Inliners International Forums General Discussion Engines Problem with a GMC 270 Crank

I have just over 14,000 miles on this engine since a complete rebuild. The rod journals were ground 0.010 under and the mains 0.020 under about 5 years ago.

Bottom end started knocking and I pulled the pan (I was driving cross country and drove with this knock for about 12 miles to the nearest town) and found that the cam looked suspect and some of the lifters had stopped rotating. I assumed that the metal from the cam had damaged a rod or main bearing. I ended up trailering it home.

I pulled the engine today and tore it down -

I started pulling rod caps starting with cylinder #1 . . . I work my way along and things look pretty good - This is #3 and is typical of what I am finding.




Bearings don't look bad at all and no signs of metal embedded in the babbit. As stated, this bearing has about 14,000 miles on it - much of which was run at ~2000 rpm with quite a few excursions to 4500 - 5000 rpm. Pretty much all the bearings looked like this . . . except one!


I get a surprise when I get to #5 when I see this.



The two bearing halves were bonded together - so I got a stiff scraper and pried the two tangs away from each other.




SO then I look at the crank journal which for the most part still looked pretty decent until I rotate it a little and clearly see a large crack. The crack had a sharp edge that wiped out the bearing and that is what the knocking was all about.







I am pretty sure that if I gave it a good yank that I could break it off right across the middle of the #5 journal.





Upon close inspection, I see another crack forming across the #6 journal, but the bearing on #6 looks good.

I am extremely fortunate that I didn't break the crank in the block and wind up screwing up my block and rods / pistons.

I am going to grab another 6 bolt 302 crank and get it magnefluxed and balanced. I am pretty sure I have one in the pile.

Anyone every had a crank crack like this? These cracks look like fatigue cracks - I am running a rebuilt 270 harmonic balancer . . . was it just a bad forging? The block / mains were line bored when I rebuilt the motor and the work was done by a shop with a good reputation.

I don't want to go through this again.

Ken,that crank journal has been repaired before(welded). You can see in your next to the last photo where there is a different layer of material where the radius blends into the original fillet material of the crank. Another way to confirm is to look down inside the oil hole in the journal. The correct procedure would have been to magnaflux it to make sure it wasn't cracked before proceeding with the repair. It looks like they welded over a crack in the radius/journal and it has come back thru the repair to the journal surface. I think they made a bad call to repair it instead of scrapping it.

The only time I saw this happen was when the rear main was put in upside down.

When you do that the oil holes are covered and no oil goes to the #6 rod. You would be surprised how long a GMC will run with know oil.

I understand that one of the military tests was to drain all the oil out of an engine (like the pan was shot full of holes) and see how long it ould run and the GMC was better than any other engine tested in the late 30's and early 40's and the military chose it.

I am positive that the rear main was installed correctly - this motor has 14,000 miles on it and the rear main looked perfect when I took it apart - #5 rod journal is the only bearing that had any damage - rod journal #6 also has a small crack but the bearing still looks good.

The thrust bearing did have some uneven wear - likely from the crank flexing under load.

This crank came from a pile of parts I picked up from an guy who saved them from getting scrapped after another GMC nut had to abandon them for lack of any place to keep them as he got older.

Here is one of two loads that I hauled from his barn. You can see the pile of cranks in the middle of the load - there were 4 6-bolt cranks and 4 4-bolt cranks and a dozen heads including small and large port heads with several H-Heads in the mix.




So I don't know the history of the crank. I dropped two more 302 cranks off today at the machine shop that grinds a lot of cranks in the Portland area - they will clean them up and check for cracks and pick the best one to grind. I only have bearings for 0.030 under for rods and mains now (Moraine 400's NOS) - so I hope the one that is M0.020 /R0.020 is good and we will go with that one.

Sort of like the guy at the swap meets who runs the engine all day with no oil pan to sell his super additive. Man does he get pissed if you reach over and crack the throttle a time or two. 4500 rpms needs oil. I don't care what you add!

Ken, Sounds like my garage. Seems like cranks (10), heads (11), and blocks(7)are stacked up like cord wood hidden under benches and behind cabinate doors.

All in a 2 car garage where I keep my 2 Motorcycles (Harley and Indian) Our LSR Roadster and my wife's Honda Civic. My racing had one consession; her car had to park in the garage. From a 65 Chevelle to a 73 Monte Carlo to 85 Pontiac Grand Am to a 2000 Honda...Next is a Smart car as I need the room.

My wife and son will have a field probably chucking then after I'm gone if I don't use them up. I'm 67 now and just switched blocks for this years run a Bonneville in October. Getting ready to build a 12 port next year and gathering parts...Things are getting expensive and my time is running out.....Good Luck

"Things are getting expensive and my time is running out....."

Ah Jeez JD, Those are the most sobering words I've heard in a long time. Makes two of us.

