Matching rod weights

[75-911S]

I have six good rods for my 2.7 that have been rebushed and "recentered" However. five of them weigh 705 grams, one weighs, 710 grams and one weighs 690 grams. Can I use this light rod even though it's outside the recommended maximum weight variance?

Jack

[Henry Schmidt]

5=705
1=710
1=690
That sounds like 7? new math?
Math aside, have the 705 and 710 rods balanced. If your machine shop can't balance these rods, send then to us.

[75-911S]

Thanks Henry. I can't add, but I can do email. I'll send you one as a follow up.

Jack

[Nine9six]

The 710 should material removed to match the 5, 705's that you have.

[75-911S]

Math was never my strong suit. That's why I became a journalist I actually only have four rods weighing 705 grams, one at 710, and one at 690.

Jack

[Henry Schmidt]

A 20 gram difference is more than most balancers would care to work with.
It may be necessary to replace the lightest rod.
We have many rods to choose from.

Because of different end weights it would be necessary to weigh both ends in order to match your set.

We have seen rods that have an overall weight that is the same but 18 gram difference from end to end.

[75-911S]

Henry,
Like I said, my rods have new bushings and have supposedly had been "recentered" or whatever it's called.
Five of them have the number 31597 19 and one is 31597 11 (the light one). I guess it's pretty obvious that the person I bought them from, mixed them up.
Anyway, without nuts or bolts my rods have the following big end - little end weights; 406/170 = 630 grams, 455/170 = 625, 460/165 = 625, 465/165 = 630, 460/165 = 625, and
445/170 = 615, for the light one.
Would have one similar to the 625/630 gram rods that you would swap me for the light one or sell me?
I thought I was all set to start putting this engine together until I did the weights. So, I'd like to move on this right away if you can help me.

Jack

[Nitrometano]

Just grab a grinder and remove material from the rod until you get the same weight from the other rods.

[Henry Schmidt]

Originally posted by Nitrometano
Just grab a grinder and remove material from the rod until you get the same weight from the other rods.

Why use a grinder? Just find a beaver to chew on them until they weigh the same, sand the old rod bearings, then take a rock and a chisel and reinstall the rod nuts.

At some point, you have to use some of the principles of proper engine assembly.
I repeat "A 20 gram difference is more than most balancers would care to work with."

[Nine9six]

Henry is absolutely correct! Issues of structural intregrity may come into play after removing that much weight.

[kenikh]

Speaking of balancing rods; is it best to do the rod balancing with the crank present or does it matter? I just bought some 2.2 rods that need to be refurbed, balanced and shot peened, but need to know if I can do it now or wait until I acquire the crank.

[Henry Schmidt]

All rotating parts should be balanced separately then those parts that can be rotated together should be.
In short rods can't be rotated with the crank and bob weights for this type of crank are rarely if ever used.

Balance every thing separtately , then dinamic balance crank, add pulley, then flywheel and last pressure plate w/bolts & washers.

[911SCfanatic]

Originally posted by Henry Schmidt
Why use a grinder? Just find a beaver to chew on them until they weigh the same, sand the old rod bearings, then take a rock and a chisel and reinstall the rod nuts.

I love it!

[75-911S]

After all this discussion, I still need a 2.7 rod in weight group 8 that weighs about 710 grams with the nuts and bolts. Or one in weight group 7 that weighs about 705.

Thanks.

Jack

[snowman]

Just have all the rods ground down to the 690 number. BUT it isn't just the TOTAL weight its the big end and the small end. All the big ends must weigh the same, and finally all the small ends (ie total weight must be the same). If the weight is mostly from the big end its no big deal, just do it. If its that much from the small end, you need another rod, doubtfull.

Rods are weighed on a balance, the rod horizontal, supported by a bearing or pin on the small end and a scale on the big end. All the big ends are weighed. THe smallest one becomes the standard and all the heavy rods are ground on the big end untill they are equal to the lightest one. Next the small end is ground untill all the rods equal the lightest rod, total weight, which is exactly the same thing as weighing the small end and matching the rods, only easier.

Weights are catorgized into recprocating and rotating. The weigt of the piston and the small end is part of the recprocating, the big end is part of the rotating. The sum of the piston and the small ends must match every cylinder, the big ends must match. That simple.

You can grind a whole lot off the big end, thats between the two rod bolts, just do so evenly and finish up with some fine sand paper and finally a new shot peening.

[Henry Schmidt]

Originally posted by 75-911S
After all this discussion, I still need a 2.7 rod in weight group 8 that weighs about 710 grams with the nuts and bolts. Or one in weight group 7 that weighs about 705.

Thanks.

Jack

Send your rods and I'll match your set.

[Henry Schmidt]

Originally posted by snowman
Just have all the rods ground down to the 690 number.

I REPEAT, 20 GRAMS IS TOO MUCH!!!

Replacement is the proper coarse of action.

[75-911S]

Sounds good Henry. I'll send them off to you with a note as soon as I can.

Jack

[snowman]

Just why is 20 grams to much??? All depends on where you take it off the rod and how. Out of 700 grams its only 2.8% of the total! If you cant' get this much out of a rod without hurting its strength, you maybe shouldn't be working on them.

And on the small end you can always add 10 to 20 grams by welding some weight inside the wrist pin. I have tig welded a entire bolt inside the wp without distorting the pin and distrubing the fit. It may not sound nice but it is solid and will not fail.

[Wayne 962]

Hmm, what no one has mentioned is that these rods are available from the factory in different sizes. Someone might have replaced a rod in this engine without checking the weights. You can get them in lighter weights, but as Henry said, it's better to mix and match than to grind.

As for more material removal, since they were available as a stock rod in the lighter weights, I would probably say that grinding them down to a lighter weight would not sacrifice the structural integrity of the rod.

-Wayne