Spun a Main bearing

[Shblack]

contractor was using our Komatsu WB-140-2N backhoe to move some stuff on our remodel, I wasn’t paying attention to how much he was using the machine, unfortunately a leak at the rear main seal got worse (I saw the trail it left) and it ran out of oil & the engine seized. Not in a good place to work on, on the wrong side of a big trench, outriggers out, bucket down, boom out.

So we removed the front drive shaft and dropped the pan, a shard of aluminum in the oil gave a clue as to what was in store. The rear main bearing has spun, carving a groove in the cap and saddle in the block and the crank has a crack starting to form. Had a set of main & rod bearings which we installed, hope to get it running to the point where we can baby it and get it out of the muddy field it is in and into the shop.

So what to do? It’s a Yanmar S4D106, which is fairly common. Found a number of cranks on eBay, so it’s just a matter of which one. Bigger problem is the block, is it a lost cause? I have read of a number of cases where folks have used JB weld on main bearings with some success. Is this just a bad idea - not sure what I have to loose. This is a farm machine, gets used a few hours a month - but really miss it now it’s down.

 

[Tenwheeler]

BAD IDEA!!! Fix it correctly or park it.

 

[Nige]

+1 on the above. either fix it right or, a) dig a hole and bury it or b) use it as a boat anchor ..........

So what to do? It’s a Yanmar S4D106, which is fairly common.

I have no knowledge of this model but if as you say it's that common of an engine then why not try to find a complete one for sale (doesn't have to be a runner or 100% complete) that you can plunder for a block and a crank to form the basis of your rebuild..?
And I would strongly suggest that as the one you have now was run out of oil that you look very closely at the condition of the oil pump while you're at it.

 

[Shblack]

Thanks for the advice, will start searching for a donor engine, found on in Ga, but it threw a rod. Since I am very new to engine rebuilds in general and diesels in particular, could you educate me on why the scoring on the surface the bearing rests against is lethal?

 

[Wes J]

The bearing is just a thin shell, it needs a solid surface to support it. If there are voids behind the bearing, it can also overheat due to a lack of conductive heat transfer.

Fixing a spun main is routine for a good machine shop. They will machine the journal and build it up with spray weld, then line bore all the journals to the correct size and location.

 

[Shblack]

Been meaning to give an update, here goes. The machine was in a poor spot, soggy backfill between the machine and the road, outriggers and boom out. So our first strategy was to try and change the main bearings in the field so it could limp out and get it to the shop. Put a set of bearings in, unfortunately, when we would tighten down the #4 cap the engine would get tight, I am guessing the crank was bent as well as being cracked.
So we waited for things to dry out a bit, then got another hoe in, lifted up the outriggers, and drug it to the road and got it on a trailer.
Got it in the shop, stripped it down and pulled the engine, sent the block to the machine shop. Word was that the block was salvageable, but 3 bearing caps were beyond repair. The bearing caps are only sold as part of the block (machined together), however I found a junk engine in Georgia (torn up cam bearing), was able to get a set of caps and a crank. Shop put the caps on, line bored the journals, honed the cylinders and good to go. Bottom end of the engine is back together, heads are back from the machine shop, waiting for a new oil pump to arrive Monday to get everything together.
One discovery upon pulling the engine was that a plate that ties the flywheel to the torque converter input had fractured. The broken part is made of 2mm thick stainless, is star shaped with a center bolt ring that goes to the flywheel and each of the 6 points bolting to the torque converter (the 6 points sheared off). So even if we had gotten the engine running in the field, we wouldn’t have gone anywhere! Upon examining the plate, I see fatigue cracking around the center bolt pattern and signs of old fractures where the tips broke off. So my guess is the plate was well on the way to failing, and the inertia of the rotating drivetrain when the engine stuck finished it off.
So what is the function of the plate? I am thinking that it gives some degree of axial and lateral compliance to accommodate small misalignments between the engine and drive line. Given the fatigue cracking in the plate and rather sudden failure of the engines rear main seal, is there another problem lurking here? BTW - drive line components are made by Carraro, and buying parts from Carraro is much less painful than getting them from Komatsu.

 

[Wes J]

It's a flex plate that allows for minor misalignment between the torque converter and flywheel. It's very common for them to crack. They are usually pretty cheap. See it all the time in forklifts.

 

[Shblack]

So having this come apart after 3,000 hours isn’t a big concern? Plate is $323.00 from Komatsu, on backorder, no due date available. Off the shelf from Carraro for $153.00. Naturally, there were several versions used on this machine, we ended up with the wrong one the first time, trying to get the right one now. Only known gap to getting the machine back together. Any thought why a flex plate is used rather than a spline?

 

[Ronsii]

Like Wes in the above post says, it so there is give when rotating, if they used splines the lateral movement would either not be enough and cause damage to the crank/TC and/or the splines would wear excessively and fail rather quickly.