Team Chevelle banner
Cancel
Create thread New Garage Forums

Change con-rods without balancing engine

Jump to Latest
1 - 15 of 15 Posts
S

Spang

· Registered
Joined
·
102 Posts
Discussion starter · #1 ·
I have new forged pistons, block machined and crank balanced.
When I checked my old rods today they were both bent and twisted quite a bit.
Now I need new rods and it is time to buy stronger ones.
But how much difference in weight (big and small end) is acceptable whithout rebalancing the crank?

And how much bend or twist is acceptable on a con-rod?
I am not going to use my old rods, but is curious if there is any general rule out there.
 
#2 ·
I was told from my engine shop plus or minus 5-10 grams total. Basically you have to get identical rods that each one ways the same as the last ones. I got no info on rod bend or twist other than if you suspect it change it.
 
Save
Reply Quote Like Like
#3 ·
How are you determiming the "bend and twist" in the rods?
 
Discussion starter · #5 ·
It is a street/strip engine, used on the weekends.

I am using a pair of dummy pins, 4.5 inches long, in the big and little end.
Measure on both sides to see if it is parallel.
This I call bend.

To check twist I lay the rod on a flat surface, only supporting the dummy pins.
Rest of the rod will hang free in the air.
If it is twisted, the dummy pin in small end will lift off the flat surface in one end.
This I call twist.
 
#6 ·
Believe it or not, they could have been machined that way, especially if they have been re-worked.

As a Machinist, I can tell you that many methods of "rebuilding" rods leave a lot to be desired.
 
#8 ·
It's almost weird how easy the stock rods bend if side loaded or twisted. Their strength depends almost entirely on being loaded near perfectly straight in either compression or tension. start any twisting or bending moments and it's all over.
 
Save
Reply Quote Like Like
Discussion starter · #9 ·
If you build a high horsepower engine you definite want stright and true rods.
But when you have an old low performance engine, is there any rule of thumb of how much difference is acceptable?

You check small 2-stroke engines this way, and a 0.0008 difference in both ways is ok.
But they have roller bearings and 3-4 inch between centers.

I suspect a V8 rod is not that critical, but how much can you accept in a daily driver or a performance setup?
 
Discussion starter · #10 ·
If you change rods or pistons in an engine, how much difference in weight is acceptable?
Is higher or lower weight components preferable?

I assume it is some difference on a low and a high performance engine too.
 
Discussion starter · #11 ·
My connecting rods weight is 828g today.
591g in big end and 237g in small end.

The best rod I easily can lay my hands on today is 815g.
I do not know weights in big and small end.

Is this an acceptable difference in a supercharged 700Hp street/strip engine?
 
#13 ·
-/+ 5 to 10 grams doesnt sound safe at all
most are matched to within +/- 2 grams. and @6000 RPMS those 2 grams are more like 300 pounds getting swung around so do yourself a favor and get that entire assembly balanced!
its only around 125 bucks and will save you from whipping wrenches if something does go wrong.
 
Discussion starter · #14 ·
Yes, it is supercharged. A procharger centrifugal supercharger.

Plus or minus 1 or maybe 2 gram seems to be what to be seeking for in a high performance engine, 5-10g in a mild bigblock.
About twist and bend of rods, it is not acceptable att all.
Not in a high performance engine.

I am going to use new, lighter and stronger rods and have the crank balanced.
Or use Eagle I-beam rods that weights 830g.
Think it is possible to have these to my right weights in big and small end of the rod.

I was hoping I could assemble this engine using new connecting rods right now, but it seems I will have to wait and assemble it later.

Now I will assemble an old 454 instead, so I have an engine at all.
Scratched cylinders, cast pistons and crank and so on.
500+ Hp whitout the supercharger is the goal if the engine was ok.
Is it going to live forever, or not?
 
#15 ·
not. but you might be able to limp it around till you can get a real engine built.
 
Save
Reply Quote Like Like
1 - 15 of 15 Posts
About this Discussion
14 Replies
8 Participants
Last post:
Tom Mobley
Team Chevelle
posts
5.3M
members
131K
Since
1998
A forum community dedicated to Chevrolet Chevelle owners and enthusiasts. Come join the discussion about restorations, builds, performance, modifications, classifieds, troubleshooting, maintenance, and more!
Full Forum Listing
Explore Our Forums
Bench Racing Performance Engine Chevelle Tech Body Shop