273 Rocker Arm to Shaft Clearance

[Torqueflite]

I've looked in five different factory and general service manuals spanning from 1957 (the LA273 shares a similar rocker arm as the A-block exhaust) to 1966 and cannot find any mention of or spec for rocker arm to shaft clearance. I have a pair of factory 1965 273 forged rocker arm assemblies with 30K original miles on the engine that I'd like to run if possible. Aside from the rocker arm tips and the thrust side of the shaft being polished from use, I find no wear. However, the clearance seems wide at .003" total. The rocker arm bores are consistent across all 16 and are not out of round. The shaft measures the same diameter where the rocker arms ride and at the towers. Because of no noticeable wear and confirmed low miles on the assemblies, it looks like the factory clearance was or was close to .003".

When I have the shaft in a vice and rock the arm straight against the shaft (not twisting but rocking to check clearance), there is noticeable play from the .003" clearance.

My question: Is this .003" clearance acceptable for a 6,500 rpm 450 HP build, or should I be looking to have the rocker arms bronze bushed and honed to the shaft, which will be far cheaper than a quality set of roller rocker arms. If they do need bushing, what is an ideal clearance? Thanks for any input.

 

[rustycowll69]

You need clearance to allow for heat expansion of steel shaft and room for oil to flow. It's far more important to get the orientation of the shaft correct than worrying about this clearance. If you get that wrong, expect a lot of problems. I think 'you're trying to pick the fly s#$t out of the pepper', as my late friend used to say.

 

[RustyRatRod]

To run 6500 RPM, I would send them and the shafts you plan to run out to Gary at Rocker Arms Unlimited and let him bush them. He will also correct for any geometry differences.

 

[gzig5]

Probably be cheaper to chrome the shafts to build them up to be a little closer to the rocker bores. But, I wouldn't be afraid to run them as-is. Member Rocket has custom made shafts for sale that may be a better fit.

 

[4spdragtop]

Maybe call Hughes? They sell new shafts reasonably priced( single grooves), maybe they can offer insight. I think Rockets are single grooves also.

 

[Torqueflite]

You need clearance to allow for heat expansion of steel shaft and room for oil to flow. It's far more important to get the orientation of the shaft correct than worrying about this clearance. If you get that wrong, expect a lot of problems. I think 'you're trying to pick the fly s#$t out of the pepper', as my late friend used to say.

And I'm supposed to trust the input from someone who knowingly eats fly ****? I'll pass.

 

[Torqueflite]

To run 6500 RPM, I would send them and the shafts you plan to run out to Gary at Rocker Arms Unlimited and let him bush them. He will also correct for any geometry differences.

Thanks. I've used Rocker Arms Unlimited before on A-block 318 rocker assemblies, and they would be the ones I'd send this 273 assembly to if it's warranted. Their bushed and chromed reconditioned assemblies are much tighter than this .003" clearance, which is partially why I asked my question in the first place knowing if the assemblies required .003", Rocker Arms Unlimited would build that in. I might call Gary and get his input.

 

[Torqueflite]

Probably be cheaper to chrome the shafts to build them up to be a little closer to the rocker bores. But, I wouldn't be afraid to run them as-is. Member Rocket has custom made shafts for sale that may be a better fit.

Thanks. RAU will bush the rockers, true up the tips, and machine/chrome the shafts for about $400 after shipping both ways, so it's not a huge cost considering the thousands going into the rest of the build. However, if it's unnecessary, it's unnecessary. I agree that my instinct is to go ahead and run them since they appear to be factory clearances and I find no signs of adverse effects on the valve train, but I thought I'd pick brains. I might PM Rocket for options/pricing.

 

[Bewy]

There is no need to go to chrome shafts because the factory shafts are hard steel.
These engines lasted for years with factory shafts, zero problems. Thermal expansion is not a problem @ 0.003" clearance with both parts expanding together & both have the same expansion rate.
The most important thing has not been mentioned. Which is NOT to over torque the shaft mounting bolts which could deform the shafts & cause the rockers to bind.

 

[gzig5]

There is no need to go to chrome shafts because the factory shafts are hard steel.
These engines lasted for years with factory shafts, zero problems. Thermal expansion is not a problem @ 0.003" clearance with both parts expanding together & both have the same expansion rate.

