Matching rod weights
 

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

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.

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

Jack

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

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

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.

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

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."

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

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.

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.

I love it!

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

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.

Send your rods and I'll match your set.

I REPEAT, 20 GRAMS IS TOO MUCH!!!

Replacement is the proper coarse of action.

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

Jack

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.

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

Grinding is not the problem. As with anything else, it's excess grinding that I question. Wanye is correct when he says the factory had many weight groups for rods. That's why a quality shop (mechanic) will match weight groups and grind to balance.


As for Snowman's claim that I shouldn't be working on these engine because I'm not a butcher shows both his style and intelligence.

A good matched set of used 2.4-2.7 rods is only worth about $200 and I won't compromise to save $200.

Let's review: match from similar weight group or weld a bolt into a wrist pin. You choose.

Here we go with the rock and chisel again.

It's a small sandbox - let's all play nice together (this means you too, snowman). Both of you have both proven your acumen to all of us here. No need to ruffle feathers, no matter who started it...

-Wayne

"Can't we all just get along?" Rodney King

Now we know what the R in Wanye R Dempsey stands for.

Sorry the comment wasn't ment to be directed specifically at Henry, but if your into grinding ANY weight off a rod, you need to do it intelligently. 20 grams is the weight of 20 one dollar bills, not a whole bunch, heck some production engines arn't even balanced any better than that (20 gms). Whats the Porsche factory limit on reciprocating weight? 5 grams? I think the crank is 15 gm-cm or something like that.

well give me the "caveman" award as I did this to mine and 7000 miles later its still revin past 7000 rpm on a reg basis.Also beavered off a bunch on the pistons.........I would have to say it works very fine for me. Nothing against H.S (respect him) and Snowman.....but if you have the stuff to weigh it...........you can just as well beaver some off. The rod will never ask you if it cost a lot or not to make him match his brothers

My main point was trying to take some of the MYSTIQUE out ot the Porsche engine. IT AIN"T MAGIC, its just a hunk of metal and metal is metal, Porsche or CHEVY. Well not quite Chevy(the materials much better in a Porsche). You machine engines, no magic involved. ALL basic engines are the same.

To make a point, many mechanics will scrap a block because a single cylinder is hosed. What about a $20 steel sleeve??? Generally dosen't apply to air cooled Porsches but does to almost everything else. The steel sleeve is BETTER than any stock cast iron cylinder, by far, yet certain people will scrap a otherwise perfactly good block because of it. DUMB?? or just a way to make a couple extra bucks?? Don't get suckered because of MYSTIQUE.

Porsche or any other mfg dosen't grind rods, they just sort them into groups. Why, because they have LOTS of rods to work with, and they do not want to pay anyone ot grind and re condition a rod. That simple. NOTHING to do with quality.

And if your worried about failure, whats the weakest point in ANY engine???? The con rod bolts. first, the con rod bolts second, and the same third. Rods only fail after the bolts in most cases. Thats why Carillo rods are built around the sps bolts. If you doubt this contact Carillo.

I agree with ya Snowman,,,I know of course H.S. way of grinding off the bottom of the cap will be more refined then my method.....but the results will be the same -shoot for 0 gram diff and all will be well.

Wow, Snowman, you are brave!

I consider myself to have reasonable weldig skills, and more importantly know some really world class-welders ( literally).

How anyone can weld into a pin without a drastic change in the metallurgy beats me..

And, surely the point is not whether we can geta way with taking 20G off one rod...its whether it is sensible, when rods are plentiful and cheap..

I know I would not do it...unless foced to..but then I have dozens of rods to choose from..

Kind regards
David

MY LAST COMMENT ON THIS FOOLISHNESS.

"Ollies won't grind 20 grams" Ike
Costa Mesa R&D " 20 grams if you have a weight pad to grind, otherwise 6-10 grams depending on which end" John
EMS "Won't try 20 grams" Bill
Riddle Machine ( Rimco)"7-10 max" Greg
Aasco " 20 grams, NO NO" Dennis Aase
Back yard "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." Snowman

We will replace the rod to match because we are a professional engine shop with hundreds of rods to choose from.

