[Ragtop Man]

A few years ago, a local guy was selling off his grand dad's leftover Pontiac parts. Bought a 4" crank from him, but did not check it as carefully as I could have. Saving grace it was a good deal (beer money) and now have a good runner coming so this would be for a future build.

They don't look like they would be a problem, but have never played with these before and defer to the experts.

Many thanks!

[steve25]

Well one thing is for sure, if it passes a magnaflux test it’s going to need to get cut if it’s not too far undersized already due to that level of rust it has .

There looks to be a lot of mass taken out of those outer most counter weights.

Makes me think that the crank was balanced for Aluminum rods.
If that’s the case then running steel rods again will call for adding Malory metal to get it balanced again and that will be a big added cost to the balance job.

[Half-Inch Stud]

There may be some lightweight strong steel rods out there, that are lighter than most AL rods. I forget the MFGr.

[mgarblik]

None of the pics really show what kind of crank this is? 4" what? It looks like a stock PMD crank but no pics of any numbers or an N or anything. I assume it's not an Eagle cast from the one pic of an oil hole.
Obviously, it was re-balanced for a pretty light rotating assembly. So have to assume it was raced in some form. I guess if I wanted to use it, my approach would be to measure it first. If it is STD or maybe .010. you can proceed. Then have it cleaned and magnafluxed. If it passes, then have it ground and balanced. Most likely looking at the counterweights, it's going to need 4 pieces of Mallory metal added back to the counterweights and welded in. Probably have to figure $600.00 to clean, magnaflux, grind, polish and balance that crank, at least around here. May not be worth it. If your going to run super light pistons.pins and rods, you may not need to add that much weight back. Mallory metal is expensive, typically 75-100 per piece installed and welded. Also be aware that -.020, -.030" bearings are very hard to source and will be for quite a while. All the bearing manufacturers are saying no -.030" bearings for about 2 years. -.020 will be spotty.

[b-man]

Find another crank, that one’s much too corroded along with the other issues mentioned above.

Be happy you have next to nothing invested in it and move on to something better.

[Schurkey]

Tap some or all (as needed for appropriate balance) of the drilled "balance holes" and install fully-threaded steel bolts with Loctite 271 and some "activator" for security. Cut the bolt-heads off.

Some guys would pound-in unthreaded steel rod with a mild press-fit, then weld to the crank. I'm not a big fan of welding steel to iron, but I guess they get by with it.

Any way you look at it, the rest of the crank needs to be suitable for re-use--no cracks, no excessive rust, not bent, rear main seal surface still usable, etc.

[hurryinhoosier62]

Originally Posted by Ragtop Man

A few years ago, a local guy was selling off his grand dad's leftover Pontiac parts. Bought a 4" crank from him, but did not check it as carefully as I could have. Saving grace it was a good deal (beer money) and now have a good runner coming so this would be for a future build.

They don't look like they would be a problem, but have never played with these before and defer to the experts.

Many thanks!

If it passes NDT and is on stroke use it. Be sure to tell the machinist you use to touch up the radius on all journals.

[hurryinhoosier62]

Originally Posted by b-man

Find another crank, that one’s much too corroded along with the other issues mentioned above.

Be happy you have next to nothing invested in it and move on to something better.

Bart, I have successfully reground far worse. I agree with Mike; the crank is going to need 4oz. of Mallory metal to rebalance it.

[b-man]

Looking back on the machine shop labor on my own 3.25” main 4.00” stroke 421 crank which was a standard crank with no rust on the journals or anywhere else:

Crankshaft Grind-H.P. Middle Spec - $129.00
Crankshaft Heat Treat - $195.00

These are 2015 prices at a well-respected shop that now no longer does work for anyone except their racing customers. I sent them a perfect untouched crank, your mileage may vary.

With the spotty availability of undersize bearings and just from the looks of it I’d pass on putting any more money into that crank. I’ve never done automotive machine work but I still can see that starting with a better piece would be a wise move.

Just because you already have it doesn’t mean you have to use it.

[mgarblik]

Can't disagree with your thoughts on this one. The cost of a better core crank for this project is probably $125-150 tops. The problem around here is finding one! They just aren't laying around in people's garages and junk yards anymore. I had one heck of a time finding a good 400 crank recently. Fortunately, a forum member helped me locate one. A 455 crank is a little harder to find yet. Biggest cost on the OP's crank, assuming it's a good core would be fixing the balance.

[Half-Inch Stud]

Mallory metal would be for heavy factory steel-reinforced slugs and stock rods.
Threaded Steel plugs (staked) makes the most sense.

Shot-peen or/and electrolysis to remove rust would brighten your day. 10/10 makes like new with improved radius on fillets.

Machined rod shoulder looks like an ArmaSteel trait. Ident might have been ground off huh.

[Ragtop Man]

Thanks to all on this one - appreciate the collected wisdom.

Agree that this would be a lot of project for a plain old 4" crank, so it will stay where it is in the garage for now.

Like the idea of staked in steel inserts to get back to normal balance, and for sure, a mag/x-ray would be a necessity before starting.

I don't recall 4" cranks ever being in good supply, the CL posts for the last several years rarely show them. There are a few 455 arms out there right now, though.

The 421 that came from Massachusetts landed at the freight terminal today (Fastenal for the win) so we're going ahead to get the ole gal moving under its own power for now.