Chrome plating was suggested not for wear resistance, though it significantly enhances that, but because a slightly under-size shaft can be built up to size or built up and ground to size very accurately, and the cost isn't crazy.

Typically for a hydrodynamic plain bearing the desired clearance is about .001" per inch of shaft diameter. In the case of the rocker arms, they work differently than say a crank main bearing because they don't fully rotate and the oil supply is intermittent, not constant. The stock stamped rockers aren't even enclosed and have significantly less bearing surface on the rocker shaft than a 273 type would. How much clearance do you think are on them? They seem to rattle around a lot and it doesn't cause any issues. The spring pressure is going to be the deciding factor IMO. I think the OP could get away with .003" on the suggested build, but it would be a good idea to get feedback from RAU

 

[RustyRatRod]

Remember too, these rockers were skewed all over the map as far as being consistent with ratio. Gary at RAU will fix all that in one shot when he bushes them. He will correct for proper geometry and even change them to 1.6 if you want. There's just no down side. Course there's the cost. I guess it all depends on how far you're going with the build. Plenty of builds have run them as is with no problems at all.

 

[rustycowll69]

There is no need to go to chrome shafts because the factory shafts are hard steel.
These engines lasted for years with factory shafts, zero problems. Thermal expansion is not a problem @ 0.003" clearance with both parts expanding together & both have the same expansion rate.
The most important thing has not been mentioned. Which is NOT to over torque the shaft mounting bolts which could deform the shafts & cause the rockers to bind.

Gray Cast Iron has a linear expansion rate of 5.8 (times 10 -6) where low carbon steel is 6.5 (times 10 -6). This is linear expansion per each degree change in Fahrenheit. The steel expands more than the cast if both would be heated equally.Aug 3, 2016

 

[Torqueflite]

Chrome plating was suggested not for wear resistance, though it significantly enhances that, but because a slightly under-size shaft can be built up to size or built up and ground to size very accurately, and the cost isn't crazy.

Typically for a hydrodynamic plain bearing the desired clearance is about .001" per inch of shaft diameter. In the case of the rocker arms, they work differently than say a crank main bearing because they don't fully rotate and the oil supply is intermittent, not constant. The stock stamped rockers aren't even enclosed and have significantly less bearing surface on the rocker shaft than a 273 type would. How much clearance do you think are on them? They seem to rattle around a lot and it doesn't cause any issues. The spring pressure is going to be the deciding factor IMO. I think the OP could get away with .003" on the suggested build, but it would be a good idea to get feedback from RAU

Your comments about clearance in relationship to shaft diameter are partially why I posted, and, to be clear from my end understanding you weren't responding to me, I understood your suggestion of chroming to tighten the tolerance. The rocker shaft is a similar diameter as a piston wrist pin, and the wrist pin gets splashed oiling compared to the rocker assembly's pressurized. While I understand a wrist pin/piston dynamic functions differently than a rocker arm and the piston is aluminum--such as you don't want a piston flopping around or rotating in the hole due to excessive wrist-pin clearance--my concern with excessive rocker arm clearance is that the rocker arm will have more freedom to push back and forth perpendicular to the shaft and "rock" in and "X" pattern on the shaft as the forces of the pushrod and spring work on it, thus changing the fulcrum point and the rocker ratio and changing the sweep on the valve stem through the "X" pattern movement. As you have said, I doubt the .003" clearance will impact performance much on this build, but to me it's worth the discussion. FYI, spring pressure is rather mild on this build: 120 seat, 333 at .480" lift.

Remember too, these rockers were skewed all over the map as far as being consistent with ratio. Gary at RAU will fix all that in one shot when he bushes them. He will correct for proper geometry and even change them to 1.6 if you want. There's just no down side. Course there's the cost. I guess it all depends on how far you're going with the build. Plenty of builds have run them as is with no problems at all.

Proper geometry is my central concern with the .003" clearance. The lack of wear proves to me that the assembly will hold up just fine if I run it as is. However, as I describe in my response above, the excessive clearance will allow the rocker arm's fulcrum to shift and its tip trajectory on the valve stem to change which in turn will change the ratio. Your point about the forged rocker arms being inconsistent in ratio is well taken, and the compounding change in ratio from the excessive clearance isn't desirable. The harm to performance remains unknown. If we were talking about a $1,000 investment, it would be worth dynoing the engine with the factory assembly and again with the reconditioned, but in this case the dyno cost would almost pay for the reconditioned assembly.