Are you talking about the tunnel at the end of the light? As long as it's the timing light I'm with you. I'm only 66 and that's the route I'm taking. I hope I don't leave all this junk for my wife, of 42 years today, and my boys to sort out.

Beater, make sure to bring your PU into Reno next week as I'll have some 4 banger parts for you to haul to Carson City, we'll see if she lets you live until the 43rd. year. We're doing 40 this year.

JD - heads stacked like cord wood is a good description of one section of my shop!

I heard back from the crank grinder today - he said that both of the cranks that I gave him had soft bearing journals (Rockwell 20) and he said that they liked to see journal surfaces closer to Rockwell 50 - 55.

He said that it appeared that the rod journals had been damaged due to a spun bearing and excessive heat after already having been ground once before.

Is this something that anyone here is familiar with? I have had a couple of people tell me before I took the crank in that the journals are surface hardened and that if I go to much undersize that I will lose the harness.

I'd be interested in your thoughts on this.

I've run into that with some european engines I built for some road race guys before. It was highly recommended by the manufacturer that the crankshafts be re-nitrided if you reground them even just .010 undersize from standard.

Ken, first time I've heard of case hardening for those cranks.

I was under the understnding that the cranks were heat treated with more of a through hardening, with some anealing afterwards for ductility.

Nitride with a good cryotreat should fix any issues you are having.

Well - mac from the Old GMC Truck website offered to send me a crank that he had tucked away. It had standard dimension journals and came from a low mile 270. He dropped it in UPS this morning so it should be here Monday or so. Hopefully the 3rd crank is the charm!

Crank thrust surface needed welding and grinding but everything else came out great at 0.010 under - will be balanced along with flywheel+pressure plate and harmonic balancer this next week - hopefully I can get it put back together over the next couple of weeks of evenings and have it back on the road before the rains hit.

Like many things on the internet, I ran across this post while looking for something else. I realize it is nearly 10 years old, but hey... better late than never.


I experienced a similar failure several years ago on my 44 GMC, with a 270. The engine had been rebuilt by a PO sometime in the past. Unknown mileage - although based on internal wear, I don't think it had a ton of miles on it since rebuild.

For a week or so prior to the failure, I thought that I heard a very, very faint and intermittent clunking noise under acceleration. Then, one day under normal driving, there was a large sickening bang, followed by a severe knocking noise.

Dropping the oil pan revealed that the crankshaft had broken just aft of the #3 main bearing journal. The fracture extended through the counterweight up to the #5 rod bearing. Unsupported, the crankshaft section with the #5 and #6 rods wobbled and cracked the rear main bearing cap in several places. It also cleaved out a piece of the camshaft, which punched a hole in the block.

Inspection of the crank revealed that it had been ground 0.020" undersize, and whoever ground it made sure that the journal to throw intersections were nice, square 90 deg corners - no radii at all. In looking closely at the failure, it appears that the crack started at the #5 rod journal, then propagated across the oil passage and down to the #3 main bearing. My theory is that this was caused by the failure of the crankshaft grinder to maintain a radius along the journal corners, which caused a stress concentration at the rod journal, resulting in a crack which grew until the point where the crank lost enough strength that it finally failed.

WRT any surface hardening, I had always wondered about any possible ramifications related to loss of surface hardness when grinding these crankshafts. So, I talked to the metallurgy professor at the community college where I teach part time. This guy actually worked as a metallurgist for GMC on cranks and other engine parts in the 70s and 80s, so I trust his judgement.

Always on the lookout for interesting projects for his students, he offered to let them section the crank, and characterize the area of the fracture, along with some of the other journals. Their findings were that it was just a steel forging, with no appreciable post manufacturing surface heat treating.

I asked if perhaps any surface treatment could have been removed when the crank was ground, but he did not seem to think so. His opinion was that the heat treatment techniques of that era, if they had been applied, would have likely extended deeper than 0.020". He also felt that it was unlikely that any "topical" surface treatments such as nitriding, would have been applied to a crank of that era - assuming it was a stock crank, which I believe it was (It had a PN on it)

On a side note, I also let his students section the broken piece of camshaft, as I had also wondered about the effects of cam grinding. His response was extremely surprising - even to him. Apparently, the camshaft in that engine was cast iron with absolutely no surface heat treatment whatsoever. He kept asking me the mileage on the engine (Which I did not know) as he felt that it was unlikely this cam would have gone even 10K miles, before it experienced severe wear on the lobes. This cam had "USA" cast into it, but was not marked "GMC" nor did it have a PN cast into it like on other OEM camshafts that I have seen. So I guess the message is buyer beware, when purchasing old pats for these engines.

Best Regards…

[img]https://photos.app.goo.gl/X5YSkoPFkGJ84Za66[/img]


[img]https://photos.app.goo.gl/ECyLyxEpp2aNfF6W8[/img]


[img]https://photos.app.goo.gl/hsajj6QDV6tCcUQA9[/img]


[img]https://photos.app.goo.gl/GdGshv7tgw6pKVMfA[/img]