What to do with crank imperfections?

[Lordofthepings777]

Recently I’ve begun rebuilding my 400 mopar engine. This is my first rebuild, so I’m learning as I go watching many videos online before trying things.

As the title states, my crankshaft had a few dings in it. I polished it at home with sandpaper and oil before installation (400, 800, 1000, 1500 if I remember correctly), and I thought that would be sufficient. Turns out the scratches were deeper than expected, and one in particular left a bad mark after testing with bearings and a piston. I already ordered a new set of (2) bearings, but I’d like to know your advice on what I should do about the crankshaft? Yes the scratches are fingernail deep, but they don’t catch much. Is it worth a try to sand them out flatter? Or should I try sanding the whole lobe again till it’s more smooth? I don’t wana take off too much material to where the bearings don’t sit right. Let me know your advice.

As specified, I’m not a professional and this is my first rebuild. I’m not going for performance, so I’d prefer to keep the machine work to a minimum. Thank you.

 

[Justin Plant]

Stop where you are and have it machined, then match the bearings to the new rod and main journal size. Please don't put it back together. You'll eat the bottom end in no time if you do.

 

[CBODY67]

Problem is that you NEED to know the OD of the journal after your proceedings! All of the removed metal means more oil clearance with the bearings!

For best results, take the crank to a competent shop to get it turned "10/10" with new bearings for that size of journal. That way, everything is known good and to factory clearance specs. AND should last another 100K+ miles with decent oil change intervals.

CBODY67

 

[Lordofthepings777]

I’ll probably end up calling my local machine shop and seeing what they recommend. I hate to do it as I was ready for reassembly, but need to do things right the first time.

 

[Toolmanmike]

I’ll probably end up calling my local machine shop and seeing what they recommend. I hate to do it as I was ready for reassembly, but need to do things right the first time.

If it's standard, they may be able to get those marks out by turning it .010 and polishing it. Then you'll know it's right.

 

[FinFury]

I dare to be different. If it is a budget rebuilt stock motor I would just fix those nicks and call it a done. Based on the marks on the bearings, those are still protruding and would require more sanding. One could carefully use even a fine file to knock the tops out and then polish those areas again. Ive seen significantly worse cranks that have worked just fine in a stock motor. And yes, I know it is not perfect but does everything have to be?

I guess that those nicks came from the rod bolts when removing or installing pistons? You can make cushions to rod bolts from pieces of fuel hose.

I agree with the aboves that verify it has standard sized journals. I would guess so based on the scuff marks on the bearings

 

[Big_John]

I’ll probably end up calling my local machine shop and seeing what they recommend. I hate to do it as I was ready for reassembly, but need to do things right the first time.

IMHO that crank needs to be turned. Yea, you could polish the high spots down and send it, but I think you already tried that and those bearings show that it needs more. If I were building this engine, I would send it out and have it ground .010"/.010" and be done with it. There are things that you need to do right, especially since this is your first engine, and I think this is one of those things.

Some of the nicks look like they are from banging the rod bolts against the crank. Those were preventable and be sure you don't repeat that at assembly.

 

[Lordofthepings777]

I dare to be different. If it is a budget rebuilt stock motor I would just fix those nicks and call it a done. Based on the marks on the bearings, those are still protruding and would require more sanding. One could carefully use even a fine file to knock the tops out and then polish those areas again. Ive seen significantly worse cranks that have worked just fine in a stock motor. And yes, I know it is not perfect but does everything have to be?

I guess that those nicks came from the rod bolts when removing or installing pistons? You can make cushions to rod bolts from pieces of fuel hose.

I agree with the aboves that verify it has standard sized journals. I would guess so based on the scuff marks on the bearings

Yes. When I removed the old rod bolts that’s what caused these nicks. I really wasn’t thinking while disassembly since the motor was locked up and I wasn’t even sure if the parts were still good. I should have been more careful, and I’m sure it’s a mistake I’ll only make once. Thank you.

 

[68PK21 440.6bbl]

Yes. When I removed the old rod bolts that’s what caused these nicks. I really wasn’t thinking while disassembly since the motor was locked up and I wasn’t even sure if the parts were still good. I should have been more careful, and I’m sure it’s a mistake I’ll only make once. Thank you.

With a old engine with unknown mileage and maintenance what you need to do is measure everything, get yourself some micrometers both inside and out and measure for spec clearance and most importantly 'out of roundness'. Don't just drop the crank off at a machine shop as they will just most likely 'upsell' you without going through the time of measuring everything as it's time consuming. You can just grind 1 or more journals as I believe I've read in the service manual that there are stampings for the engine block to identify which journals are undersize. And what is the purpose for the engine rebuild? Are you just planning on using the car for cruise in shows/Cars & Coffee or intermittent parade type driving or driving to far away events via the Interstate Highways (as the car was designed for) or are you just getting it running just to 'flip' it? It's your choice and pocketbook, dropping engines off at todays machine shops can get you a very expensive and unnecessary big bill with all the upsell machine work that's not really needed for a around town daily driver or car showoff event. Nick's garage goes through this with his Zero decking and he got caught up with a constant leaking rear main seal on a 383 that was 'line bored'(?) some of this unnecessary machining can open the door for further problems down the road if not done right.
Fine if your planning on building a blown drag car, but for regular intermittent use... ? <just say'in>

(edit)
About the motor being 'Locked Up' what was the reason for that? Rust in the cylinder walls? How bad is that?
In a recent save a vintage motor rebuild video the very competent mechanic/machinist... you know a mechanic that does his own machine work and has the equipment to do it although he's mostly a Ford shop, he explains the level of rust in cylinder walls and when is the time to 'sleeve' the cylinder.


.

