Stovebolt.com Forums The Shop Area The Engine Shop Pushrod clearance hole?

Pardon me, I don't have a clean engine apart where I can examine it.
IIRC the pushrod clearance (and oil drain) holes in an 18 bolt head are 9/16". McGurk suggests enlarging them to 3/4" for bigger pushrods, but is this true for both the 15 and 18 bolt heads?
The holes directly below them in the block deck look considerably bigger, but:
1. are they?
2. how big, 11/16"?
3. are they accurately aligned with the head drains, or just bigger so they can be sloppy?

Thanks (yes, this is for the book)!!

I will check that for you when I get home from work tonight. I have a block and head apart so should be no problem. btw, I have 3/8 smith brothers push rods in one of my motors. Its close but they don't seem to be hitting the head or anything, even with a .492 lift cam. I am going to adjust the valves on that motor this weekend and will double check no clearance issues.

Hey Jeff,

I have both an early and late 261 short block handy and the pushrod holes are 5/8" dia. in both. Holes in an 848 head are .550"dia.

Excellent, thanks!

Checked three blocks, a 59 261, 55 235, and a 56 235, the block holes were 5/8. Holes in the head (55 5913, 57 848 and 59 848) as measured from the bottom of the head were slightly smaller at 9/16.

Thanks to everyone. What I'm working on: can Leo Santucci's idea of "breaking" that very long pushrod into separate components (as he did on a 292) to stiffen them up be transferred into the 235? His book has a picture with minimal text but you can guess how it's done. Looks like possible with lots of work, would allow more aggressive cam lobes.

Kirby’s intermediate lifter pushrods set up, interesting. Here is the challenge, at least as I see it. Those heads can’t breath much past 4800 rpm and maybe not even that deep, even with big valves and some porting. The cam I have in one of my 261 motors is 280 degrees and .492 lift. Its an aggressive street cam. I use smith brothers 3/8 moly push rods and their matching ball adjusters and did not enlarge the holes any and four years and last night, no indication of any clearance issues. have a Clifford cam 290 and .512 that I ran years ago before I knew that bigger is not better. Ran that with the smaller TRW replacement push rods and no issues but I would suspect that the higher lift might be close. That being said, the head can’t use the lift. So while it may be possible, the design of the ports is the limiting factor, at least in my opinion.

I agree, the head casting will always be a restriction. In quite modern fashion, the intake valve is very large (area per inch of displacement) and well placed (not near the cylinder wall) exactly as John Kaase suggested. The valve also lifts into open chamber volume with no masking, so more lift is not as useful as in a shrouded chamber (such as Gen-3 250).
However: Spintron has made a liar out of much trust in pushrods, especially big block (BBC, BBM, BBF) with tall decks and 10-11" pushrods. Even 7/16" are willow wands with big valve springs. IMHO much of the success of the LSX is due to the raised cam CL making the 7.4" pushrods nearly immune to bending. Crude math: a 7.4" pushrod (all other variables removed) is about 280% as stiff a 10" pushrod.
My intention was to allow much higher rate of lobe lift (inches per degree of rotation) to create a larger "window" without late IVC closing and big OL. The .990" tappet face makes this possible (as opposed to the .842" Gen-3 tappet).
Making an intermediate pushrod resting inside the head's relief hole allows the lower pushrod (which only reaches from the tappet cup to the block deck) to be less than 1/2 the original length, which increases stiffness 10 times vs. a 5/16" replacement - bending is no longer present.
Another factor is use of (aftermarket, obsolete) high ratio rocker arms such as B&B, which locate the rocker pushrod adjuster closer to the shaft, which reduces maximum pushrod diameter. Using an intermediate allows an upper pushrod to be much smaller for clearance with bending risk.

If I follow your thinking here, and I think I am, it seems to me the exhaust would benefit the most. Funky chamber, shrouded valve. The exhaust has always been horrific in terms of flow and efficiency, made more challenging by the two end ports as singles and the center ports As siameased. So the wider lifter May support a quicker ramp and a longer duration on the exhaust side. Very interesting. A flow bench, cam grinding machine, and a dyno would come in handy right about now.

If you want an extreme rate of lift, why not use a roller lifter? The same multi-piece pushrod setup could be used, or just extend the lifter body up a few inches into the pushrod chamber. Iskenderian used to sell a flathead Ford V8 cam/lifter setup with a radius on the bottom and a groove in the side of the lifter. It had to be pinned into the bore to keep the radiused part of the lifter aligned with the cam lobe. It was a short life race option, but it allowed for an almost square cam lobe. I think Isky called it a "404"- - - - -indicating valve lift in thousandths.

Isky also made cam setups that used mushroom-shaped lifters that had a much larger foot than the body of the lifter. The block had to be spot-faced with a special flycutter to allow the lifters to be inserted from the bottom and rise high enough to install the cam. Those cams had extreme lift rates, as well.
Jerry