Grey motor pushrod investigations

Engine discussion.

Grey motor pushrod investigations

by Harv » Thu Jul 16, 2015 10:28 am

Ladies and gents,

The notes below are from a small side project I did looking at grey motor pushrods. I got interested after Ellis recently had some new pushrods made. Hope the info below is of interest.

Three different types of pushrod were offered for the Holden grey motor.
• Part number 7401247 up to FJ engine 178123,
• Part number 7408422 through to FB engine B65279. I am uncertain as to why this change was made or the difference in the pushrods as no change was made to the lifters (anyone know why?), and
• Part number 7416291 through to the end of the EJ. This pushrod came about because the lifter ball seat was raised 3/16”, and the pushrods shortened by the same amount to an overall length of 10 1/8”.

Image

The humble grey motor pushrod has a relatively simple job to do. At one end, it follows the lifter, moving up and down as the lifter itself rolls over the camshaft lobe. At the other end, it is moving up to push the rocker (and open the associated valve), and then following the rocker back down under valve spring tension. In an ideal world, the pushrod is just a simple vertical column of steel under a cyclic compression load. Steel is damn strong under compression – it takes around 150 megapascals (22,000 psi) to get steel to yield and mushroom out.
Unfortunately, the humble pushrod is far from ideal. Interaction with the lifter at one end, and the rocker at the other, can put bending (rather than compressive) loads on the pushrod. This is particularly true for the rocker end of the pushrod, as the rocker moves in an arc.

Image

The Holden grey motor pushrod is relatively robust. However, it is not uncommon to bend them. Note that the damage seen is a distinct bend in the middle of the pushrod, rather than the mushrooming that would be seen in purely compressive failure.
Along with the interaction with the lifter and rocker, there are other sources of bending forces on a pushrod.

a) At high rpm, the lifter can jump off the end of the cam, leading to increased loading on the pushrod when the lifter smacks back down onto the cam lobe. This type of loading can also occur at the other end of the pushrod due to valve float (when the valves are moving so fast they don’t quite seat) and valve bounce (where the valves are moving so fast they smack the valve seat then bounce open a bit).
b) Sticking valves (from poor valve clearances, poor tolerances when machining, coke build-up, or corrosion from an engine that has sat a long time) and backfires can all make the valve (and rocker) want to be in a place that the pushrod isn’t ready for, leading to increased loading.
c) The standard valve spring pressure for a grey motor is 98-110 lbs. (valve open) and 48-54 lbs (valve closed). Valve spring pressure is often increased in order to remedy valve float. This is done by replacing the standard grey motor valve springs with stiffer springs, either single or double. Whilst the stronger springs help keep the valve closed, they also impart additional loading on the pushrod (and can also lead to increased valve bounce).
d) For increased flow, a common modification is to increase the valve opening by regrinding the camshaft to give increased lift. Care needs to be taken that the increased lift does not put the selected valve springs into bind. Spring bind (or coil bind) is where the spring has compressed as much as it can… but the cam wants to lift the valve more. This puts a significant bending load on the pushrods.

One option that is used in some grey motors to prevent pushrod bending is to convert the pushrods to larger diameter units. The larger diameter resists bending better, though can lead to an increase in weight. Bear in mind that at the nominal stock grey motor redline that a pushrod is accelerating from standstill and back to a dead stop again 75 times a second. It takes energy to undertake this acceleration and deceleration – the heavier the pushrod, the more energy lost. To regain some of the weight of larger diameter pushrods, the pushrods are often made hollow. There is then a trade-off between the strength and weight gained in greater pushrod diameter, and the strength and weight lost in hollowing them out.

Aftermarket pushrods are also often made of steels that have been alloyed with chromium and molybdenum (chrome moly, moly, CrMo or chromalloy). Chrome moly steels have a better higher temperature strength than steel, and slightly better corrosion resistance. However, at room temperatures they are not a “miracle steel”, despite the legends that surround them. For example,

a) One rumour is that chrome moly steels are lighter than carbon steels. This is not correct – they are both roughly the same weight (about 7.85 times heavier than water). A tube of carbon steel weighs exactly the same as a tube of chrome moly steel of the same dimensions.
b) Another rumour is that chrome moly steels are stiffer than carbon steels. This is also not correct – they both have roughly the same resistance to being bent (a Young’s Modulus around 210 gigapascals or 30,000,000 psi). A tube of carbon steel will bend just as much as a tube of chrome moly steel of the same dimensions.

Where chrome moly comes into play is that it has a higher yield stress (around 435 megapascals, or 63,000 psi) than carbon steel (around 280 megapascals or 41,000 psi). If we imagine our carbon steel tube above, we can apply a force to it and it will bend, then snap back like a spring once the force is taken away. If we keep increasing the force, eventually we permanently bend the carbon steel tube. If we do the same thing with the chrome moly steel tube, it will take twice as much force before it permanently bends.
So in short, a chrome moly component is no lighter, nor any stiffer than a carbon steel one. It just takes twice as much punishment before it permanently bends.