 

[Torqueflite]

Gray Cast Iron has a linear expansion rate of 5.8 (times 10 -6) where low carbon steel is 6.5 (times 10 -6). This is linear expansion per each degree change in Fahrenheit. The steel expands more than the cast if both would be heated equally.Aug 3, 2016

First you bring up your inclination to eat fly crap rather than address the known feces in your meal and now you are expounding on expansion rates of cast iron. Neither the shaft nor the rocker arm is gray cast iron.

 

[rustycowll69]

First you bring up your inclination to eat fly crap rather than address the known feces in your meal and now you are expounding on expansion rates of cast iron. Neither the shaft nor the rocker arm is cast iron.

Shaft is steel, rockers are CI. You would not want to use like materials rubbing against each other.

 

[Bewy]

Post #12.
Assuming room temp of 68* F & r/shaft & rockers get to 178* F [ probably a lot less than this ] = 110* F temp change. Being generous with the numbers.
Shaft is about 0.88" diam from memory. The lineal increase is.....drum roll please.....0.00044". Less than half of one thousandth of the available three thou clearance. And that is NOT allowing for the expansion of the rocker. As I said in post #9, I do not see a problem.....

 

[RustyRatRod]

Good lord.

 

[rustycowll69]

I dunno. 0.003" is an acceptable clearance(admittedly, on the loose side) for a connecting rod bearing, what makes the rocker arm to shaft clearance so far outside the norm or more critical than a con rod bearing clearance? Hey, if it means that much to you, spend all you want. It makes no difference to me. At what point, why don't you just get some Harland Sharp's and call it good enough?

 

[Rat Bastid]

I dunno. 0.003" is an acceptable clearance(admittedly, on the loose side) for a connecting rod bearing, what makes the rocker arm to shaft clearance so far outside the norm or more critical than a con rod bearing clearance? Hey, if it means that much to you, spend all you want. It makes no difference to me. At what point, why don't you just get some Harland Sharp's and call it good enough?

Because the rocker shaft isn’t 2 inch diameter. While the standard .001 clearance per inch of diameter is still used, with modern oils you can easily get down to .0009 or even .00088. I know that’s picking the fly poop out of the pepper but it can be done.

 

[Bewy]

What Rat said.
We are dealing with a steel shaft that is less than 1" diam & everything is in proportion when dealing with thermal expansion. I would venture that the smaller gap will improve oiling of the two surfaces. If you have a large gap the oil will just run out either side at the bottom of the shaft [ gravity ] without being cycled around the shaft. Like the stamped rockers with the sloppy clearance; only the stamped rockers do not have a 'top' so no oil reqd there.

 

[rustycowll69]

Meh. Agree to disagree. The rockers being CI are probably limited with respect to lifts, rpm's & v. spring pressures, anyway. If you're wanting a precision part for a high dollar, ultra high HP small block, I would think it'd be more cost effective to buy a Harland Sharp(as an example) set.
Now, if it's a 273, and for some reason you want precision stock rockers and are willing to throw a bunch of money at tightening up the rockers, by all means knock yourself out. I just think it's a waste of money, but that's just me. You do you.
Years ago, people used to talk about "blueprinting" engines. In short, the meaning behind that term was basically, minimizing tolerances and maximizing clearances.

 

[Bewy]

The same style rocker from the same company [ Chrysler ] were used on Max Wedge engines that were raced in gruelling road races & drag racing; & did not have a history of failing or breaking. Also went to 6500 rpm.

 

[rustycowll69]

The same style rocker from the same company [ Chrysler ] were used on Max Wedge engines that were raced in gruelling road races & drag racing; & did not have a history of failing or breaking. Also went to 6500 rpm.

So you're claiming these were all bushed from the factory and ran much tighter clearances than 0.003"?

 

[Bewy]

[1] No. Where did I claim the factory rockers ran tighter than 0.003" clearance??
[2] No. Where did I claim the all factory rockers were bushed from the factory??

 

[Bewy]

From the Jan 1963 issuer of HR magazine. Page 33 on the then new 426 Max Wedge.

" Rocker arms are cast malleable iron..."
"..have run hundreds of times at the drags & will exceed 7000 rpm without audible valve float. Chrysler designed the valvetrain to be stable at 6500 rpm & it fills this order with plenty to spare."