polished rods; maybe not.

[Anonymous]

I,ve been seeing, in various places recently, people advising the polishing of connecting rod beams. The idea is to remove those crossways grinding marks, left by the factory after they clean up the foundry flash, to eliminate stress-risers.

However, the way I learned it, many decades ago, was that a forging has a hard "skin" which adds to its fatigue-strength. To polish a rod, you start with a grindstone, then hit it with a sanding drum using progressively finer grit rolls, until you get to a surface finish you consider to be "polished". Of course, you are careful not to overheat the part at any time. But to work your way through those factory grind-marks, you also are removing much or all of that forged-in hard skin, and very likely shortening the fatigue-strength of the part. Therefore, as I learned it, don't polish rods or other forged parts unless you are willing to pay for the additional step of getting them shot-peened, which re-creates the hardened skin. And since the big-name shot-peeners in this town didn't inspire my confidence that they knew what the hell they were doing, I've never polished rods after that first experience. Anyway, for a street engine, magnafluxing, re-sizing, and good rod-bolts should suffice.

Or am I wrong? I keep seeing contrary advice.

 

[Anonymous]

Polishing rod beams was once a big thing, but all the forgings used to be a lot uglyer than they look now, so now everyone just double shot-peens the beam section after a good magnafluxing or zyglowing.

 

[rbohm]

8) polishing rods can actually improve fatigue strength as you elimintate stress risers when polishing. you should still have the rods shotpeened afterwards though.

 

[Thad]

Want to find a shot peen company that you feel; you can trust? Look for a FAA certified engine rebuilder.

 

[Anonymous]

Thad

Thanks Thad, good idea. The rods I had done years ago came back looking like they had been blasted with shot that was several sizes too large, under way too much pressure. They didn't break, but they probably wouldn't have anyway, without peening.

Hey Thad, if you missed it, I gave you an attaboy back on the "balance 300 crank" thread (I think it's called a thread; I'm still fairly clueless about this medium). Thanks again.

 

[Thad]

Sorry, missed it, was in transit coming home.
Thanks
What are you plans for your 300, street or Mean street or full boogie race?

 

[Anonymous]

That is why you have to shot peen a rod after polishing it. Shot peening makes the surface metal more dense thus regaining the rods strength. We do quite a few sets of 200 rods every month, shot peened, polished. resized and arp bolts. We also balance the rods to within 1/2 a gram.

Keep in mind that doing this to the rods shaves almost 10 grams off the rod and most of the current replacement pistons are 10 to 20 grams lighter than stock so rebalancing should be done.

Todd Fields

 

[Anonymous]

hey powerbrakes
Im thinking about getting my rods isotropic finished would I need to get the shot peening done after or before?

 

[Anonymous]

Thad,

I'm re-engining my '79 E-150 "Trailer Special" van; 351W, C-6, 9-inch with Traction-Loc, PS, PB, A/C. The V-8 was getting tired when I got the vehicle, so I bought a timing chain set for it. But I put off installing it one day too long . . . .

I really like vans, and am surprised to see little discussion of vans in these sites, particularly when several of you are racers. When I raced boats years ago, and crewed for guys racing bikes and cars, the pits were full of vans. What happened? Is there a good website for ford vans? I don't mean the rolling boudoirs of the old van magazines, but utilitarian haulers and campers.

The one bad thing about vans, and it is very bad in the generation of van I'm talking about, is trying to do any work under the hood or hatch! So after fighting and cussing to get the manifolds and heads of the V-8, I decided I'll discard that engine entirely and go with a simple, light, compact 300-six. I've always prefered sixes anyway.

So to finally answer your question, Thad, I'm building a van engine, a torquer and not a revver, so it doesn't need any fancy rod-prep. I only brought up the subject because I'd seen people advising polishing without mentioning the further step of shot-peening. I believed this was a bad idea, maybe, and I see that "powerbrakes" and some others think so too.

Except for the one time, I never polished/shot-peened rods for my racing outboards, some of which revved into five figures. Two-stroke rods usually don't fail unless you break a big-end bearing retainer or skid the rollers. In fact, I wonder how many 4-stroke racing rods actually fail by bending or snapping the beams in the absence of some other serious problem. I would guess not many. That said, if I were using stock rods in a high-winding 4-stroke, I'm sure I'd do all of the prep, but I think the crucial items are the rod bolts and the notch where the bolt-head seats.
And a reliable supply of non-aerated oil!

