Thread: 400 SBC Rod Bearing Failure

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Well, I have been away from the shop for a while but have gotten down to it with this engine. The side clearances on the rods are between 15 thou and 18 thou. It seems that I might have been running the rod bearing clearances a bit tight, because when I pulled the caps on the rest of the rods, I found a problem. #s 3, 4, 5, & 6 looked fine and had a clearance of about .002 (perhaps just slightly under, but well over .0015). #s 7 & 8 were just starting to pull the bearing material, and had a clearance of .001.
I have cleaned up the crank (the first and last throws had trace scratches), and now have a clearance near .002 (using the plastigage shows just under .002, but well over .015).
Hoping that this solves the problem.
Thanks for the advice and tips.

400s' have a larger stroke than a 350 and therefore produce more compression. A common problem is that people tune with 4 degrees too much ignition timing which causes predetination and takes out both rod and main bearings as you described. Maximum timing is 34 degrees when fully advanced which means the timing needs to be dialed in somewhere above 3000 rpm.

Well, I got it together, in the boat, and running. I set the timing to 4 degrees BTDC, and the idle to 650 RPM. (It runs like a dream at that RPM). On initial start up cold, the oil pressure went up to about 50 -55 PSI @ 850RPM, and about 65 - 70 PSI @ 1500 RPM. After getting it good and warm, (about 175 which is normal for this marine installation), I reset the idle to 650. Oil pressure at that RPM hot was between 25 and 30 PSI. At about 1800 RPM (the max you can take it on a trailer with a garden hose supplying cooling water), the oil pressure ran about 50 PSI.
I welcome opinions on whether that sounds about right.
I have put about 2 hours on it so far, and these numbers have been consistent.
Also, is 4 degrees BTDC a correct initial setting for that engine? I have some conflicting info on this, and would welcome your input.
Next step is to get it in the water, and start breaking it in.

Oil pressure sounds great.

4 degrees is, I think, the recommended static timing for a SBC. Most guys will dial in a little more (10-12) to improve throttle response, then limit the total advance to somewhere around 34-36 degrees.

I would pay particular attention to TyphoonZR's post about total timing. If you were not the person who built the motor and have no idea about the squish distance (piston to head clearance at TDC), then stay on the conservative side of total ignition advance to prevent the motor from detonating and knocking the bearings out again. With a tight squish (0.035" to 0.040"), you can run a full load of ignition timing on pump gas.

Well, I have a bit over 20 hours on this engine now. I have changed oil on it a couple of times, and been very careful breaking it in.
The idle is set at 650 RPM, and runs at that warm (about 175 F). On initial start up cold, the oil pressure still goes to about 50 -55 PSI @ 900RPM, and about 65 - 70 PSI @ 1500 RPM. The oil pressure at idle (650) now runs about 15 - 18 PSI after a run at cruising speed. After running slowly for a while, it recovers slightly. Oil pressure recovers to about 25 PSI at 1000 RPM, and at 3500 RPM, I am running about 55-60 PSI after it is hot and has been running for a half hour or so. This has been consistent. I have been running Mercruiser 25W 40 oil (after the initial break in period).
It appears that opening up the clearances was the trick. Thansk to all who contributed ideas and advice.