The Stovebolt Site Forums The Shop Area 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

Having a flow bench, I would suggest finding one and flowing the head with and without a valve, you may find the air just cannot get to the chamber no matter how much lift or open time you have. I bring this up is because more valve lift doesn't always make for better running engines, it's the amount of flow the ports will handle.
Low lift flow is more important to throttle response then maximum air flow is. The valve is at partial lift twice per cycle, only once at full lift. The more low lift you can get, the better the engines run. Air speed is also very important, the faster you can get through the port, the better the engine will run, crappy port design and air speed do not go together. Bigger holes equal slower air speeds, so that doesn't work either.
Once in the cylinder, the air has to get out, look for 75-85% exhaust flow to intake flow or the engines will not run as they should. To much exhaust flow is as bad as not enough.

A shop vac and a digital manometer makes a pretty good field-expedient flow tester- - - -just lock the valve open at whatever lift you want and monitor the pressure rise in the port.
Jerry

Curiously enough, many of the cam lobe shapes in major cam mfg. "library" designed for use with mushroom tappets were originally for a very obsolete engine: the Ford Model "T" (a sidevalve L4) with very bad breathing but very compact and rigid valve gear. The Model "T"'s .970" tappet foot was re-cycled for use in other engines.
This means that an existing lobe profile design for the .970" mushroom is safe to use in the stovebolt and GMC engines with their .990" tappets (a narrower tappet would dig in to the side of the lobe). A wider foot will not change the lift or duration, but will allow far more violent acceleration in lobe design - the rising lobe contacts the leading edge of the tappet foot significantly ahead of a smaller tappet, and has a longer path before passing under the trailing edge.
Compared to the usual SBC/Gen-3 .842" tappet (frequently used as the model for current stovebolt cams - ask your grinder what it's based on), the .970" tappet allows about 15% greater maximum velocity.
The roller wheel on a tappet has 2 functions:
1. anti-friction, no break-in, much less chance of scuffing, etc.
2. completely changes when & where the tappet moves vertically w/r/t the lobe's rotational position; a flat tappet (solid, hydraulic or mushroom) always follows the highest point of the cam lobe, while a roller tappet (solid or hydraulic) always follows the closest tangential point on the lobe (which will differ slightly with roller wheel diameters).
The downside is very expensive heavy tappets and a billet cam.

As an aside: a solid (no moving parts) but radiused tappet has been used for many decades in English motorcycles (Triumph, BSA, Norton). This allows more choices of lobe shape, acceleration etc. as described for roller tappets, but of course still has sliding friction, break-in problems, life expectancy etc. This was dealt with by use of very light components, low ratio rocker arms (which reduces the spring load on the tappet) and frequent oil changes!
Triumph also used at least 2 radii for the contact surface: “Std.” with ¾” radius and “R” with 1â…›” radius. As described, an “R” tappet will produce slightly more duration than a “Std.” tappet on the same lobe. These could be interchanged to make minor tweaks in the engine's "tune up" without replacing or re-timing the cam.
Harley-Davidson used an asymmetrical mushroom tappet from the 1941-52 in the 45" race engines. This had a .731" stem (body diameter) but the "foot" was over 1" in length.
Obviously, any non-rotating tappet needs an alignment fixture.
Yes, they both developed a flat spot in the contact surface half-way along where the lobe peak landed.

Chet Herbert was the only one I knew to have ever made a commercially offered set of roller lifters for an early inline chevy six. I have only seen a photo of one set and have never had a set in my hands. Chet reground a pair of cams for me a good many years ago. They were really good cams. He used to advertise in Hot Rod that he would grind any cam with any grind for $39.00. He offered that service at that price well into the middle 1980s. I actually had a very pleasant telephone conversation with him around 1983 when I wanted to get a roller cam and lifters for the chevy but alas, no longer available and too expensive to start production over so I was stuck with a flat tappet mechanical cam. Interestingly enough, I have run isky, a howard, a couple of Clifford cams (which I believe were ground by Erson), prior to having my current cams ground by Schneider. The Herbert cam was way better then the Howard and isky, and on par with and maybe even a little better then the Clifford depending on the grind. The Schneider cam that is in my 261 is a pretty good cam, really good actually. That being said, a roller lifter would be pretty nice if you could get them to live in a street motor and take advantage of the lift profiles that may be possible with that kind of cam. Indeed, all those cams were a learning experience, trial and error as I never had access to a flow bench and dyno. So I go back to my earlier comment that a flow bench and dyno would be handy right about now, lol.

As an edit, I was looking through some of my old literature and it looks like McGurk also offered a roller lifter.

Chevy makes millions of roller lifters for their production engines- - - - -they're even hydraulic. Why not sleeve down the lifter bores in a stovebolt and have somebody grind a few different cam profiles for dyno testing? Even a reground stovebolt flat tappet cam would probably survive enough dyno time to get an idea what the grind needs to be, then contract for a small production run of dedicated roller cams.

Doesn't everybody have a flow bench and a dyno like mine?
Jerry