There are basically three reasons to provide an undercut radius - the first most obvious being geometrical clearance for a mating part - the previously mentioned illustrations are good.

The other (2) are:

(1) To avoid a design factor called "Notch Sensitivity" - the harder a part is, the less tolerant it is of grooves, corners and changes of section. A soft mild steel bar is therefore less sensitive to a "notch" than a quenched and tempered chrome/moly bar.

Anyone who has cut glass or seen it cut knows about the effect of the scribed notch on the hard unmarked surface. Or maybe "don't ever leave scratches in a hardened rod or crankshaft journal".

(2) To mitigate design factors called " Stress Concentrations" - where the internal stresses of a loaded part are forced to travel around geometric features or be forced into a smaller section. This chart shows the "old school' way of estimating this effect and sizing the parts accordingly. There are times when necking down the intersection - to provide the increased radius of the undercut is just enough to make it work - and last.

Today - my engineering tech's use a computer design aid called FEA (Finite Element Analysis) to get similar (and more accurate) information when designing parts with very complex geometrical shapes - changes of diameter, corner radii etc.

As can be seen in these graphical representations - just about any technique that allows increasing the radius that the internal load or stress must "flow" around - is good.

 
 Steve Metzger       Tucson, Arizona

As I understand it, it's the lines of strain that get concentrated at a sharp transition that stress the material beyond its strength. When that happens, the metal crystallizes where the strain lines concentrate and a crack starts. The crack causes a further concentration of strain lines at the end of the crack, and the metal cracks more, continuing until the remaining metal is overcome by the strain. I would have expected to see the head of the lifter fall off. That lifter shattered, like it was brittle.



Edit; cryogenic treatments help prevent the fracturing of steel by improving the structure of the grain; the conversion of coarse austenite to Martensite and the improvement of the Martensite to finer grained Martensite. I wonder if giving the lifters a cryo treatment would help prevent the fracturing.



Also, was there enough of the right kind of ZDDP in the oil? ZDDP is a high pressure lubricant.

Best regards,



Paul Menten

Meridian, Idaho

Good thread guys!  Steve, Thanks for the engineering graphs.  Great stuff !

 

A question for Barry or Royce.  Was the lifter badly worn on its face before it shattered?  Any wear factor simply makes the lifter face thinner and brings a breakage issue to the forefront.

Lorena, Texas (South of Waco)

This doesn't sound like a wear problem, but I wasn't there. Like to see photos of the cam and lifter to see the truama.

Frank/Rebop

Bristol, In ( by Elkhart) 

You and me both.  Steve, although it hurts my little brain I love the stuff you share with us.

   God Bless.   Tim                           http://yblockguy.com/

350ci Y-Block FED "Elwood", 301ci Y-Block Unibody LSR "Jake", 312ci Y-Block '58 F-100, 338ci Y-Block powered Model A Tudor

tim@yblockguy.com  Visalia, California    Just west of the Sequoias

Very good stuff Guy's.

I thought Royce mentioned there were no evident marks on the cam other than scratches from after the lifter failed. They are not amature builders and I am sure they used the proper assembly and break in lubes. It just looks like the lifter shattered.

Royce, Since Jerry doesn't visit here, ask him to look for marks on the bottom of the rocker where that particular lifter failed to see if there is any evidence that the rocker that broke didn't get jammed under there.

55 Vicky & customline

58 Rack Dump, 55 F350 yard truck, 57 F100

59 & 61 P 400's, 58 F100 custom cab, 69 F100, 79 F150, 82 F600 ramp truck, 90 mustang conv 7 up, 94 Mustang, Should I continue?

There was no wear pattern of any kind on either the lifter or the cam lobe other than where the lifter stem scored the cam after it broke..Plenty of ZDDP. Jerry used Valvoline racing oil plus a can of Comp Cams break in oil supplement.



