Broken Crankshaft

[Destroked 450]

About a year ago I found the crank had broke in my 4000SU with 350 hours on a complete overhaul

Can't mow hay because it's raining so I figured I do a little mechanic work and pull the engine down.
Tractor ran fine other than having low oil pressure after the engine warmed up.

Previous owner had stumped the oil pan and broke the oil pump off.
The crank had to be turned down .010 on the mains and .020 on the rods.

Notice the large fillet radius on the replacement crank

The much smaller radius on the broken crank indicates the machine shop ground the crank wrong.

Said it had been too long so I get to eat the whole cost of repairs, luckily I got a good deal on a good used crank, I'll polish it myself.
They won't be doing any more crank work for me.

 

[Bern]

When I saw a broken 3-cylinder crank, my first thought was "That's pretty rare". I've seen plenty of 6-cylinder cranks do that when the front
dampner fails, but I don't think I've ever seen a 3-cylinder Ford crank break.

Your explanation makes total sense now that you explained it. There are good machinists, and there are bad ones - kinda like the ones who
will surface any flywheel until it's flat.

 

[Dean]

Bingo, DS.

I expect that the OEM fillets are rolled. Such cranks cannot be ground.

4.2 L Buick V6 and all turbo 3.8L V6 engines have rolled fillet rod journals. The rod journals cannot be ground.

Dean

 

[Destroked 450]

Bern this is the first 3 cylinder crank I've ever seen break, cranks in these engine are tuff, hardly ever need anything done to them.
I did see a 256 four cylinder crank from a A62 wheel loader with a crack in it, operator had ran it over 3000 hours without changing oil until it finally stacked a bearing.
Never worked on the larger 6 cylinder tractors, not many of them in this area, 7710 was a very popular tractor around here, like to have one myself.

Dean these cranks can be ground but the machine shop has to reshape the edge of the grinding wheel for the larger radius.
Apparently something they failed to do in my case.
A friend had a Case diesel crank reworked at the same time as mine by the same shop, he also broke a crank but his was noticed right away.

 

[RBnSC]

No doubt grinding any crank weakens it. If stress broke this crank don't you think it would have broken
on the back end of the third journal not the front side of the first journal.
Ron

 

[Hobo,NC]

Good info thanks for posting it...

 

[Destroked 450]

Grinding a crank doesn't really weaken it to a great extent, I seen a rod journal on a 3406 Cat crank turned down .050 to save the crank, Cat offers bearings in that size.
But the fillet radius must be maintained at all times which requires shaping the side edges of the grinding wheel to match the radius so that it is maintained during the grinding process.
If the radius is ground away (as was in my case) the crank is destined to become two pieces of junk (as what happened in my case).
The radius eliminates a sharp edge, sharp edges are where cracks start.

 

[Destroked 450]

I don't know why it broke where it did, this is the first one of these cranks I've ever seen break, but I do understand the reason why it broke.

 

[ErnieD]

Did the shop assemble the engine or you? In my world low pressure after warm up is wide bearing clearance. The shop should offer
to refund the grind job; if not the crank they ruined. You could offer to drop the crank off at their nearest competitor...

 

[Destroked 450]

I do my own engine assembling, always have.
I was working in a open shed in Feb putting it together, when I found there was a oil pressure issue I started second guessing myself about the bearings, with the crank turned .010 mains and .020 rods I was wondering if I had gotten the wrong bearings and had overlooked it because I was cold.
Wasn't the case, I had overlooked the job they had done grinding the crank, which should have been a non issue.

 

[Dean]

ALL cranks with rolled fillets will break if ground.

You doubt?

Read the service manual.

Who knows what the Ford/CNH service manual says, regarding hundreds of issues. The GM service manual regarding hundreds of thousands of such issues is FAR more authoritative.

Read it.

Dean

 

[RBnSC]

How many of those crankshafts are out there running that have been turned "wrong" that are running every
day. Do you think that every shop but the one you used knows the proper way to grind the radius? I think there was a flaw in the forging.
Ron

 

[Destroked 450]

Sorry Dean but it's not a GM gas crank and it's not a rolled radius.
I appreciate the info on the issues GM had with some of their cranks, something I didn't know, but there's enough steel (not cast) in this 3 cylinder crank to make 2-3 V6 GM cranks.

Grinding the crank under sized is the recommended procedure in the Ford factory service manual
Service manual gives size recommendations for cranks ground to as much as .040 undersize, but specifies the fillet radius size of .120-.140 must be maintained.
When I removed the crank for the overhaul it had already been turned .010 on the rod journals, but it had been done properly.
If simply grinding the crank was the issue it should have broke sometime during the 3 years my neighbor owned or the following 6 years I had it before it blew a head gasket and I found a scored cylinder from a broken ring. That's when I tore it down and the crank was ground a second time, the second time was when the wrong radius was ground into the crank leading to it's failure.

Grinding the crank isn't what caused the failure, grinding the wrong fillet radius into the crank was the cause.

 

[Dean]

It's your crank. Do as you like.

Dean

 

[Destroked 450]

(quoted from post at 21:16:39 05/29/18) It's your crank. Do as you like.

Dean

I understand your opinion and that's ok, but the replacement crank is standard and cleaned up with a light polish so it shouldn't be a issue.

 

[Bern]

I agree 100% with you Destroked. Grinding Ford cranks is something I've done several times in my career, and have never seen one fail after
the fact. The only Ford cranks that I've seen fail were in 6-cylinder engines with 2-piece (i.e. failed) front dampeners.