Machining crankshaft .040 on rod journals
 

What is everyone’s opinion on running a crankshaft that has been cut .040 on the rods? My N crankshaft is a little rough and the crank grinder thinks it will take .030 or .040 to clean it up. I’ve always ran std or no more than .020 under on the rods. .040 sure seems like a lot. The engine is a 68 WS 400 and will likely never see over 5,500 rpm and not be over 400 horsepower.

Just make sure you have bearings in hand before doing a grind that large. Not many companies offering bearings in odd sizes anymore.

In the years before crappy Communist Chinese cast cranks, guys took OEM Chevy 400 cranks, and cut the mains 0.200 (TWO HUNDRED thousandths, not twenty thousandths) so they'd fit into 350 blocks to make "Stroker 383" engines.

.040 is nothing, IF YOU CAN GET BEARINGS.

I have a Chevy crank at the machine shop right now, waiting on an offset grind that's gonna need 0.050 rod bearings when it's done. (Assuming I can get .050 bearings in a semi-post-Covid world.)

Do not worry about it, I have had a .030-.030 crank running hard forever.
It is very important to make sure your main bearing oil hole lines up exactly with the hole in the block on the tab side of the bearing. Or you will starve your rods from oil at high RPM.

He is only talking about his Rod bearings here.
I have done this on many of my motors right off the bat since it cuts down on both the Bearing speed ( heat reduction ) and the need for oil volume at the same time.

It also allows for a nice fat strength increasing radius to be ground in from the journal to the Crank cheek.

Just make sure during mock up assembly that you do not have to narrow the rod Bearing to clear this now bigger radius.

Going .040 on the crank isn't a problem when it comes to the strength of the crank. The issue is getting a good quality bearing at that size. Definitely make sure the bearings are available before grinding. When I say available, not that they're listed, but actually in stock.

Clevite/Mahle has a listing for .040 undersize bearings for a Pontiac engine. That doesn't mean they are very available. However, a SBC STD rod bearing is 2.100 which is the same as a Pontiac bearing -.040. That doesn't mean it will automatically work. You or your machinist, will need to verify ALL the dimensions of a SBC bearing if you want to go that direction and make sure the bearing tang, width, chamfer and radial wall thickness is all the same and can be safely used. the .040' undersize crank journal is not an issue, the correct bearing is of course. I think the problem can be solved. Much less of an issue than a main bearing issue where bearings are very limited for a Pontiac engine.

I think you are mistaken with your decimal point. Stock Pontiac rod journal is 2.250

Yes, I thought something didn't look right there. Sorry. So an option would be to grind the crank -.050 and use BBC rod bearings in STD size. I don't see why that wouldn't work if -.040 Pontiac bearings are not readily available. Again sorry for the incorrect SBC information.

As long you have a good radius is the big factor.,

You would actually have to use a rod with a BBC big end, dimension wise, to use that bearing.

Thanks for the replies everyone. It sounds like as long as I can get some bearings, it really wouldn’t be a big deal turning one .040 on the rods. I’ll be sure to get some bearings in hand before the crank gets ground.

My 020 BBC Rod Bearings were only avail without the Rod-Pin hole. So Imhad to make fixture and Drill each Lower. Looks good.

Thought the surface hardening (tufftriding ?) only went .010 deep.

Correct, Eagle and others offer a 6.635 BBC rod that would work well.

The 6.635" BB Chevy rod journal has a 2.200 journal diameter.
I have used that rod journal on several of my special Kellogg/Moldex forgings
with a 3.5" stroke. Joe at Moldex NITRIDES the journals after he makes the rod
journal modifications.

Marty Palbykin did a 473 cid engine built with a 2.100" Chevy SB rod journal to get
the stroke he needed for his 740 HP naturally aspirated engine before he went boosted.

If the crank work is proper people have run less than 2.00" rod journals in engines making over 1400 HP boosted. No worries except finding correct bearings for the Pontiac rod journal modifications.

Tom V.

ps and when the engine failed, the rods did not fail, or the bearings, or the pistons.
THE 455 BLOCK 4 BOLT STRAIGHT BOLT MAIN CAPS FAILED ON THE #2, #3, & #4 MAINS.

Factory cranks like the N crank ( and all others ) the OP is running are not surface hardened .

I have had several Factory cranks Nitrided by Joe at Moldex (and his dad "Whitey") when they worked for the Moldex Crankshaft Owner (before he sold out to Joe a few years ago).
Mr Bob Gillian was his name and he was a crankshaft artist and expert.
He ground every type of crankshaft you could imaging in his lifetime. Automotive, Military, Aircraft, 1 cylinder, 2 cylinder, all the wat to 16 cylinder engines. They believed in Nitriding a crankshaft.

RIP Bob & Whitey.

Tom V.

Consider a full Re balance on that crank and whole assembly. Grab some plastic gauge.

This^^^. Built a .030/.040 under 389 years ago and it was just fine. Pontiac cranks are super beefy and cutting undersize is not an issue.