I am theorizing that the lifter broke where the stem meets the foot. The foot was then shattered when it hit the spinning crank and ricocheted around the inside of the motor

Thanks Royce.  I think must of us are just grasping for something other than a faulty lifter but from the description, that’s the best scenario for what happened without any more information.  Unfortunately in the case of the lifters, you do end up putting them in an engine on blind faith and hope for the best.

Lorena, Texas (South of Waco)

I had a long talk we Vern Schumann today and the subject of Jerry's lifter failure came up. It is his opinion and I agree that the likely cause is tight valve guides. This happens far more often then you would think. In a stock Y-Block you would just fold-up the pushrod but in a race engine with high quality pushrods something else will give up. The broken rocker is also an indication the something was seriously wrong.

A lot depends on the type of guide used in the head as to the required stem clearance. I never use stainless valves in iron guides so no need to talk about correct clearance there. Iron guides can destroy stainless valves even with chrome plated stems. If using a thin wall bronze guideliner we shoot for .002" clearance on the intake valves and .0022" on exhaust. Some people will tell you that is too loose but experience has shown that guides with .0015" clearance will likely cause trouble in iron heads.

If you are using .502" OD bronze guides I would recommend even more clearance. .0022-.0025". The bronze is not dimensionally stable until many heat cycles.

When it comes to sizing valve guides my motto is: If the guide has a couple 10,000ths more clearance than might be required the customer will never know it. If it has a couple 10,000ths too little he will hate you.

I think that Jerry should disassemble his heads and have someone carefully check the valve guide clearance!!!!

BTW, he was not using the aluminum heads I sent him.

It is too easy to blame the broken part for a failure. How often is a broken connecting rod really an oiling problem?  A broken ring or pounded out rod bearing a detonation problem? In other words, a rod bearing failure could be caused by an ignition or A/F problem. If Jerry had only broken a lifter it would be tempting to blame the lifter but with a broken rocker arm also, something in the valve train was binding up.

I have sold 1000s of the Schumann lifters and lifters from the same manufacturer (Vern does QC for the other vendor) with an extremely low failure rate. I did replace 2 for a shop that broke during engine assembly! I suspect the "mechanics" broke them by beating a tight cam gear on and using a lobe/lifter for a back stop.

Also, if you are rotating the crank without the full cam assembly bolted on allowing the cam to slip back while rotating, 2 lobes can be forced between lifter heads. I would guess that some lifters have been broken this way.

Now you have my 2 cents worth.

http://ford-y-block.com 

20 miles east of San Diego, 20 miles north of Mexico

John.  Thanks for the input.  I have never seen a flat tappet lifter failure here in the shop on any engine that was a result of cam/lifter metalurgy or the oil.  When there have been failures, it’s always been another factor such as lifter bore clearance, valve spring pressure, coil bind, retainer to seal clearance, rocker arm geometry, inadequate pushrod clearance, valve stem clearance, camshaft end play, rocker arm to shaft clearance, inadequate prelube, connecting rod to cam lobe clearance, and the list goes on.  Because I do get to run in engines built by other shops or individuals on the dyno, I remain cognizant on how easy it is for the simple failures to take place.  It typically takes overlooking a single detail among a myriad of items that must be thoroughly checked to propogate a failure.

 

As far as bushed rocker arms, I’ve had repeated breakage problems with those on the FE engines.  The bushing reduces the wall thickness of the aluminum around the shaft and just makes the rocker more prone to breaking.  Because of this, it’s difficult to just assume that the rocker and the lifter are automatically related.  Age of the rockers is also a player and any aluminum rockers that are eight years or older automatically become suspect also.

 

Bronze wall guide clearance has been a topic in the past and it will be interesting to see what Jerry’s and Royce’s final assesment is of the heads once they are pulled apart and examined if not already done so.  I still see bronze wall guide clearance problems cropping up in the circle track engines and this is normally traced back to an inexperienced shop or new personnel doing the head work.  Learning curves can be expensive to say the least.

Lorena, Texas (South of Waco)