Adjustable rocker arm convertion?

[monsterbronc]

My tired ole 258 heads got valve issues, and I want to swap in the 4.0 head, the problem is Ive already got .050 milled off the block, and will most likely mill the 4.0 head before I install it, and Ive already spaced the rockers .050.

custom push-tubes dont seem expensive till you have to buy 12 of them, at 4-5$ a piece, it can add up. but a stock set of used rockers from a bone yard would be far cheaper I would think. I want to go with adjustable rockers like on a SB Chevy, I know they wont fit, but just loosen till they tick and tighten them half a turn, you're done. But How about the 1.7 ratio rockers off a GM inline 6? they are self centering rockers similar to the ole small-blocks, and a 1.7 ratio compared to the 4.0's 1.5 ratio is sure to be beneficial, Has anybody tried this yet?

Or might I end up with spring, seal, or piston clearance issues?

[rollguy]

I don't think that valve to piston clearance is an issue, but the springs may coil bind if too much lift from the higher ratio rockers.  You could find the spec's of the cam you are using, and the installed height of the springs and do the math.  There may be enough room to run the higher ratio rockers with room to spare, but I would check first.  Who knows, that extra lift might add performance!

[monsterbronc]

Ok, Ford 400M pushtubes are roughly 0.125 shorter, so take the 0.050 off the block, and 0.050 off the head, that leaves me with .025 (ish, Ill have to mic) and the range for preload was pretty forgiving except when I put in the cam, I was already just over the spec, hence the shims. Ill have to check out the preload specs again, but 0.025 should be well within my safe zone.

http://www.compcams.com/Technical/Instructions/Files/145.pdf

Yup, 0.030, +/- 0.010. Im gonna have to measure the difference in lengths though to be sure, but this should put me right at 0.025 preload, hold on and Ill research it.

[monsterbronc]

Hmmm? stock pushtube length for a 258 is 9.7 inches

Stock pushtube length for a ford 400M (P# D1AZ6565A) 9.51 inches

so the ford pushtubes I scrounged are 0.19 shorter,  Darn  they might be too short

[eaglefreek]

The best way is to get a pushrod length checker. Trend makes a good one. Not sure of the price, you have to contact them to buy one. http://www.trendperform.com/index.php?go=tools Then you can order custom length pusrods. The real precise guys will either use shims under the rockers pedestals or machine some off the the pedestals mount to get the rocker geometry correct and then measure for pushrods. Adjustable rockers are available from a few sources. You can check the web for that info.