As a side note, compare this with a titanium alloy like Ti-6Al-4V. It is almost half as light as carbon steel (4.43 times heavier than water), will bend twice as much as carbon steel (a Young’s Modulus around 115 gigapascals or 17,000,000 psi), but will take three times the beating before deforming (a yield stress around 880 megapascals or 128,000 psi).

So what happens when we take a solid, mild steel pushrod and change it to a hollow chrome moly pushrod of different diameter? There are a few things we are playing off here:

a) A hollow pushrod is lighter than a solid one. This relieves stress on the reciprocating valve train.
b) A hollow pushrod will take less load than a solid one. It will bend more, and permanently bend before a solid one will.
c) A smaller pushrod diameter will take less load than a larger diameter one, and permanently bend before a larger diameter one will.
d) A chrome moly pushrod will still spring like a steel one… but will take twice the loading before it permanently bends.

So how do we balance this up? Modelling a pushrod that is being bent in compression is not an easy thing. Equally, we don’t need answers to seven decimal places. The easiest way to model a bending pushrod (and accurate enough for what we are doing) is to treat the pushrod like a simple beam being bent:

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Assuming a pushrod with outside diameter D, and for the hollow ones with an inside diameter d,

a) The bending load on the solid pushrod is inversely proportional to the diameter4. For the hollow pushrod, it is inversely proportional to (the difference in inner/outer diameter)4.
i.e. stress α 1/D4 for the bar, and stress α1/(D4-d4) for the pipe.
b) Weight is proportional to the diameter2 for the solid pushrod, and proportional to (the difference in inner/outer diameter)2 for the hollow one.
i.e. weight α D2 for the bar, and weight α(D2-d2) for the pipe.

As examples, taking a solid 3/8” pushrod and increasing the diameter to 7/16” will halve the bending stress in the pushrod, but increase weight by a third. Replacing the same 3/8” pushrod with a 7/16” hollow pushrod with 0.080” wall thickness will reduce the stress by 40% (less than the 50% of the solid 7/16” ones), but will reduce weight by 20% (compared to the 33% increase in weight of the solid 7/16” ones).

One trick that the racing fraternity has been using for some time is to cut the ends off old hollow pushrods, and insert a thinner tube into the two old ends. This was recently done by Ellis, using some 0.035” walled chrome moly tubing. Looking at Ellis’ new pushrods:
a) original solid pushrods are 0.252” outside diameter
b) replacement pushrods are 0.372” outside diameter and 0.035” wall thickness (0.302” internal diameter).
Ellis’ pushrods have only one third of the bending stress than the original GMH pushrods, and will be some ¾ of the weight.

An alternative would be to take the standard GMH pushrods, cut the ends off and slip the ends into a larger chrome moly tube. The resultant pushrod strength (and weight) would depend on what tube wall thickness is used. As a guide for this scenario:

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The graph above shows that tube wall thicknesses of 0.024” and thicker would have the same or less stress than the standard pushrod, whilst wall thicknesses of 0.052” or thinner would have the same or less weight than a standard pushrod. In short, if you want to take a standard grey motor pushrod, cut the ends off and insert them into hollow chrome moly steel tubes, the wall thickness “sweet spot” range is between 0.024-0.052”.

As a comparison, looking at some typical pushrod manufacturers:
• Manton make pushrods in 0.035-0.168” wall thickness (3/8”-7/16” outside diameter),
• Trend make 0.080-0.188” (5/16”-3/16” outside diameter), whilst
• TrickFlow make 0.080”-0.135” wall thickness (5/16”-3/8” OD).

Cheers,
Harv
 
Posts: 487
Joined: Sun May 04, 2014 7:52 am

Re: Grey motor pushrod investigations

by Harv » Fri Jul 17, 2015 10:42 am

Some clever detective work from the FE/FC forum guys has hunted down the explanation of the early humpy pushrod change. The pushrods were shortened by about 2mm, because the valves were changed to incorporate a stem oil seal, and the valve stems were lengthened by around 2mm. At about the same time, the insert in the inside of the cam follower was deleted, although the newer cam follower was dimensionally identical to the old one. So after FJ engine 178123, the inlet valves change (from part number 7401248 to 7408420), the exhaust valves change (from 7401249 to 7408421) and the valve stem oil seal gets introduced for all future grey motors (part number 3835333), along with the slightly shorter 7408422 pushrods.

So in metric terms:
a) early humpy pushrods up to FJ engine number 178123: 264 mm
b) middle series pushrods from FJ engine number 178124 to FB engine number B65279: 262 mm
c) late grey motor pushrods (with their own different lifters) from FB engine B65280 to the end of the EJ: 257 mm

Cheers,
Harv
 
Posts: 487
Joined: Sun May 04, 2014 7:52 am


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