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Hoping that someone on this forum can point me in the right direction. I'm putting together a 970 motor for occasional use in hillclimbs and have just been checking the rods.

To start with these looked pretty well matched, with overall weights of 711, 713, 708, and 705 grams. However on checking the end-to-end weights the differences are a bit larger at 525-186, 533-180, 528-180, 533-172 grams.

What tolerances are advised for end-to-end rod balancing? Does it really matter?

Mine were balanced by a shop to 0.5 grams of each other.

They'll work surprisingly well as they are however the better matched they can be, the happier the crank will be, although on a 970 it becomes less fussy.

Thanks for the feedback guys.

Red - sounds like your race shop does a good job.

Spider - I think you may be right, may not be so essential. I am in two minds here as once I start balancing them then I will probably end up getting them lightened, polished, and shot peened.

We normally do them to 0.1 of a gram.

If you lighten , balance , resize , shot peen and fit new ARP bolts it will probably be cheaper to get a set of H beams

525-186, 533-172 grams.

Good luck with these two.. Reminds me how much I hate lightening and balancing rods....

Hallo and A MERRY CHRISTMAS & A HAPPY NEW YEARE everyone.
Rods 970S. I/WE have built a couple of very successful 970S FIA pre-65 engines that I raced the few last seasons. If we take value for money I agree that they are fine enough for sensible road use. But for competition you should prepare them in the sense that that other authors has described above.
BUT! I always have had brand new in box witch is no guaranty that they basically are OK. First check that the seats for the main bearings are 100% round at the right measure. If that is not take care about it and it does not matter if they are done by a gold smith becouse thats where the problem will start if any place. Than shave of and get a reasonable even surface to up to a point it will prevent rev related damages. You can rev all way up to 8.000rpm trouble free ( race engine ) and in tight situations even more.
In the end you will see that new 970S rods, all machining, weight adjustment and ARP leads you to the conclusion it is better, easier and cheaper to buy new H type rods. ( I can not use them due to APP.K ).
Tommy

All useful feedback, slightly depressing to read that 251ENG is balancing rod sets to within 0.1gram and my set weights are over 10 grams out. Hey Ho!

Having checked the rods for overall dimensions it turns out that the machining of the little ends on rods 1&3 is different to rods 2&4. Width across flats is 29.8mm for 1&3, and 28.7mm for 2&4. If I correct this for 1&3 by milling to the same dimensions as 2&4 this removes about 3.5 grams.

That should give me 525-182.5, 533-180, 528-176.5, and 533-172 end to end rod weights. Still miles out but I can then equalise the big end weights relatively easily, again on the mill. Then it will probably be a case of selective assembly with suitably heavy gudgeon pins in 3 and 4 to get it all within a gram or so.

PS. Would be great to go for a set of H-beams but I don't think Santa will be that generous.

Pulled sets with more that 10g variation in to balance before .

You want to try doing some Austin Healey 3000 rods , they weigh over 1kg each and sometimes are miles out

The worst situation is where you have rods that have been massively lightened and are a long way out , you just run out of metal to take off without compromising strength . Small bore A series rods can be the worst because the beams are so thin .

251 ENG wrote:We normally do them to 0.1 of a gram.

That's persistence!!

251 ENG wrote:
If you lighten , balance , resize , shot peen and fit new ARP bolts it will probably be cheaper to get a set of H beams

I'd agree. Having done a few sets, I doubt that I ever bother again. The hours don't justify it - unless comp regs don't allow aftermarket rods. The aftermarket rods are nearly always stronger and lighter right out of the box !