 

[CBODY67]

Agreed, there are differences in machine shops. Many of the 1-2 person shops CAN be run by older, more experienced builders who have seen enough to know what needs to be done and what is not completely necessary. Their equipment will not generally meet the "recommendations" of the Saturday morning cable car shows, but it will be well-proven and work well. These older guys usually can glance at something and KNOW what needs to be done, by observation. They can "read" the main bearings upon disassembly and know if the block line hone really needs to be "touched". As well as observing the front main bearing for drive belts that were too tightly-adjusted!

Then there are the shops (usually with lots of newer equipment) that want to "touch everything" to make sure it is "right" so their build can be the best it can be. Not unlike the prior "blueprint builds" or similar. Blocks decked, mains honed, deck plate bored, etc. All of which can make an engine that can go well past 100k miles with OEM-level parts and good motor oils, but for a cost.

I suspect that my late machine shop operative would have looked at that nick and advised to just get out his die grinder to knock-off the side ridges, a little polish action on the journals cleaning, and said "That's all it needs". That will just be a place where more oil can be held. Now, if he was building a dirt track or drag racing engine, a "turning" would probably be done if the customer wanted it. After he noted cues of the customer as to if he was "a racer" or "a cruiser", too.

Would it be best to do a "10-10" deal, with appropriate bearings? Sure. But would that be totally necessary? Probably not for most individuals. ONE thing my guy had come to despise with many crankshaft shops is that after the journals are re-sized, the area beside the journal bearing surface needs to be re-done too, which he could look at and tell if it had been done correctly. In some cases, from particular re-griding shops, he rejected several before he got one he could use and be confident with. BTAIM

I completely understand the desire to "shade tree" the engine rebuild. It can be a great learning experience, too. I hope all works well for you, too!

IF you back-track to the 1950s and 1960s, people "did what worked" and learned to look for "evidence" of what was wrong with an engine during the tear-down phase. Back when if a salvage yard engine held good oil pressure, it was "a good one". If it later needed a valve job, just one of the things about buying a used engine!

Just some thoughts and observations,
CBODY67

 

[Big_John]

I really wasn’t thinking while disassembly since the motor was locked up and I wasn’t even sure if the parts were still good.

About the motor being 'Locked Up' what was the reason for that? Rust in the cylinder walls? How bad is that?

You asked the questions that I was going to ask.

 

[Retired auto tech]

The reason we regrind a crankshaft is to reestablish the correct clearance between the bearings and the crank, typically.0025" to .003". This clearnce is responsible
for both the correct oil pressure in the system and the rolling hydrostatic wedge that is what actually keeps the parts from touching. If clearances are too large there will be excessive leakage of oil at all bearings making it impossible to have correct pressures. This also applies to the cam bearings.

For example if the crank is worn as little as .005" we now have a clearance of .0075 to .008 with a drop of 10 to 20 psi when at operating temperature.
This can be approximately checked using plastigauge on the mains and throws. However checking cam bearings requires both inside and outside micrometers.

Crank polishing is not just the act of sanding but is done with a specific tool somewhat like a long belt sander while the crank is spinning on the crank grinder to not only establish the smoothness (surface finish) but also the correct direction of the "peaks" of the metal surface. This is done to provide for oil retension and prevent the metal surface from damaging the bearings on start up and break in.

Other querstions that come to mind are have you measured the cylinder bores for taper and out of round? If outside of spec you can end up with piston slap and poor ring sealing and even ring breakage. How have you cleaned the block? Did you remove all of the oil galley plugs and core plugs and use hot soapy water at a
car wash and then blow everything out with compressed air? Any residue left in the engine will very quickly destroy all of your hard work. Have you checked the block deck and heads with a straight edge and feeler gauge for warpage? If too far out of spec it may be difficult to get head gaskets to seal.

Do you plan on reuseing the cam and lifters? If so then only if you kept the lifters in proper order to go back on the lobes they were on and none of the lifter bottoms are dished or damaged and all lobes are good. The timing chain is garbage, replace with a new double roller. Also inspect the harmonic balancer and replace if the rubber has deteriated.

Valve stem seals will be hard as a rock and need replacing and the guides need to be checked. If not too bad they can be knurled saving the cost of replacing.
Worn guides and seals are the main cause of oil burning. Inspect valve faces and seats. If in good shape you can maybe hand lap them but a proper valve job
will give better sealing.

Inspecting, measuring and attention to detail will result in success.

 

[fury fan]

I'm neither a .0001" machinist nor engine builder. I'm not an economist or insurance salesman, but I'll speak to it.

As @Retired auto tech said, you need proper clearances and the hydronamic wedge for your crank to live.
These are the rod bearings, and those take a hammer-pound every time the cylinder fires. (the mains are more forgiving in this regard)

It was one thing when you were blissfully unaware how well your polishing was going, and the remaining scratches.
You have positive material on a surface that needs to be true and round, not just smooth.
You can knock it down, sure, but what shape will result? What shape do you have now?

Consider how much work you've put in so far, how much more to go to assemble the engine.
Time and effort to put it into the car, install to the trans, final connections, tuning, etc.
If that bearing shits the bed within a few hundred or few thousand miles - then what?
Lather, rinse, repeat.

The economist asks - how much did you spend and lose on non-reusable gaskets, fluids, etc? Those costs are your discount on the machining work.
The insurance salesman asks - How much is it worth to not repeat all this effort, how far did the debris travel, and did you damage the crank even further? The machining cost is the insurance against that. (and against a lot of time in re-work)

I'm a believer that not everything must be done 'the right way' as if that's the only path to success.
But I would definitely get this crank done.

 

[MEV]

Nobody local to me turns cranks anymore, thats one of the reasons i wound up with a stroker. Beautifully finished crank and rods fully balanced and shipped to my door. The added power was just a bonus.