But now what is "GASSED250" referring to? "Isotropic-finished" is a new one on me. And then we could talk about cold-stabilization . . . .

 

[Anonymous]

I a assuming isotropic finished is an aussie term for stress relieving. Stress relieving basically aligns the grain structure in the metal to relieve stresses in the metal from the forging and or casting process. Unless you have a motor that is producing over 1.5 hp per liter and or has unusally high piston speed it is not nessacary. The basic procecess of shot peening and polishing is good for motors that are going to see 1-1.5 k rpm over the stock redline. 200 motors have such a short stroke they have a slow piston speed. The changing of direction of the piston going upward in its travel to going downward in its travel is a lot less violent in a short stroke motor than say a 500 inch pro stock motor with a 4.125 inch stroke. Polishing the rod may remove all the stress risers where cracks can start but it takes away the surface strength of the metal. Shot peening strengthens the metal by shooting steel balls at the metal and every time a little ball hits the rod it creates a divot which makes the metal more dense at that point thus strengthining the surface of the rod.
Balance is another crucial point to take into consideration, most factory engines of the sixty's and seventy's were balanced to within 50 grams. For every gram out of balance at 4000 rpm it is almost 250 lbs of force acting against the reciprocating assembly. All of our motors are balanced to within half a gram. Like Thad said faa cretified engine rebuilders must adhere to strict quality control, like inspecting thier shot weekly, as the shot wears as it is used and needs to be replaced occasionally. The size and the shot material also has strict tolerances that nust be adhered to.


Todd Fields

 

[Anonymous]

Thanks Todd. That sounds like a good guess on the terminology.

Do you think we should start a separate thread on cold-stabilization? That is an interesting topic, and controversial, and it is interesting that it should be controversial.

 

[Anonymous]

Cold-stabilization? Is that another term for cryogenics?
All this rod talk (he he)... is interesting and something I have been thinking about recently. I'm getting ready to start a rebuild and was wanting to balance the rotating assembly. Being I have never done this on a multi-cylinder engine and all the talk of balancing your rods to within a half gram. What is the correct method of shaving weight off of your rod? Would you shave the beam or where specifically do you like to grind or shave the rod? Shot peen... Then, Do you do the same thing with the pistons? Same question...Do you shave the piston skirt? I would think you would want to balance each recipricating unit or each rod,bolts, wrist pin and piston as a unit. Now, All that being done, do you then balance the crank? Maybe tungsten inserts? or are they fairly close? And then... As long as I'm going nuts with my grinder anyway, How about the trick where you take off the rough edges of the crank to reduce windages? Anything special I should look out for other then just using common sense in removing the rough edges? or should I just leave all this stuff up to common $$cents and the pros? I've just always liked going nuts on my two strokes and I have time. At least I have a lot more time than I do common $$cents. :roll:
Dave

 

[Anonymous]

Now that I wrote all that stuff... Most of these answers are in the thread: Balance A 300 Crank...Oh well, Answer if ya want. I love this kind of info.
Dave

 

[Anonymous]

Hi Speedy, hey this thread has about run its course, I think. Why don't you start a new one on DIY balancing?

No, don't grind the beam. The squarish pads on the bottom and top of most American-made con rods are called balance pads (at least, that's what people I know call them) and provide a place to remove weight without weakening the rod. I'm short of time just now, so I could only make a real quick search on the subject, and I didn't find an introduction to balancing site, though there surely is one somewhere. Some of the veterans here can probably help with a source. If not, I or someone will tell you what we know.

One thing I have come to believe is that balancing is worth the effort and money for any engine you rebuild and plan to keep. You'll hear that factory balance is good enough for street and mild performance use. But somehow when you put in new pistons, grind the crank and (especially) re-face the flywheel, imbalances creep in. Nothing is more irritating than to road-test a new engine and find a vibration! It isn't something you want to try to live with, because it can blur the image in you rearview mirror, cause bolts to back out, cause main bearings to pound out and crankshaft seals to leak; oh, it's a pain! Being particularly dense, it took me two experiences to fully learn this lesson. Luckily, the first one was sufficiently rectified simply by balancing the flywheel + clutch.

