How to polish rods?

[Yugo_Turbo]

How to do it?
I did some reading about it,but it left a lot questions...

Is it "allowed" to use die grinder and such tools while polishing for rough finish and then fine finish it with hand?
I know that rod must be polished along it, not across, and I'm a bit scared that circular motion of sand paper on die grinder will damage the rod and create those small scratches we don't want.

[SirYun]

Hi,

the main reason for polishing would be to try and eliminate points of crack initiation. (think of plate glass cutting)

(based on Advanced Engineering Design "lifetime performance and reliability" Van Beek, TU Delft 2006) factors influencing fatique strenght are:

1- Stress raisers
2- Surface finishing
3- Material
4- Dynamic load
5- temperature & environment (not really an issue for a connecting rod..it's not over 450 celcius or covered in caustic sludge)

1:any sharp edge or corner ( be it as a result of machining or sometimes using letter/number punches) is bad: adding a as large as possible radius chamfer usualy about halves the stress.

2:
surface finishing:

rough finishing lowers fatique strenght by introducing micro scale stress concentrations (basily what happens with #1 but on a very small scale).

the reason for not grinding across is chances are that you introduce a crack across the con rod (which is how they tend to break anyway) .

the higher the material strenght the more critical the surface finish becomes. a machined part made of 1800MPa steel will have a aproximate fatique strenght reduction factor of 0.5 vs a 0.7 for a polished finish.

for a 400MPa steel those numbers would be 0.8 vs 0.9.

ergo polishing lower grade of steel will do less than doing the same to higher grade steel.



#3 is more or less a constant..you could try heat treating but there are concerns about the warping. Sometimes the rods are heat treated without the caps because of this. I know this tactic is often used for 998 mini racers.

get a better rod it you feel your application needs it/ you can afford it.

the Cunningham rods as sold by GC are basicly textbook examples on how to do it right.

#4 dynamic load..excessive clearances increase this a lot.

as for the actual question:

I would first see if you are not solving a problem that does not exist ( do they break under the conditions you plan to use them? including missed gears an such).

what kind of material are the rods made of. elimination of stress risers is of course a good thing but the gains of polishing can be limited on a low grade rod material.

a good surface finish is not really influenced by the way you power the polishing (elbow or die grinder) as long as you know what you are trying to achieve and avoid.

I hope this is of use.

thank you
Joost

[Yugo_Turbo]

Oh, thank you sir, just fantastic post.

I would first see if you are not solving a problem that does not exist ( do they break under the conditions you plan to use them? including missed gears an such).

Let me explain the goal with engine I'm building.
It's based on Fiat 1.3 turbo block, but engine will be N/A, so there is beneffit of having those lovely oil jets.
The engine will be used in light road and weekend racing purposes(light road cause Serbia doesn't have any real circuits so it's more like some parts of road which are used for our National racing championship).
I'm trying to build engine to be DIY as much as it can be, uisng a lot stock Fiat/Lancia parts so it will be cheap to run and maintenance.

The reason why I want to polish the rods as you've said, is to increase their strenght, beacuse this engine will have to be reved pretty high and the transmission which will be used(Zastava Yugo's) is notorius of missing gears so there is danger of damaging engine.
Of course rods will be lightened and balanced to help the engine to rev up easier.

So, if I understand you wright, I can use die grinder.

Thank you very much for all usefull informations.
I'll try to findout of which material are these rods made and post some pictures before and after polishing.

[SirYun]

hi,

If you have a way of testing the hardness (rockwel) of the rods you might be able to sort out the good ones from the bad. Personally i would try heat treating the rods after grinding and polishing just the critical points on both rod and cap. there are other options but i don't know enough about those (shotpeening and the like)

Do invest in good rod bolts (ARP comes to mind, expensive but for some engines a set of relatively cheap US V8 bolts can modified and used for two engines. as for uno engines i just don't know) and read up on proper technique for torqueing (prefererably by measuring bolt stretch). i was quite shocked to find out how 'accurate' normal readings are.


thank you Joost

[Yugo_Turbo]

I doubt that there is posiblity of testing hardness anywhere in Serbia.
Even if there is, it's not available for normal people like me.
I will ask around :)

I dont have enough knwoledge about heat treating.
What kind of process is that and is it posible to do it at home?

There was some thinking about shotpeening, I'm trying to find someone who is doing that.

About bolts.
I had some terrible expirience with bolt, no on rods though but on cyl. head.When I was installing head on, new bolts were bought of course but...they were just awfull, over galf of them couldnt be tighted up enough and streched significantly, so allmost all of old bolts were reused and they are showing o problems with stretching.

Best regards!

