Engine 289s W/ Gt40 Heads. What Pushrod Length Did You Use.

Discussion in 'Classic Mustang Specific Tech' started by ks65stang, Sep 9, 2012.

  1. Attached are a few pics of my GT40 heads off a 93 302 cobra installed on my 289 w/ Scorpion roller rockers. My stock rods are too short. I just got an adjustable push rod and I'm gonna mess with it till I get a good wear pattern on the valve tips. Any suggestions on what length I should start with or you used?

    Also what budget friendly push rod brand do you suggest?

    Installation instructions for Scorpion Non-Adjustable Rocker Arms:
    Working on one cylinder at a time, watch the intake pushrod while rotating the engine in the normal direction of rotation. When the pushrod moves up and then all the way down (intake valve just closes) install the exhaust rocker. Make sure the radius on the pedestal mates with the radius on the rocker fulcrum - the flat on the fulcrum goes up against the head of the attaching screw. Lube with Scorpion Cam & Lifter Installation Lubricant (part numbers SRPAL4-1 or SRPAL8-1) and lightly snug the rocker arm attaching screw with your fingers until the up and down looseness of the rocker is taken up (zero lash). Setting your torque wrench to 18 to 20 ft. lbs., turn the screw until the torque is reached. The screw should turn one half to one full turn between zero lash and torqued. This will give the proper lifter preload. If there is less than one half turn, a longer pushrod is needed. If there is more than one turn, shimming of the rocker stand or a shorter pushrod is needed. Scorpion Performance offers shims in .020” thickness, part number SCPRS.020 (32 pieces).

    Attached Files:

  2. My push rod length at zero lash and no pre-load on the lifters is about 6.87" and per my camshaft instructions acceptable preload is .030-.060" on the lifters. The Scorpion tech guy said .030-.080" of lifter preload is acceptable.

    I ordered the push rods below.


    Push rods specs:
    Comp Cams #249-7828-16
    High Energy Pushrod Set
    • Ford 255-302ci 1968-85
    • 5/16'' Diameter
    • 6.936'' Intake; 6.936'' Exhaust (+.060'' Long)
    • End Type: H-H
    • 16/pkg.

    This puts my push rod length now at 6.93 and preload on the lifters at 0.060" (the max). I may shim the pedestals up .020 if I feel needed.

    I just wanted to post incase anyone else decides to run GT40 heads on a 289. If I have issues I will post.
  3. Curious, did you change pistons to get your compression back or are you counting on it being lower? I looked at doing this to my early 302 ,but the huge chamber would have reduced my compression, so I opted to go to 289 heads and get a bump in compression.

    Let me know if those pushrods work out, I know if I were building a 289 pr a 302 those heads would be a good choice if I was able to pick my compression with piston dome or dish.
  4. You always want to measure the correct pushrod length. Spend $20 on an adjustable length pushrod checker, mock up the valvetrain, and then buy the correct one. Tolerances of the components parts are quite loose. What worked for one person cannot be assumed to be correct for the next, especially when using 20-40 year old parts. The engine may have been decked, heads milled, etc. It's one of the fun detailed things you should do, like degreeing the cam, checking piston to valve clearance, making sure the oil pump pickup is the correct distance from the bottom of the oil pan, etc. That's where you get the real value out of doing it yourself: satisfaction in knowing you checked everything and it is spot-on.
  5. I trusted my machine shops opinion on the pistons.....

    My original pistons where dished about the size of 2 half dollars side by side and maybe 1/8" deep (photo attached). My new pistons are .040" over due to the new bore and flat tops. My machine shop says this should make up the difference in compression and from eye balling it, I would say he is correct but I have no exact measurement or science here. If I loose a little compression that will be oK, its a street car and I would like to run 87 octane anyways.

    I will post a photo of the new pistons when I do my compression check w/ clay hopefully this weekend. Wish me luck...

    Attached Files:

  6. Did my piston to valve clearance test and the exhaust valve barley touched my clay and I got a minimum of 0.134" of clearance on the intake. See the attached pics.

    Do you think I could have gotta away with 1.7 ratio rockers? I may see if I can get a deal on 1.7's in the mean time and sell the 1.6's. I got a steal on the 1.6 rockers anyways, paid $195 for a new but open item on eBay.

    Attached Files:

  7. I assume you are running hydraulic lifters? If so, did you convert to solid or install checking springs on the valves prior to doing the p-v clearence test? Just making sure, because the hydraulic lifters can collapse under pressure from the valve springs and give you a false reading.

    To answer your question about higher ratio rockers - maybe. You could certainly try them and see. Be certain that the clearance numbers you are getting are correct before buying 1.7s and keep the 1.6s until you know the 1.7s will work. I like your idea of trying the 1.7s though.
  8. I was gonna convert to a solid lifter for the clearance test but a Scorpion tech said that it would be inaccurate due to the lifter seat can be at a different heights than the hydraulic lifter. So I extended my adjustable push rod to bottom out the lifter on a flat side of the cam and then checked the clearance. I was a little worried about doing this wondering if it would damage the lifter? I have another lifter set that came w/ my rebuild kit if I need them.
  9. Sounds like your clearance numbers are correct. No - you didn't damage the lifter.