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Balancing Con Rods

Practical Engine Building

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Looking for some advice.

I have been steadily purchasing engine parts for a new engine rebuild, one issue is that the forged Rods are not available anymore, however a few years ago I chanced across some old stock Arrow forged rods which i purchased along with some nice Omega forged pistons to suit.

Fast forward to this weekend as because of lock down I have finally got round to start preparing the engine for build... and to my horror I have found that this box of rods where clearly the left overs after the engine builders had mixed and matched sets to get the weights identical. Bottom line is I have 2 rods that are 550 grams and 4 rods that are 558-559 grams.

The rods are way over spec'd as I have a set of identical rods from the same company that are 2mm longer and yet over 30 grams lighter as the webbing is 1mm thinner (we run the longer lighter rods to 950bhp reliably and the set are within half a gram of each other)... i cant use these rods due to there length as I'd end up with 11:1 CR and a piston 0.7mm out the block plus they would need a full refurish.

So the question I have, is it even possible to remove 9 grams from a forged rod or am i looking a total waste of money for the pistons and rods?

I think you should be able to do that, it just takes time to grind / measure / repeat. You have the time -- get grinding / measuring..

Depending on where the mass is higher, might end up removing more for end to end balancing.I'd suggest that, if you do do it, you carefully compare the rods, even make measurements, to see where the material may be removed to make them similar. There are, I expect, several on-line guides to removing material from OEM rods and even improving performance rod balance, with the places it is safe and places it isn't to remove the material - even the direction used for the removal can affect the fatigue strength as doing it wrong can introduce stress risers - flaws in the surface - as a general guide, along, NOT across the beam and ends.

Considering the rods are (should) be made from the same material, with the same maching tools and programming, that is a BIG variation! I have heard of others who had issues with ARROW rods, including big end roundness, but that seems rather excessive.

Thanks for your responses and advice

I carefully measured the Rods and identified the extra weight on the two offending items was in the H beam sections themselves, I didn't want to try to thin that part down so I took the decision to buy a brand new set of the longer versions.

Leason learnt here was check and measure the parts you purchase prior to buying specific matching items.... I now have a set of rods, pistons and a machined oversize block that I'm not going to use. expensive leason.

Ouch! Can you return the longer rods - you should be able to get a set machined to the slightly shorter length you require for only a little more money. Even if it's more than 'a little' it should still be cheaper, as you can use that block and piston set rather than buying new parts.

I looked into getting new short rods and decided it simply was not worth spending more money to stick with.... for a start the longer rods are actually 30grams lighter and proven to the 1000bhp level I'm, targeting... plus I want to rev the engine 1000rpm more than previous and the longer ratio is Better for that and the weight is very important as I did the calculations and the forces are insane within this particular package. (79.5mm stroke and 89mm bore)

Ive actually purchased a brand new crate engine as a base (better to start from zero overbore anyhow I guess), bought the new longer rods along with JE pistons to suit, then balanced them all to within 0.5grams just because I could.

The assembly has been balanced in block (as its a V6) and is now ready for me to strip, re-clean and do the final assembly, so i'm fully committed to this route now.

Never hurts having spares on the shelf.

incidentally a friend is also balancing his version of the engine and has just found the rods in his engine has run since new (built 2005 and it's a competitive race car) has 6grams differences between the lightest and heaviest so it seems common for these engines..... its seems an awful long way out to me, but then Im new to building my own engines and i'm following the HP academy courses to learn so hardly an expert.

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