[SirYun]

Hi,

heat treatments are an industrial process and definitely not something you can do at home!


maybe a differnt way of looking at it:

Can you afford to blow up engines ? if the answer is NO i would strongly suggest steering well clear of high revving engines. If you want a dependable high RPM engine it will need proper parts and those cost money..

600 UKP for a set of custom connecting rods is a lot of money but it is not expensive.

An engine will just about run forever at the factory rev limit ( most factories have very demanding tests..equivalent of 10.000 km at full throttle or the like). And as italians are not afraid to use the acellerator pedal all the time i suspect Fiat engines are tested to a higher standard.

i would polish as it is a good idea in theory but leave it at that. just keep the RPM's down to a sensible level!

the main reason why an engine will fail is mis-shifts.

So you can either try and make a engine mis shift proof..which is neigh on impossible.

If you shift UP from 3 to 2 instead of to 4th gear as intended at 7200 rpm using a for example a ford MT75 box the engine will end up turning >11.000 rpm.

a decent soft cut rev limiter will save you from this one.

Downshifting from 4 and hitting 1st gear from only 5000 rpm will generate 14.000 rpm. and No the rev limiter will not save you this time.

So maybe the gearbox (and operator) is where you should be looking.

[Yugo_Turbo]

I doubt the engine will be revved over 7000rpm.
The rods will be polished and shotpeened, if I found someone to do it for me, of course.

Talking about gearbox.
It will be taken care about that.


Thank you once more.
I'm going to polish my rods now :)

[Yugo_Turbo]

Little update.

I've started working on one rod, just to try how it will go, interesting job, less interesting then head porting but less anoing though.

As it was mentioned, the con rods will be lightened and balanced.
And I have some questions about that.
Take a look at first picture.
Can metal be removed from areas painted red?

[Yugo_Turbo]

Well, after a lot of work...and especially time...I ve done it.
The rods are polished, lightened and balanced.
I'm currently doing the final polish with 220grit sand paper.

I have one question...
can those tiny scratches shown on pictures be dangerous- do I need to sand them down?



Best regards,
Sasha!

[ICI]

Hi Sasha
You can see how I did mines : phpBB3-2/viewtopic.php?f=4&t=745

[Guy Croft]

Sasha, hi

my slow dial-up on the present server still making it very difficult to do replies, but it should improve soon.

Looks like you have done the polishing right in the important areas, the radii under the pin boss, the flanks of the I beam section, the root radii where the beam joins the big end and around the machined section where the bolt sits.

I'd finish off by a good scrub with 3M Scotch-Brite, coarse grade (green or black) and then fine (grey), you can use light oil too, gives an even better result. All metal conditioning by abrading produces scratching of some form or another, and of course the finish varies according to the grade of abrasive. If you're going to plish rods, as you have done, you're trying to end up with no visible grooves in the surface where a crack might propagate. And that crack is going to be caused by tensile stress or bending load.

Here's 3M's website, they have distributors everywhere, like I do - get the real thing, I don't want to hear about 'cheaper alternatives', members, thanks!

http://solutions.3m.com/wps/portal/3M/e ... ageHandler

Good race rods are finished with 'omni-directional' or 'non-directional' polishing, no particular direction, the tool generates a random dispersion of millions of smooth microscopic ridges, rather than a whole series of scratches lined up right across the critical bending stress regions!

Finish off the rods by dressing the cheeks of the big end region both sides with a fine carborundun stone and oil to give a 'ground' finish - good for oil retention. For that I recommend either Norton Abrasives, their listing of stone is a pdf download and is at:

http://www.ind.nortonabrasives.com/Data ... 0000003962




GC

[Yugo_Turbo]

Yes, I know houw is it, I have 28.8Kbps dial-up connection :(

Thank you, for kind words about the rods, I really tried to do the work wright using help of your book and a lots oh googleing :)
Ok...I will finish it with scotchbrite...ofcourse 3M, I also use their products, they are a bit more expensive, but you know what you pay for.

Good race rods are finished with 'omni-directional' or 'non-directional' polishing, no particular direction, the tool generates a random dispersion of millions of smooth microscopic ridges, rather than a whole series of scratches lined up right across the critical bending stress regions!

Interesting...I've heard something about that...but never acctually seen it.

cheeks of the big end region both sides

Is that region around bolts?
I supose it is.

Thanks a lot, once more.

[Guy Croft]

No, those are called grinding stones or more correctly 'mounted points'. The stone for dressing will need to be a fine rectangular one, like you'd use for sharpening a knife..

The rod cheek is the bit that runs against the crank web, see arrow, photo below. Never, readers, polish the cheek, it will have no oil retention and score quickly.

GC

[Yugo_Turbo]

Oh...I know on what you think.
Thanks...I'll post pictures of finished rods soon with some numbers :))