21:33 PM

Quote from: Canoe on September 10, 2012, 04:32:45 PM
If getting a 258 block rebuilt,

its already apart, that should save alot of work

amcinstaller · September 14, 2012, 07:23:06 PM

also, you cant use the old 258 lifters on the 4.0 cam. once a lifter is used on a cam, its mated to that cam, and that lobe. either get new lifters or use the lifters on the same lobe they were on in the 4.0

Canoe · September 17, 2012, 12:04:53 PM

Getting all new lifters and rods.
He'll be looking at the 7120's rockers, to grind or new.

carnuck · September 17, 2012, 12:13:45 PM

Or yellaterra roller rockers if still available

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run a 258 with a 4.0 cam with the 258 head?