Rod weights - Engine Balance

[Jeffery]

For whatever reason, I blew up a piston and bent a rod in my 383.

5.7 rods, 4.030 Hyper pistion.

This is not a race motor, its in a Jeep street machine that will probably never see 6000rpm.

The crank is internally balanced for the assembly that's in it now.


I need to replace a rod and a piston. How far out can I go on weights before it is a problem?

I weighed the old rod, 588 grams. If I put a lighter rod in, say 558 grams, is that going to be an issue?


Keep in mind, this is a patch job, and were it a race motor I would be putting in all new matched components.

[usafracer]

Yes that's a huge difference

[Krooser]

Bolt it in and forget about it... you'll never know the difference. For the first 100 years of the automobile factory tolerances varied so much it wasn't unusual to see 50-100 grams difference per hole.

An old buddy of mine broke a piston in his SBC 383 late model back in the 80's... two nights left in the season and the motor was tired and no $$$ to fix it. Sooooo he bored the bad hole on the floor in his filthy garage with an old Kwik-Way boring bar, scavenged the trash for a usable piston (it was from a Ford!), had a machine shop hone the Chevy rod to fit the Ford piston pin and he put it back together.

Different pin height meant way different compression... different weight to be sure... used piston rings... won the feature that Saturday night...

That old motor won't know the difference.

[dave41]

if you bent a rod , you probably pushed that journal back. vib. now

[Krooser]

dave41...???????

[dave41]

journal back

dave41...???????

BENT the CRANK.

[Krooser]

I'm just wondering how prevalent that is... broke a few rods but never had any cranks bent. Unlikely in a street motor IMHO.

[MasterSbilt_Racer]

I'm just wondering how prevalent that is... broke a few rods but never had any cranks bent. Unlikely in a street motor IMHO.

I have bent multiple rods and the crank be fine.

[Krooser]

That's what she said....

[powerslide]

You can use a thicker or thinner wall pin depending which way you need to gain/lose weight. Its not a perfect way but i would think its better than being 30g total off. As a disclaimer i've never tried this lol

[Fox_hunter42]

hpyer pistons don't like water. Maybe you had a head gasket issue??? I agree, install what ya got

[Krooser]

Ancient post but I took my 383 crank into the shop last week to have one journal repaired. It tossed a rod and took two more with it... and three pistons.

The shop owner asked if the crank was bent... he put it in his fixture and we found out it was bent .018... guess I never blew enough motors to think it would bend that bad! Good quality billet crank, too!

It's fixed now...

[Lizardracing]

There's the right way to do and the way it's been done time and time again.

In street beater I wouldn't hesitate or even thing about it twice. I've seen all kinds of crazy stuff run and run and run.
Bent cranks, bend rods, mismatched pistons, heads, bad bearings, missing/broken rings, cracks in the block, flat cams, etc...

In a all out race prepped engine the tighter tolerances add to the longevity or the performance and sometimes both but the window for that stuff in a street beater is MUCH bigger.

Is it "right?" Who cares if it works!

[Krooser]

I got my crank back from the grinder and I asked if it will need to be rebanced after the repair and regrind. He told me not to worry that the thicker bearing shell in the new undersize bearing will negate any difference in the crank bob weight.

My engine machinist said the same thing so I guess I'm ok.

I also recently bought some spare stock rods to replace the ones that were damaged in the last explosion.

I have a scale here so I weighed them all.... eight stock unbalanced rods and four that were in my engine when it failed and but survived. The stock rods varied from 740 grams to 785 grams. The ones that were previously balanced were from 725 to 735 grams. So the stock ones varied 45 grams yet still gave miles of good service in a stock application. The 'balanced' rods varied more than I imagined... perhaps we are so geared toward perfection today that most would consider those numbers unacceptable in one of today's race engines.

The PM rods in my son's SBC B-mod engine were all within 5 grams before balancing... I would attribute that to the PM manufacturing process being more precise than the OEM's had 40 years ago.

[fastford]

you have to weigh both ends of each rod , if the big ends varied that much ,45 grams, i would be concerned.