Pushrod length and lifter preload question: '68 Cougar 302 Retrofit Hydraulic Roller

[CaliCougarXR768]

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!

[Royce]

I did this exact retrofit installing a 347 stroker engine in place of a 289. The shorter pushrods were described in Summit’s catalog so all I had to do was order them. I did not need to know the exact length.

[9F91H581221]

If the block or heads have been decked, or a valve job done that changes stem tip height, you should definitely check for proper pushrod length. Your rocker tip to stem pattern will be affected by the rocker height on the stud, preload won’t change that. If you have non-adjustable valvetrain, you might need to shim the stands to adjust. With studs and nuts or poly-locks, you can move each rocker up and down to optimize contact pattern. Longer push rods won’t affect the tip pattern unless they are so long, it bottoms out the lifter—–then you have other problems as well. But, that is a lot more than .030”. I’ve never used a pushrod length checking tool to check rocker tip to valve stem tip contact. Perhaps you can, with light valve springs. Some of these tools are aluminum and it might not stand up to opening heavy springs. Either way, if you are happy with the pattern with your rockers in the position they are in, measure the tool length and add the preload measurement. With the hydraulic cam, If there is no preload, it will make noise and leak oil internally. Using the search at Summit’s website, you can filter down to (most likely) get the exact push rod you are looking for.

[CaliCougarXR768]

Thanks for the responses, folks. I should have added that I am using my stock rockers. I did not know that the contact patch on the valve is a bigger surface area due to its more sloped patch. Here are some pictures.

I got the numbers wrong in some of the photos; all lengths are 6 inches, plus several thousandths. I tried 6.300, 6.248 (85 FORD 302 Hydraulic roller height), and 6.270, which is a size my engine shop gave me. To me, it seems like the patch is hitting the outside of the valve stem, so my thinking is I should go with the 85 Ford height of 6.248. What do you all think?

[9F91H581221]

Lay a straight edge across the top of the valve stems. See if the tips are all the same height. If they are different heights, you might need 16 different length push rods. Or, if none of the patterns go off the edge of the valve, find the pattern best centered and order a set that length. With the adjustable rockers, you can find zero lash and add a 1/4 turn of preload. It will be fine.