Thread: 400 SBC Rod Bearing Failure

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I have a 400 SBC chev (4 bolt) that I marine converted. I got it from a local car guy, and it was supposed to be rebuilt. I plastigaged the bearings, and all looked well, although # rod bearing was at the minimum clearance within spec. I installed it, and it ran well with great oil pressure, until a got a knock. I pulled it out, and #1 bearing was fried. The bearing material was pulled with copper showing. The only problem I could find is one I overlooked last time I had it apart. # 2 rod bearing is on backwards. The piston is in right, but the rod chamfer is facing # 1 rod instead of facing the crank throw side. It didn't hurt # 2 rod bearing at all except for some wear along the edge of the bearing. Two questions:
1) Would the #2 rod installation cause the failure of #1 bearing?
2) Are these rods offset .060 in the pistons? (In other words, you can't just turn it around, can you?) I figure the mark on the piston is there for a reason. Any help is greatly appreciated.

You'll have to turn #2 rod around on the piston. Seems strange that the one next to it would go, but the tight clearance might have done that.

Originally posted by jwallis
I have a 400 SBC chev (4 bolt) that I marine converted. I got it from a local car guy, and it was supposed to be rebuilt. I plastigaged the bearings, and all looked well, although # rod bearing was at the minimum clearance within spec. I installed it, and it ran well with great oil pressure, until a got a knock. I pulled it out, and #1 bearing was fried. The bearing material was pulled with copper showing. The only problem I could find is one I overlooked last time I had it apart. # 2 rod bearing is on backwards. The piston is in right, but the rod chamfer is facing # 1 rod instead of facing the crank throw side. It didn't hurt # 2 rod bearing at all except for some wear along the edge of the bearing. Two questions:
1) Would the #2 rod installation cause the failure of #1 bearing?
2) Are these rods offset .060 in the pistons? (In other words, you can't just turn it around, can you?) I figure the mark on the piston is there for a reason. Any help is greatly appreciated.

If you put a rod in backwards you should have noticed this when you turned the engine over.I would say by doing this your rod side to side clearance is greatly reduced.A marine application is very much like a circle track application and you would need a min. of .015 of an inch.The chamfered side always goes towards the thrust side of the crank.What was your rod bearing clearence?

The engine didn't seem that tight, but I might not know what I'm talking about on that. When I got the engine, the main bearing bores were out of line (had it line bored). Then, it was really tight.
The bearing clearances on # 1 and #2 were between .001" and .002" (a bit over 1.5 thou) measured with a good micrometer and confirmed with the plastigage.

It's not really possible to put a rod in backwards because the outside of the rod is chamfered to clear the radius between the crank pin and web. You would have a hard time turning the engine over if you were able to get one tight.

Originally posted by jwallis
The engine didn't seem that tight, but I might not know what I'm talking about on that. When I got the engine, the main bearing bores were out of line (had it line bored). Then, it was really tight.
The bearing clearances on # 1 and #2 were between .001" and .002" (a bit over 1.5 thou) measured with a good micrometer and confirmed with the plastigage.

For a marine engine this seems a little tight.I would say .002 to .0025 would be a little closer.You never did say what your rod side to side clearence was.

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.