Don't buy a used Cat 315 excavator until you've verified the swing bearing condition and the engine serial number against the known emissions update cycle. That's the single piece of advice I wish someone had shouted at me in October 2021 when I was staring at what I thought was a 'deal' at a Ritchie Bros. auction. I skipped that step, and it cost me roughly $4,200 in unexpected repairs within the first four months of ownership.
How I Learned This the Hard Way
My background? I run a small site-prep crew in central Texas. We do mostly residential pad prep and some small commercial work. I handle equipment purchasing for our fleet, which at the time consisted of a beat-up Cat 305 and a rental credit line I was getting tired of using. In late 2021, I decided we needed a mid-size machine: the 315.
Everything I'd read about buying used excavators said to check the hours, look for leaks, and run the machine. That standard advice is fine—or rather, it's incomplete. It's what I followed. The machine looked clean for a 2016 model with 4,200 hours. It tracked straight. The hydraulics felt smooth. The undercarriage was at 60%.
I bought it for $68,500.
The surprise wasn't a catastrophic engine failure. It was the swing bearing. Three months in, the upper structure had developed a noticeable clunk when rotating under load on a side-slope. That's when I learned the difference between a 'tight' swing bearing on flat ground at the auction lot and a failing one on your job site. (Should mention: I asked the mechanic who inspected it—he pointed out the play was subtle, and on flat ground, it can be hard to spot.)
That repair—a bearing swap plus labor since I don't have the tooling—ran $3,800. The other $400 was a seized idler adjuster I missed because I didn't test the grease zerk on the track tensioner.
The 6-Point Checklist That Now Exists Because of That Mistake
I've documented 14 significant mistakes across the equipment I've bought since 2017. Roughly $23,000 in total wasted budget. The checklist I now use for any used excavator purchase, especially the Cat 315 series, is based on the three most expensive errors (the swing bearing being #1). It's saved us an estimated $8,000 in potential rework over the last 18 months by preventing us from buying three problem machines.
1. The Swing Bearing Test (The One I Missed)
- Don't just rotate on flat ground. That's a basic function test.
- Do position one track on a 6-inch block (or a curb edge). Rotate the upper structure 90 degrees so the boom is over the raised track. Then slowly swing left and right. If you feel or hear a thunk, or see the boom drift slightly at the start of rotation, walk away.
- The conventional wisdom is that a little play is normal on a machine with 4,000+ hours. My experience with this specific series suggests otherwise: the Cat 315 uses a sealed bearing that, once it develops measurable play, tends to get worse exponentially.
2. Engine Serial Number Verification
The Cat 315 (especially 2015-2018 models) had a mid-cycle engine update involving the fuel injection system. Some earlier machines in that range had issues with the DPF regeneration cycles causing prematurely high idle hours. I now check the engine serial against Caterpillar's service database (as of November 2024). If the engine falls in the pre-update range, I budget $1,500 for a potential injector replacement or factor that into my offer price. I should note that not all pre-update engines had issues—but I've personally documented two failures in my network on that serial range.
3. The Idler Adjuster (The $400 Sore Thumb)
This is a quick one: locate the grease fitting on the front idler. Try to pump grease into it. If it won't take grease (seized zerk) or if grease immediately leaks out the seals, the adjuster is shot. It's maybe $200 in parts but labor is a pain because you have to remove the track. I missed this one because I didn't have a grease gun with me on the first visit.
I went back and forth between bringing my own tools and relying on the seller's 'inspection.' Ultimately, bringing a $15 grease gun is worth more than the $400 this mistake cost me.
When a 'Good Deal' on a Cat 315 Isn't Worth It
The checklist isn't infallible. It won't catch everything—a failing hydraulic pump can sound fine at low idle and only show symptoms under sustained load. But it catches the expensive surprises. If a machine fails any of these three points (swing bearing, engine serial concern, idler condition), my rule is simple: pass on it, or adjust your offer 10-15% lower to account for the known risk. If it fails two of them, I rarely even make an offer.
At least, that's been my experience with the Cat 315 series. For a larger machine like a 320 or 330, the checklist items shift slightly (the swing bearing test is still crucial, but the engine issues are different). Your risk tolerance might be different, especially if you have in-house repair capability. I don't, so I'm more cautious.
