Here's the engine.... Brand new 383 w Scat crank, 5.7" X rods, Hypereutectic 9.7:1 pistons, #186 heads, #472 intake, List 4053 Holley, HEI ignition, Melling hi volume oil pump.

Here's the story..... Primed oil pump and started engine which ran great. No vibrations, no leaks, 60lb oil pressure droping to 20 at idle, everything ran great except for the choke. It didn't want to idle very well while cold. I took it to a very reputable garage to have the carb/choke worked on and they adjusted the choke, set the floats, rejeted the primary 3 times, changed my vacuum advance from manifold to ported and set my timing to 16 deg initial. It ran great for about 125 miles and then I started hearing a knock at about 3000 rpm, but only on acceleration, not when I decelerated. This knock became real worse real quick and after towing it home and disassembling the engine I find the remains of a spun #1 rod bearing, which caused the #1 piston to hit the head. A piece of the piston was broken off completely. It didn't seem to hurt the head, but the valves will have to come out of #1 and the seats cleaned. The crank is trash along with the #1 rod and piston.

My question is: Was this caused by detonation or not? If not, what would cause the rod bearing to spin? The oil pump was primed before the engine was started. The assembly of the engine was done very meticuously. I know rod bearing clearances were set to .0025. After the tune up I did hear some pinging, but not bad, and I attributed it to a slight exhaust leak they said I had.

Thanks for any ideas you may have.

[This message has been edited by kurt6325 (edited 10-08-2001).]

One thing to keep in mind is #1 is furthest from the oil pump. You could uspect an oiling problem if you see some unusual wear on say # 2 or 3 rod bearings, close to the problem. Did the mains look OK? Oil passages could be blocked. Seems like a complete lack of oil would have made itself known sooner than 125 miles. I'd hate to think about it, but did this noise appear for sure after you got it back from the shop? Sorry to hear about it. Must be severe fustration.

I checked the #1-2 rod journal and the #2 rod journal is fine and the #1 rod journal is trashed beyond .030. During assembly all the side clearances and end play were checked and set correctly. Plastigage was used to check all bearings. The bearings were Clevite and rod bolts were ARP. The main bearings and all the rest of the rod bearings all look fine. This is in a 4 bolt main 010 block. The block was blasted and cleaned, threads were all chased and passages cleaned. Bored and honed to .040 over with torque plate. There was no audible pinging or knocking before the shop tuned it up. The only reason it ever went to the shop was for carb/choke adjustment.

The odd thing about the broken piston is that it doesn't look to be burnt. I would think that if there was detonation that the small piece that was broken off would not be there at all. When the head was removed the broken piece was in its place on the piston. Couldn't hardly even see the crack. If there was detonation, shouldn't it have been melted out the exhaust? The piston broke when it hit the head, which was obviously after the bearings were gone.

The shop documented that when the timing was set to 16 deg on ported vac that my total timing was 34 deg. To me that doesn't sound all that bad, but I don't know how much advance I was getting at part throttle from the ported vac. Could this engine have been detonating and pounded out the bearing without burning up the piston?

The main reason for my concern as to why this damage happened is that the owner of the shop tells me that if this was their fault then they will make it right. My view is the same but if it is not their fault, then they should not have to pay for it. Fair is fair.

kurt,

Let's take a look at the timing side of this. How much mechanical advance was built into your dist.? How much advance was built into the vacuum canister? When the canister source was manifold, most of the vacuum advance was factored into the initial. Give us the numbers and we can go from there...

I don't know all the timing numbers. The distributor is a Dave's Small Body HEI and I'm told by Dave that the mech adv. should be completely in around 2800 rpm and shouldn't be over about 30 deg total. The timing was set to 12 deg on manifold vacuum after the engine was initially started. It ran OK with this except for the needed carb adjustments. I never checked the mech and total timing. Dave Ray tells me that 16 deg on ported vac is a sure fire receipe for detonation.

If I-man built the distributor then, according to his past posts, it should have 10* vacuum and 14* mechanical and the initial should be 12 to get 36* total. This is what I have read from all his posts.

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68 RS, Ash Gold,pwr brakes,pwr windows,Hounds tooth,355/T350,MSD 6AL,3200 stall Dynamic converter,3.31:1 posi,Autometer guage pod

[This message has been edited by CA420 (edited 10-09-2001).]

What bothers me about this was your initial oil pressure...a brand new engine with a hi-volume oil pump and only 20 psi at idle??? I have built a lot of engines in my lifetime and have never had a fresh engine with idle pressure that low...even with a standard oil pump. The current 3 year old short block in my truck with a standard volume pump still puts out 35 psi at a hot idle with 10w-30 oil. Did you have the rods resized and/or checked for roundness? One way to tell if you was running into detonation is examine your spark plugs closely...do you see specks of grey on the porcelin? I am assuming that these 9.7-1 pistons are somewhat dished? Detonation is a possability, but I dont think that was your problem.

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375hp 78 Chevy truck
77 Chevy Nova
95 Chevy Lumina 3.4L
and building a 78 Nova

The rods were all resized and balanced. All big ends and small ends were balanced to the same weight. All the other rods and journals look perfect except for some scratches due to all the schrapnel floating around after the #1 bearings went south.

My oil pressure may have been as much as 30lb at idle. All I have is the factory console gage to go by.

I did examine the spark plugs when they came out. I could not see any signs of alum or glitter. But I don't have the correct light to examine them either. I did save them all and marked what hole they came from. Maybe I should take them in someplace and get them checked.

To finish my thoughts from the post above.... Not all Dave's curves were exactly the same but using the numbers you provided (12 initial 30 total = 18 built in), yours would look something like this after the shop tuned it.. 16 initial + 18 = 34 total and maybe 12 more vacuum at part throttle. That's about 46 at part throttle... I would guess that would be why you heard some pinging after the tune up... Did any of the other rod bearing have any funny wear marks at all? I would think if the problem was caused by the detonation there would be tell tale signs of all the rod bearings getting hammered some...

All the other bearings and journals don't look too bad except for the schrapnel scratches.

I would like to know if the timing is advanced that far, could that pound out a rod bearing before it burnt up a piston?

The melling oil pump is a high volume-high pressure pump but I didn't have the high pressure spring in it.

I don't know enough to really say detonation caused it! Any audable pinging is bad and there is such a thing as silent knock as well... Detonation is when the plug fires too soon. When this happens before the piston reaches TDC you can imagine the force placed on the rod and bearing. Of course like shooting at a moving target, you lead the target a bit. Advancing the timing BTDC is the same thing... You probably already know that though!!

With no other bearings being pounded on I would think the one that failed may have been pre-conditioned to fail. The pinging you heard may have been enough to not cause damage to the others and just enough to clobber #1... At this point I'd have a hard time putting this on the shop that did the tuning...