Now opinions vary but some opinions are more valuable than others.

http://forums.pelicanparts.com/uploa...1128355986.jpg http://forums.pelicanparts.com/uploa...1128356060.jpg http://forums.pelicanparts.com/uploa...1128356073.jpg http://forums.pelicanparts.com/uploa...1128356088.jpg

PS: Now I know who's been chiseling cam nuts off.
http://forums.pelicanparts.com/uploa...1128356215.jpg

I would NEVER chisel a cam nut off. I have used a 1" impact gun though. Those of us that do not have personal Porsche junk yards might want to save a couple of bucks without jepordizing the engine. That great big fat hump on the bottom of the rod is just excess weight. Show me a rod that has failed at this point and I might change my mind. Of course, if you just happen to have what you need, just laying there collecting dust, and you can get rid of it, and make more money doing so, why do anything else.

As to welding a bolt into the wrist pin to add weight, I didn't even think it would work before I tried it. I found a bolt that had to be lightly pressed into the hole (after having cut the head off the bolt) and then with a TIG welder set fairly low was able to tack the bolt in several spots. Nice thing about a TIG is you can keep the overall heat down and yet penetrate enough to hold. Since the weld only has to hold the almost zero lateral force on the bolt to keep it in place, it can be tiny. Clearly not a first choice but if you have say 7 perfactly matched rods and one that is 10 grams lite in the small end, OR one piston is 10 grams lite, it can be a very reasonable thng to do. After welding the pin fit into the rod just like it did before welding, nice and smooth, no binding and no discoloration for more than half the wall thickness of the pin. And on top of that the weld is only on the very inside edge of the end of the pin. So did I hurt the pin or not? Will the bolt fall out? never.

Another comment on balancing. Even though we all tend to balance things to less than one gram, what is the PRACTICAL point, ie the one that REALLY makes a difference? Should squirter design be condidered in balancing? After all oil may weigh several grams and depending on the time in the engine cycle, there may be a very big difference in the ammount of oil on each component. THen there is G forces and oil sloshing around to consiser. Next there is differing ammounts fo carbon buildup on components.. Perfact static balance is nice but whats for REAL?

75-911S did it right and now he has peace of mind knowing his repair was completed to industry standards.

http://forums.pelicanparts.com/911-engine-rebuilding-forum/246512-infamous-20-gram-too-heavy-rods.html

I picked up a set of 3.0 rods from Supertech last Thursday. All balanced to 1/2 gram...Henry IS a pleasant and smart business man. He likes Porsches, and wants to help all us garage type mechanics. His shop is very nice and for what he charged me, I feel I got a deal.:D

I LIKE MINE AT 0 GRAMS.....but I am just picky....in the end it will not matter.
A smart man he is, we are all just diff/and we all have diff priorities.
This not meant to be sarcastic, but if you are going to be 1/2 gram off.... for the $$ you spent you could have bought a scale and been closer then that, right?? Just trying to see the reason for that direction??

All test equipment has an operating tolerance. Even a 0 measurement has a +or- tolerance. To pretend that 0 is exactly 0 shows a lack of basic understanding of engineering principles.

Are you sure you want to bring your knife to this gun fight.

BTW: It's not that the rods weigh the same, it's how they were made to weigh the same that is important.

Cheers.

No I am not trying to do battle...just education,..If I shoot for 0 grams- nothing is purrfect, but would I not be closer then starting out at a 1/2 gram off??
Why are you always threatening me with your proverbs....let it go already. I apologize for ruining you day with a question even.

You really don't get do you?
Our goal is 0 gram difference and the results are 0.0 +or- .5 grams (the factual limit of our equipment) This is a scientific statement of weight.
Your statement 0 grams is meaningless . It could have been weighed on a bathroom scale so 0 really means 0 grams +or- a pound.

Who is educating who?

thats why I asked the question.....the only dumb Q. is the one not asked in my book

Theres a reason that Porsche dosen't balance their rods to 0.5 gram, cause it don't make any F difference, thats why. Nice to say you have done this, but just the oil sitting on the pistons will make a liar out of you in the long run. Say well you do the best you can, well you can do 0.001grams if you want to, I can even buy scales that do better than that, but does it matter??

It is difficult to control the fluid dynamics that exert influence inside the engine. What we can control are the parts we install and the method and tolerances we use to prep these parts. It's easy to say " it don't make any F difference" it is not so easy to take pride in what you do and the service you perform.

Taking 15 or 20 grams off of one rod would take quite awhile, taking it of of most of the rods could keep you tied up for hours.
And you would have weaker rods when you were done. I have never had a problem finding a good used 911 rod in the proper weight group either in my stock or one phone call away for not to much money.