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Howard, how about some details of what you’ve built here. I’m taking guesses as to why it bent a pushrod based on generally discovered problem areas. You must treat this bent pushrod as a serious problem till you work out the cause. Putting in a fresh rod and recranking if not starting the engine is a receipe for disaster.
1) Bad pushrod that had a manufacturing or material flaw, a one of a kind random event or a generic problem in all the pushrods made by that manufacturer in that batch.
2) Piston meeting open valve and slamming it shut, the reasons for this could be
a. The piston has a dome or insufficient relief to provide clearance during the overlap phase of the cam. The flip side of this is the cam or change to 1.6 to 1 rocker arm has increased lift to where the piston and valve get into each other.
b. The heads or block deck have been milled such that resultant closeness of these parts has eliminated the original clearance of the valve and piston.
c. The valve stem is bent or there is insufficient clearance to the guide causing the valve to stick or bind slowing or preventing its return to the seat in time to miss being hit by the piston.
3) Pushrod binding caused by several things:
a. Increased cam lift or the use of a high ratio rocker has changed the geometry enough to where the pushrod is being bound by its guide that’s now too small to provide clearance. With the valve a max lift there should be at least .060 inch clearance between the pushrod and its guide walls as defined in the direction across a line drawn from the intake to exhaust sides of the head That’s good for heads with cast-in guides or with sheet metal guides. The same must be held for later model engines with self-guiding rockers, where the pushrod passes thru its passage hole in the head, it also needs at least .060 inch of clearance throughout the lift cycle. Also where cast head guides or sheet metal guides are used, the push-rod must be hardened and if only hardened on one end, that end must be up. Also a note about smoothness of sheet metal guides, it’s not uncommon for them to have roughness inside the guide-way from their manufacturing operation, get in there smooth any burrs but don’t go any deeper into the metal as you don’t want to cut through any surface hardening treatment. If sheetmetal guides bind it's time to replace them with a deeper grooved part rather than push a grinder thru the hard face.
4) The rocker arm impacting or binding on other parts can cause a lot of problems.
a. Most frequently the valve side of the rocker becomes bound up when it drives the valve spring retainer into the top end of the valve guide. At full lift there must be at least .060 inch between the bottom of the spring retainer and the top of the guide and that top is defined by whatever type of stem seal is or will be present.
b. Another problem caused by high lift cams or high ratio rockers is the collision between the rocker’s stud slot and the stud itself. At max valve lift there needs to be .060 inch clearance between the slot and the stud as measured on the valve stem side of the slot. This is where a wire gauge is most handy as it can be passed into this gap from under the rocker. I recommend getting some .060 inch wire as it can be cut and bent into some odd shapes that are necessary for the measurements I describe here-in.
c. A place where trouble also crops up with dimensional changes from milling, high lift cams, high ratio rockers, or even changing to a roller rocker of the same 1.5 ratio (1.7 for BBC) is pushrod length. Getting a Manley pushrod checker along with an adjustable pushrod to make these measurements is a good investment. The rocker when properly aligned on the valve stem will make contact at about 1/3 the width of the stem dia on the side toward the intake. As the valve is depressed, the contact pattern will proceed toward the exhaust side of the valve stem. At half lift the rocker should be in the middle of the stem. At full lift it should be on the opposite 1/3 mark of the valve stem towards the exhaust side. From that point it should sweep back across the middle and come to rest back on the intake side of the valve stem at 1/3 the stem diameter. Each and every valve should be checked this way. The same is said for clearance measurements, check everything. If you’re using a really big cam, that is lifts over .5 inch, I highly recommend going with lash caps. These are hardened steel caps that go on the top of the valve stem. They usually require that you order long length stem valves with more distance from the locks to the stem tip to get space for them. These devices provide more area for the rocker to contact helping to keep from the situation where the rocker, especially roller tips from falling off the valve because as the lift goes up so does the sweep distance of the rocker across the stem face. They also protect the stem face from mushrooming which is a problem with aggressive solid tappet cams as they tend to really snap the rocker onto the stem through the lash cycle. The use of stem caps apways requires longer pushrods.
5) Spring and retainer clearance, this is problem area that always needs to be checked on a valve by valve basis.
a. The retainer needs to be checked for clearance to the rocker. There’s a couple problems here; first, if the pushrod is too short the lever arm of the rocker will sit on the outer edge of the retainer toward the rocker side. This can bind the rocker holding the valve open a bit or worse push the retainer down far enough to unlock the locks allowing the valve to drop into the cylinder. Second, the retainer can
become bound by the fulcrum boss of the rocker, this clearance needs to be checked for each and every valve. Again the magic number of .060 inch is a good measure of adequate clearance.
b. The retainer can be jammed into the valve guide boss before
the cam has reached full lift, this will either break the rocker
arm or bend the pushrod.Again that seemingly magic
number of .060 inch of clearance comes in. That’s about
the minimum clearance that’s safe, more is better but not
necessary, lets say .040 is iffy but doable if the lift isn't
exceeding .5 inch and the RPMs are kept under 5500.
As I previously stated somewhere, this measure is from the
bottom of the retainer to the top of whatever you’re using
for a stem seal, not simply from the retainer to the metal top
of the guide.
c. Spring clearance needs to be .050 inch between each coil at maximum lift to prevent the spring from binding. If the spring binds before max lift; either the rocker will break or the pushrod will buckle. Sometimes the stud, if pressed in, will be pulled out or if a screw in, will become bent or broken.
When dealing with multiple springs getting inside them to do
measurements can be difficult, this could mean that each
spring has separately and sequentially assembled and
measured. It’s not uncommon, especially for the engine’s
top end, for the engine builder to have to assemble,
disassemble, and reassemble the engine 2,3 or 4 times
before finally closing it up.
Take notes as you go, make a chart or what ever it takes to keep an audit trail so you know what you’ve done and measured. Trust me there are car builders and chief mechanics that won’t fire a fresh engine till they’ve read such a list and even then they only trust documentation from an engine builder they know and trust. There are some really good books on the subject out there such as the “Step by Step Guide to Engine Blueprinting” from S-A Design Books, by Rick Voegelin. This is 20 bucks well spent.
Setting valves, I do not like nor do I use these quickie valve setting techniques used an abused by the Flat Raters. These almost always get you into trouble, especially if you’re running a long duration cam, or high ratio rockers. I recommend the painful practice of bringing the number one piston to TDC firing, this absolutely insures the valves are fully closed. I do this right down the firing order which means jumping back and forth, and from front to rear of the engine, but I know for absolutely for certain that no valve was sitting on a ramp when I adjusted it. On hydraulic lifters, I carefully run each rocker to zero lash, that’s just at contact on the lifter's plunger. I don’t go back until the engine is running and hot, then I reset them for zero lash hot with the recommended amount of preload being ¼ to ¾ turn more. I never turn the adjustment more than a 1/4 turn beyond where the lifter becomes quiet. It’s better to fire up with a little ticking than to have an over tightened lifter hold a valve open for a piston to hit it. For solid lifters; I go ahead and lash the rocker to the valve stem in the middle of the cam manufacturer’s recommended range. Then go back and reset when the engine is running and hot. This process requires lots of rags and keep heavy duty aluminum foil around to make baffles to catch the flying oil.
Bogie
Edited 11/17/2006 4:46 pm by oldBogie
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