Hi community,
It’s been a while since I’ve posted here, but I have a question regarding pushrod length.
My Cougar is a matching-numbers '68 XR7. I recently rebuilt the engine, boring it .030 over and replacing the cam. The original engine had been severely neglected; the factory cam actually had chunks of steel missing from the lobes, and the cam bearings were completely shot.
I had the block machined by a local shop, installed new cam bearings, and decided to upgrade to a Melling 24110 hydraulic roller cam. I didn’t want to go significantly wilder than stock because this is a really nice, original car, and I want to keep using all the factory vacuum features. I also kept my stock 1968 cylinder heads but had a valve job done, installed stainless steel valves and seats, and upgraded the springs to match the Melling cam. To tie it all together, I installed a set of Howard’s retrofit link-bar lifters. The bottom end also got new stock-style replacement pistons, King bearings, and a balanced rotating assembly (weighted rods, crank, flywheel, and harmonic balancer).
While I am not new to rebuilding engines (I rebuilt a 289 for a '66 Mustang a few years ago), I didn’t have to adjust the valvetrain geometry on that project. Setting up this new adjustable geometry and choosing the right pushrods is proving to be a bit nerve-wracking.
The machine shop gave me a set of 6.270" checking pushrods to work with. I did the standard pattern test, and they look great. They give a nice, centered witness mark right across the valve tip. However, from what I am reading online, I need to buy a final set that is roughly .030" longer than the checking length to account for the lifter preload.
My concern is this: if I buy a longer pushrod to account for preload, won’t that push the rocker arm up and move my nice, centered witness mark toward the exhaust side of the valve tip? Should I actually look at a shorter set (like a Ford 6.248") so that once preload is applied, it lands closer to the 6.270" witness mark I have right now?
I am a bit stuck. Has anyone out there done a similar small-block Ford setup using a mild retrofit roller cam and Howard’s link-bar lifters? If so, what pushrod length did you end up with, and how did you calculate your preload vs. geometry?
Thanks in advance for your help and advice!