Crankshaft Balancing Question

General engine tech -- Drag Racing to Circle Track

Moderator: Team

Post Reply
Krooser
Expert
Expert
Posts: 999
Joined: Wed Jul 31, 2013 10:14 pm

Crankshaft Balancing Question

Post by Krooser » Thu Nov 02, 2017 6:40 pm

So I have had my small block mopar crankshaft repaired and reground.

The engine suffered a broken camshaft and was freshened... then it lasted just a few laps in a dirt modified before a rod spun and destroyed one rod journal, three rods and four pistons. The crank is an older Mopar Performance forged deal and weighs in at a hefty 67#.

I believe the engine was run with too thick of an oil and run cold. From what the PO told me the engine started to seize but the driver thought it was an ignition miss so he pushed it harder.

The crank was repaired at a local shop that does a lot of crank work... it was bent .018, straightened and the rods ground .020 under and the mains cleaned up at .010.

Will I need to have this rebalanced? How about the rods and pistons. I'll be replacing four pistons (Ross I believe) and three rods (stock bushed Mopar 340 deals). The crank had quite a bit of mallory in it... the pistons and rods are faily heavy with the piston around 700 grams and the rods around 725.

I'm taking the rods and one piston back to the shop that did the original build. He has most of the info from it's orginal build and was the one who also did the refresh.

This is the engine I want to start the 2018 season with in my dirt late model.
Last place in the B-main is better than anyplace in the grandstands...

User avatar
modok
Guru
Guru
Posts: 1293
Joined: Sun Jun 06, 2010 1:50 am

Re: Crankshaft Balancing Question

Post by modok » Thu Nov 02, 2017 8:16 pm

Krooser wrote:
Thu Nov 02, 2017 6:40 pm

Will I need to have this rebalanced?
I think no
Glen Urban

dwilliams
Expert
Expert
Posts: 923
Joined: Sun Oct 15, 2006 10:00 am

Post by dwilliams » Thu Nov 02, 2017 8:39 pm

> Will I need to have this rebalanced?

For one journal reground? That's below the repeatable resolution of my machine. Button it up and be happy.

racear2865
Guru
Guru
Posts: 1358
Joined: Mon Sep 21, 2009 11:58 pm
Location: tenn

Re: Crankshaft Balancing Question

Post by racear2865 » Thu Nov 02, 2017 8:52 pm

The weight you remove by grinding the journals will be replaced by additional weight in the bearing. basically negating any balancing. The largest problem you will have is getting the new pistons and rods to match the used parts. Good luck on that.
reed

PackardV8
Guru
Guru
Posts: 5032
Joined: Sun Jul 30, 2006 2:03 pm

Re: Crankshaft Balancing Question

Post by PackardV8 » Thu Nov 02, 2017 9:08 pm

The crank, no. The new rods and pistons, yes.
Jack Vines
Studebaker-Packard V8 Limited
Obsolete Engineering

Krooser
Expert
Expert
Posts: 999
Joined: Wed Jul 31, 2013 10:14 pm

Re: Crankshaft Balancing Question

Post by Krooser » Thu Nov 02, 2017 9:31 pm

Thanks.

If these are indeed Ross pistons they still make a similar piston but it's 22+ grams lighter which won't be a problem to match. The rods will be easy. too.

Just wanted an opinion here... I trust the shop that's going to do the work... they have been around a long time, seem to be nice people and you seldom, if ever, hear anything negative about them.

I would have had them do the crank, too, but I can't complain about the guy who did the crank work and he's 100 miles closer to me.
Last place in the B-main is better than anyplace in the grandstands...

Post Reply