Balancing rods on an I4

[226bhp]

I'm building an inline four 2ltr car engine which will see 9000rpm, how far do you think I should go with matching the weights of the rods? There is a 2g difference between lightest and heaviest at the moment.
They are lovely forged steel H beams, so I'm not enjoying taking the grinder to them!

I've looked in some engine books I have and scoured the 'net, but can't find anything.

[226bhp]

Thanks for the replies, I know plenty about how and why balancing, just didn't know what the limits were on an I4, any available info is usually about V8s which are quite different in many areas. I think i'll bring them a bit closer and put them in matching pairs, i've got a fixture which does end for end balancing.

[226bhp]

Pumaracing said:

I usually balance to within a gram but honestly I wouldn't fret too much about it. I suspect that carbon build up and varying amounts of oil clinging to things make up more difference than the couple of grams you're seeing.

I remember getting a Pinto Stock Rod engine to rebuild 15 or so years ago that had originally been done at considerable expense by a "perfeshunal" engine builder in the south west of England of supposed repute who most of the top boys in that class went to back then although god knows why. His power claims were, and I see still are, absolute nonsense and every year he supposedly "discovered" a new better camshaft so you had to come back for the latest spec build although in practice nothing seemed to change power wise.

I couldn't believe what I found in the bottom end when I stripped it. The four conrods were a motley assortment of different casting numbers from different engine years and the heaviest weighed 25 grams more than the lightest! They didn't even have a consecutive series of bore numbers on them from 1 to 4. There were like two 3s and no 2 or something like that. Nothing had been done in even the most cursory attempt to balance them.

I can only suppose they stripped down loads of engines at a time, chucked everything into a parts washer and just rebuilt the engines with randomly selected parts later. However the owners told me they never noticed any unusual vibration and the engine had run well enough for a season with nothing breaking.

It gave me pause for thought as to how much real difference all the fine detail titting around I do on every engine really makes but then you do the best you can on every job and at least you can say you left no boxes unticked.

Stock rods usually have plenty of spare meat to play around with at both ends but forged, shot-peened ones don't and you'll probably do more harm than good grinding on a shot-peened surface and introducing stress risers. I'd leave well alone other than putting the heaviest piston with the lightest rod.

25g

Thanks for all your thoughts, in the end (don't laugh) I got them within .2g. Not for any better reason than I could It only took a few more minutes and I already had the gear to do it. Some thoughts I had whilst doing it:

If you built the engine correctly you would hope the carbon build up be the same on all pistons, you would think the oil hanging off them be similar too, although you could never prove the latter.

Where you take the weight of rods isn't a common failure point (which would be the beam) I took some off where you would on a normal old stock rod, but then rounded off the section around where the bolts go through which was angled (I took the corners off) to get a bit closer on one, I linished out the grinder marks.
I couldn't find any heavy pistons as they were all practically alike.
The little ends of the rods were all alike, they are so delicate on these modern forged rods I guess there is not enough metal for them to be far away. I sussed out that you don't have to balance end for end, if you weigh the whole rod and then then the small ends it's just the big ends which need material taking off (this just meant less time spent and I only had to make a fixture for one end of my balancing jig).

Onto the head now....

[226bhp]

Pumaracing said:

Sounds like a pukka job you've done

I have to say I also find it very difficult not to try for perfection on every damn silly thing which is why I never made much money doing engines. My digital scales only display to 1g but I kept a box of No4 split shot from my fishing tackle next to the scales and they weigh 0.2g each. So by adding split shot to the scales until the display ticked over to the next gram up I could work back to the exact piston or rod weight to the nearest 0.2g. I used to kick myself every time but could never break the habit

Thanks, yes it's coming along nicely. It helps that my supplier is also an engine builder so I can get some good parts and information, having good quality bits to work with cuts down on the amount of work you have to do to get them optimal as you know.
When you can buy the scales which go to .1g so cheaply on Ebay it's daft not to invest and have a go yourself.