I'm skeptical about "streamlining" crankshaft counterweights, but include that in your new thread and let the current racers weigh-in.

-continued-

 

[Anonymous]

-continued-

Cold stabilization could be a way of preserving old racing drivers, if that were at all desireable. But what I'm refering to is a roughly twenty year old technique in metallurgy and materials science of using extreme cold treatments to effect changes in molecular structure and improve mechanical properties. It was controversial then, and I don't know the current climate of opinion. I might see what I can find and start a thread.

What kind of 2-strokes do you work on?

 

[Anonymous]

Oops, okay now I see the "Balance 300 crank" thread you refered to. I forgot about it, and blew right past it, duuuuhh . . .

 

[xctasy]

No worries, Smitty. That's what makes this site such fun!

On a RACING six, crank Knife edgeing is worth it to prevent crank windage atomising good, cooling, lubricating sump oil into a haze of 15 thou blobs in the crankcase!

That's all I've got to say.

Ciao!

 

[Lazy JW]

-continued-

Cold stabilization could be a way of preserving old racing drivers, if that were at all desireable. But what I'm refering to is a roughly twenty year old technique in metallurgy and materials science of using extreme cold treatments to effect changes in molecular structure and improve mechanical properties. It was controversial then, and I don't know the current climate of opinion. I might see what I can find and start a thread.

What kind of 2-strokes do you work on?

Cryogenic treatment has attained a measure of popularity in rifle barrel use. It is claimed to have some benefits but they are fairly difficult to prove, although opinions abound. It certainly doesn't seem to hurt anything except the wallet. I have had salesmen try to sell me saws and chipper knives that were cold treated for use in the sawmills. I never tried them and they haven't been pushing them lately, the other filers that I know that did try them said they couldn't see any improvement.
Joe

 

[Anonymous]

Oh come on, xtaxi, say more, say more!

What you say makes some sense; I can see that the square edges of the crank might tend to shear the oil. But I can't imagine that you can "streamline" a crank to have a worthwhile effect on air-drag. The big end of the rod acts like a big flat plate, and the maincaps also confuse the issue. But intuition must bow to data . . . .
Are you saying that even after you've installed a windage tray and a very close-fitting (ie., hand-made, not factory) crank scraper, you can still get an additional reduction in oil-foaming with a streamlined crank??

Joe, about fifteen years ago, while I was working as a toolmaker for Boeing, I read about how Northrop had significantly extended the working life of drillbits, endmills, etc., by giving them the liquid nitrogen dip. So I submitted a suggestion/query through channels, to the effect that those of us doing drilling, spot-facing, sawing, and related operations on composite laminates were having a hell of a time with tool-wear, and could you please investigate this technique. Eventually I recieved the usual curt negative answer that I always got to my suggestions, saying, "We know about it, we're not impressed, carry on." They used to say the same thing about Airbus . . . .

 

[xctasy]

The areodynamic profile of a knife edge crank is best when your not allowed a 60 thou close crank scarper. When it hits any oil, it ensures more drips back to the sump without becoming mist.

If the race car engineer is a crank scrapin', windage tray makin', Briggs&Stratton-fuel-tank-on-the-edge-of-the-sump building nutter, then I'd say knife edgeing is useless!

The Poms told us to knife edge cranks from the aero engine days. I doubt there would be any real stress risers to come out of an outer radius which was ground to an edge.

The extensive Cryogenic treatment of parts (ie conrods) started in NASCAR back in the early 1980's after ARP and SPS did all they could on rod bolts. Lots had been done on heat treating in F1, but soon Crower was doing vaccum forged, heat treated combos similar to what was happening for some space shuttle parts. You've got to love it. Plane engines have had special component tratements for years, now were doing it in 2004. Duh!

Cold treatments are used by clever people who have transmission componets and axles which are not as strong as they could be. It is possible to re-treat them in certain ways. I've seen the old GM corporate 8.2" c-clip positrac axles shoved in a freezer after a special treating process. I understand if it is done wrong, you get a strength drop, not an increase.